United States
Environmental Protection
Agency
«&EfV\ Research and
Development
EPA-600/R-92_ 12 5
July 1992
PB92-218338
HVAC SYSTEMS IN THE
CURRENT STOCK CF
U.S. K-12 SCHOOLS
Prepared for
Office of Environmental Engineering
and Technology Demonstration
Prepared by
Air and Energy Engineering Research
Laboratory
Research Triangle Park NC 27711
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EPA-600/R-92-125
July 1992
HVAC SYSTEMS IN THE CURRENT STOCK OF U.S. K-12 SCHOOLS
by
Jerald D. Parker
Oklahoma Christian University
Box 11000
Oklahoma City, OK 7313 6
EPA Contract 68-DO-0097, Task 2-2
(S. Cohen and Associates)
EPA Project Officer: Timothy M. Dyess
Air and Energy Engineering Research Laboratory
Research Triangle Park, NC 27711
Prepared for:
U. S. Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
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ABSTRACT
Previous studies in school buildings have shown that radon
levels can be reduced by depressurization and ventilation of soil
under slabs, but that this method is not readily applicable on all
buildings. The heating, ventilating and air-conditioning (HVAC)
systems have also been shown to impact radon levels, either by
depressurizing areas and increasing radon concentrations, or by
pressurizing and ventilating to reduce radon concentrations.
This report summarizes inf ' Urn about types of HVAC systems
commonly found in U.S. .'tcL- ¦ buildings, their ability to
pressurize and ventilate cla&.-ocm spaces, and how they operate and
are controlled. Some information is given to compare systems as to
energy usage, cost and their ability to maintain stable levels of
static pressure in classrooms and/or to adequately ventilate the
spaces.
Not all HVAC systems are capable of providing pressurization
since some have no provision for the outdoor air to replace the
exfiltration losses always created by positive room pressure. The
level of pressure attainable in a space depends upon the fan
characteristics, the duct design, the room leakiness and the method
of control of fans and dampers. Return fans and relief dampers play
an important role in some systems, and exhaust fans always work
against maintaining positive room pressures.
There appears to be no well defined trends in types of HVAC
systems being installed in current school building construction and
modifications. Some systems using reheat and/or mixing have been
prohibited or their use discouraged by local codes and regulations
because they waste energy. Capital costs appear to vary more with
locale and quality of construction than with type of system
installed.
The unit ventilator (UV) has been the most popular type of
system in American schools but its noise and operating limitations
have reduced its popularity in recent years relative to central
systems. The UV system can provide limited pressurization and
dilution through outdoor air intake but the fan must be operating
for it to be effective.
The two-fan, dual-duct variable air volume (VAV) system
appears to be considered as an excellent choice for relatively low
operating costs in future construction and should be capable of
pressurization and ventilation. All HVAC systems will have
significantly increased utility costs if they are operated long
hours during unoccupied periods and/or if they are modified to
maintain higher static pressure levels in classrooms. This is
particularly true for U. S. school buildings, many of which are not
tightly constructed (i.e., they have high passive rates of outdoor
air exchange through the building envelope).
ii
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CONTENTS
Page
Abstract ii
Figures iv
Introduction and Background 1
The Prevalence and Radon Abatement Characteristics
of Various Types of HVAC Systems 2
Description of Terms 5
The Basic Central System 7
Constant Volume Central Systems
Pressurization with a Central System 9
Pressurization Control 12
Air Distribution; Return, Relief,
and Exhaust Components 16
The Economizer Cycle 17
Types of HVAC Central Systems 19
Single Zone 19
Multizone 19
Variable Air Volume (VAV) Systems 22
Reheat 27
Dual Duct 29
Air Induction 31
Unitary and Other Systems 34
Fan Coil Systems 34
Unit Ventilators 36
Exhaust Only 38
Radiant and Free Convective Systems 3 9
Unitary Systems 39
Rooftop Units 4 0
Packaged Terminal Units 4 0
Specialized Systems 41
Kitchens 41
Gymnasiums 41
Laboratories 42
Swimming Pools 42
Restrooms and Custodial Closets 4 3
Appendix A 1979 HVAC Survey Results 44
References 47
Bibliography 50
iii
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FIGURES
1*9,
Title
Paae
1
The Basic HVAC Central System
8
2
Example of a Pressure Profile in a Simple
HVAC "system
10
3
Pressure Differential Fan Control
13
4
The Diminishing Effect of Ventilation Rate
15
5
The Control of an Economizer Using
Temperature Sensing
18
6
Schematic of a Simple Multizone System
20
7
A Typical Control Scheme for a Multizone System
21
8
A Simple VAV System
23
9
Air Handling System for VAV with Return Fan
Control Using Flow Measurement
25
10
Air Handling System for VAV with Return Fan
Control Using Plenum Static Pressure
26
11
A Simple Reheat System
28
12
Schematic of a Simple Dual Duct System
30
13
Control of a 2-Fan Dual-Duct VAV System
32
14
Simple Diagram of an Induction Coil and
Typical Schematic of an Induction System
33
15
A Simple Fan Coil Unit
35
16
Typical Component Arrangement in a Unit
Ventilator
37
A-l
Distribution of U. S. School AC Systems in 1979
45
A-2
Distribution of U. S. School Heating Systems
in 1979
46
iv
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INTRODUCTION AND BACKGROUND
The United States Environmental Protection Agency (EPA) has
studied ways to reduce radon levels in schools since 1987. A recent
discussion of some of these studies is described in the proceedings
of the 1991 International Symposium on Radon and Radon Reduction
Technology, (1). Radon mitigation research to date has emphasized
reduction of radon levels through the use of active subslab
depressurization (ASD). Although ASD has proved successful in a
number of schools, it is not reasonably applicable in all school
buildings. As a result, reduction of radon levels with HVAC systems
needs to be investigated as an alternative approach to radon
mitigation, particularly in schools with moderately elevated radon
levels (4 to 20 pCi/1) . Leovic, Craig and Saum (2) concluded in
their stud;, that "One of the most significant factors contributing
to elevated levels of radon in schools and influencing the
mitigation approach is the design and operation of the HVAC system.
The complexities of large building HVAC systems present problems
not previously encountered in house mitigation."
Brennan (3) reported on the EPA School Evaluation Program
(SEP), involving site studies in 26 schools in 8 regional locations
in the United States. An HVAC system approach was the preferred
radon reduction technique over soil depressurization in 23 of the
26 schools evaluated. The reason given was that many of the schools
did not meet current standards for schoolroom ventilation, and that
radon levels were low enough that meeting ventilation standards
would likely solve the radon problem. A wide variety of
ventilation systems were found in the SEP schools, and many of
these systems were not designed or operated properly.
An update on radon mitigation research in schools by Leovic,
Craig and Harris (4) describes efforts through 1991 to evaluate
both ASD and HVAC methods in schocl buildings. One significant
conclusion of the report was that many HVAC systems in schools were
not supplying adequate outdoor air.
Researchers at EPA desired to better understand the various
types of HVAC systems that exist in kindergarten through twelfth
grade schools throughout the U.S. This report attempts to fill the
need for a reference document that identifies the various HVAC
systems that one should expect to find in U.S. schools, the ability
of these systems to pressurize and ventilate, the strategies to
control pressurization and ventilation, and to describe
modifications that might have been made by owners to conserve
energy, and how these might affect pressurization and ventilation.
1
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THE PREVALENCE AND THE RADON ABATEMENT CHARACTERISTICS
OF VARIOUS TYPES OF JIVAC SYSTEMS
Early in the study it appeared that hard data regarding types
and numbers of HVAC systems presently installed in U.S. schools was
not readily available. Some 1979 data were found giving the
distribution of U.S. school building heating and air- conditioning
systems. These data are described and shown graphically in Appendix
A. Most of the school buildings described by that data are likely
still in use, some with modifications to their HVAC systems. Recent
construction probably has led to trends different from those of the
1979 study, such as in the wider use of cooling, of forced air
systems, and of rooftop units.
The 1991 paper by Leovic, Craig and Harris (4) reported the
distribution of HVAC system types in the 47 AEERL research schools:
45% of the schools have central air handling systems, 4 3% have unit
ventilators, 30% have radiant heat, and 11% have fan coil units.
This distribution includes schools with combination HVAC systems.
Actually only 83% of the schools were designed to provide
conditioned outdoor air, the remaining 17 percent having only
radiant heat (11%) or only fan coil units (6%) . Most of the systems
providing outdoor air were not designed or operated to meet the
current guidelines recommended by ASHRAE.
Calls and inquiries to school administrators and staff and to
consulting engineers revealed no newer quantitative data but did
show clearly that there exists a very wide variety of HVAC systems
in use in U.S. schools. While most school systems operate within
guidelines of state and federal regulation they are generally free
to select their own architects and engineers and the designs and
policies followed in building construction are locally controlled.
The types of systems might depend more upon age, size of plant and
local economics and wage scales than upon geography or even
climate, although cooling systems do appear to be more common in
the southern, warmer states than in states with cooler or milder
summer climates. They are also more common in schools used
year-round.
With air-conditioning (cooling) very often there are more
elaborate ventilation and control systems than where heating only
is utilized. Some types of heating systems, such as radiant systems
for example, have no controlled ventilation as part of the system
and depend entirely upon radiation or free convection to transfer
heat to the controlled space and depend on infiltration for
ventilation.
School boards and administrators are becoming more concerned
with indoor air quality problems and hopefully will benefit from
the current attention being given to environmentally sound design.
ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air
Quality (5) gives guidance for improving indoor air quality in
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buildings. It is useful in defining terms, setting minimum air
requirements and in describing procedures for assuring that
requirements are met. Since schools, like many types of buildings,
are occupied intermittently, the procedures outlined in the
standard for assuring air quality under such conditions is
particularly useful. The impact of Standard 62-1989 on
heating/cooling capacity and energy use in both new and retrofitted
schools is discussed in a recent article by Wheeler (6). He states
that few applications are likely to be affected more decidedly by
ASHRAE Standard 62-1989 than the school classroom. This is so
because the standard's recommended rate of outdoor air could be
more than half the total air supply needed for cooling a classroom.
Moreover says Wheeler this rate is also the minimum permissible
total air flow as well as the outdoor air component.
The poor condition of many school buildings in the United
States is described in a report of the Education Writers
Association titled "Wolves at the Schoolhouse Door" (7) . This
report states that one fourth of all U.S. schools are in an
inadequate condition and that there is an estimated backlog of $41
billion in maintenance and major repairs. It also states that
maintenance funds are the first funds cut in a budget crisis and
estimates that 85% of maintenance budgets are spent on emergency
repairs rather than continued maintenance.
There is a boom in construction and refurbishing of school
facilities in many parts of the country, with the national level
being its highest since the 1950s. The estimate of the F. W. Dodge
Group of McGraw-Hill for 1990 is that elementary and high school
construction spending is at an all-time high of $10.7 billion and
is expected to continue at near this level throughout the decade,
A very large part of the current construction projects involve
overhauls of existing buildings, most of which were built during
the 1950s and 1960s and which were of generally low-cost
construction. Many school buildings have undergone modification,
some with only quick fixes attempting to reduce energy consumption.
Many school buildings have HVAC systems that need significant
repairs (7). Concern over costs of heating and cooling school
buildings and in meeting the new requirements of ASHRAE Standard
62-1989 are the main theme of the report by the American
Association of School Administrators, "Schoolhouse in the Red -
Cutting Our Losses" (8) . Thus it is a good time to be promoting
good design for improvement of indoor air quality (9).
Regarding future trends, the requirements of local building
codes will be strongly influenced by ASHRAE Standard 90.1-1989,
Energy Efficient Design of New Buildings Except Low-Rise
Residential Buildings (10). This standard and the desire of School
Boards to have both low initial and operating costs in their
buildings will probably cause certain types of systems (for
example, reheat and dual-duct, constant flow) to no longer be
3
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built. No single type of system seems to be the obvious best choice
for all schools. The following quote is taken from page 6.3 of the
19 91 ASHRAE Handbook (11):
"No trends in educational facility design as related to heating,
ventilating, and air-conditioning systems are evident. In smaller
single-building facilities, centralized systems are often applied.
These systems include unit ventilator, rooftop and single and
multizone-type units. Central station equipment, especially
variable volume systems, continues to have wide application in
larger facilities; water-to-air heat pumps have also been used."
All of these systems have outdoor air capability that can be
used to adequately pressurize and/or ventilate a building if
equipment is sized properly.
The single most significant factor to be considered in this
study is the air distribution system of a school building and
whether that system has provision for outdoor air. Schools which
have no air distribution systems, for example ones with only
radiant heating or schools with exhaust only ventilation, cannot
(without modification) pressurize the space for reduction of radon
levels. Modification for radon abatement would require the
addition of an air distribution system, properly designed to bs
compatible with the existing comfort system or to totally replace
it.
Some existing school HVAC systems have air circulation but no
controlled provision for outdoor air. Some systems with outdoor air
have been modified to minimize (or eliminate) outdoor air to save
on energy costs,and in some systems outdoor air damper units no
longer operate properly due to poor maintenance. Brennan (3)
reported that every one of the 26 schools in the EPA SEP study had
at least one ventilation problem.
Pressurization of a building occurs when the amount of outdoor
air introduced into the building exceeds the amount of air removed
by exhaust systems. The excess air (air not exhausted by fans) is
forced out of the building through leaks in the building shell
(e.g., floor cracks, around windows and exterior doors). This
leakage of air from inside the building to the outdoors is referred
to as exfiltration. Air exfiltration always occurs under a
positive pressure condition. Therefore any system without
controlled outdoor air must be modified to provide that feature if
room pressurization is to be ensured. It should be obvious that
dilution of room air by ventilation cannot occur without the intake
of outdoor air. Room pressurization is always accompanied by some
dilution due to the required introduction of outdoor air.
After a brief introduction of pertinent HVAC terms the basic
central air system will be described and used as the basis for
comments on the different types of systems existing in schools.
4
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DESCRIPTION OF TERMS
The term central system applied to an HVAC system implies that
furnaces and chillers are located in a central area or areas in a
building and energy is distributed by air (in ducts) or by water or
steam (in piping) to and from the various zones served by that
system. A very large percentage of the HVAC systems in school
buildings are of the central system type. Where a campus,
institution or facility has its chilled water, and/or hot water or
steam generated in a single location the term central plant or
central station is used. In contrast to central systems and plants,
packaged terminal units and room air conditioners furnish heating
and/or cooling to a single space, usually without the use of
external ducting or piping.
Central units may be of the built-up type, being assembled on
site from components selected by the designer to meet the specific
application, or they may be unitary systems where the components
are factory assembled into an integrated package or packages.
Unitary systems can be split systems with an outdoor section
(condenser) being separate from the indoor section or they can be
packaged units as for example a rooftop unit. Both built-up and
unitary systems may have external duct work and plumbing or tubing
as well as some controls and electrical wiring installed on site.
If heating needs are furnished by vapor compression or absorption
cycles the term heat pump is often applied.
The system may consist of single or multi-zones. The term
zone defines an area of one or more rooms that is under the control
of a single thermostat. Central system may be further classified as
to how the energy is distributed to the various zones. The three
common types are all-air systems, air-water systems, and all-water
systems.
An all-air system provides complete heating and cooling by
supplying only air to the conditioned space. There may be some
piping connecting the chillers or heaters to the air-handling
devices. A simplified diagram of an all-air central system is shown
in Figure 1. The system shown has both a return air and a supply
air fan. Some systems have only a supply air fan. In systems havir'y
both fans the room pressurization level is determined by the
characteristics and operating speeds of both fans as well as the
positions of the exhaust air and the outdoor air dampers and the
room tightness. Negative room pressures are possible if the return
air fan overdrives the supply air fan and/or if the exhaust air
exceeds the supply air rate. With a single (supply) fan the room
pressure depends on the relative amounts of exhaust and makeup air.
As emphasized before, for a positive (above atmospheric) pressure
to exist in a zone there must be a higher rate of makeup air than
exhaust air. In such cases the zone would be exfiltrating or losing
air through cracks and openings to the exterior.
5
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In an air-water system both air and water are distributed to
each zone to perform the conditioning function. The bulk of the
heating or cooling in the space occurs in a heat exchanger coil
that may be part of an induction unit, a fan coil unit, or a
radiant strip or panel. Individual zone control is obtained by
controlling the rate of air flow across or rate of water flow into
the coil. Air or water temperatures or both may be changed to
permit control of space temperatures year around. Because the bulk
of the energy required for maintaining comfort can be easily and
compactly carried in pipes by the water, only enough air needs to
be delivered to the zone to maintain pressure levels and/or
comfort. This greatly reduces the duct size and fan power required
resulting in many cases in both reduced installation and operating
costs. Duct size can be further reduced by using high velocity air
distribution. The air is usually supplied at constant rate from the
central system and is referred to as primary air to distinguish it
from room air that is recirculated over the room coil.
If the rate of primary air supply exactly balances that
required for exhaust and/or exfiltration then the return system can
be eliminated. This represents a significant cost reduction for
installation and makes some types of air-water systems good
candidates for future consideration when room pressurization is
desired. With no air return the room would operate under positive
static pressure, neglecting any wind or stack effects on the
building. Some form of room pressure relief damper might be
necessary if the required makeup air exceeds that which will
naturally exfiltrate at the desired room static pressure.
All-water systems heat or cool a space by direct heat transfer
between water and circulating air. Heating systems include
baseboard radiation, free-standing radiators, wall or floor
radiant, and even bare pipe. Cooling systems must provide for air
flow including outside ventilation air, drain pans, and filters.
The equipment designed for this purpose is referred to as a
terminal unit. The all-water system has the advantages of requiring
less building space for the delivery system, since pipes replace
ductwork. Individual room control is easily maintained and there is
little cross contamination of recirculated air from one space to
another. Two disadvantages are that maintenance must be done in the
occupied areas and condensate removal can be a problem.
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THE BASIC CENTRAL SYSTEM
Because it is the air distribution system that plays the
primary role in radon abatement by the HVAC system and because the
central air system has characteristics somewhat in common with all
systems it will be used at first to describe the basic problems and
approaches to a solution. The discussion will deal first with
constant volume air systems. The significant and unique
characteristics of the more common types of HVAC systems will then
be given, including systems that operate with variable air volume.
CONSTANT VOLUME CENTRAL SYSTEMS
The basic arrangement of an HVAC central air system shown in
Figure 1 is taken from McQuiston and Parker (12). Tne
characteristics of these components, along with those of the
controls, the duct system and the space being conditioned and the
operating schedule determine the HVAC effect on radon. In this
report it is assumed that radon abatement by 'the HVAC system is
entirely due to room pressurization relative to the subslab,
dilution by outside makeup air, and the schedule of operation of
the HVAC system. The effectiveness of radon removal by dilution is
determined to some extent by the air distribution within the room
itself. This room air distribution involves types and locations of
air diffusers and return grills and the resulting entrainment,
mixing and stagnation that might occur within the space being
served. Any discussion of ventilation and room air dilution will
also assume that outdoor makeup air is at typical ambient radon
levels of 0.2 to 0.5 pCi/1.
The components shown in Figure l are not present in all
central air systems. Systems designed to provide only ventilation
air and not the basic heating or cooling (fan coil systems for
example) may not have an air return. Systems without air returns
provide positive pressurization to the space and are prime
candidates for consideration where radon might be a problem.
In the smaller, basic central systems the fan cycles off when
the thermostat is satisfied and more often than not the thermostat
is setback during off hours. In this case there are long periods
when the fan is not running and pressurization is lost. Most
systems can be set to operate with the fan running continuously but
these extra hours of fan time can add extra expense to the school's
utility bills. A compromise would be to determine at what hour the
fans could be turned on so that through dilution and
pressurization the radon in a space could be reduced to an
acceptable level before the space is occupied. This would depend on
radon levels, source strength, etc.
The majority of central air systems used in residential or
light commercial and in some school buildings have returns but do
7
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C3
Exhaust
air
r
r
r
r
r
09
Q-
~
Outdoor
air
Return air
fan
r
r
r
r
r
r
Return
air
Supply
air
Humidifier
Filter
V
V V-V \ fit j)
|~J Thermostat controlled
dampers
Heating coil
Cooling coil
Supply air fan
Preheat coil
Figure 1 - The Basic HVAC Central System
Reprinted by permission of John Wiley & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston and Parker Copyright ® 1977, 1982,
1988 by John Wiley & Sons, Inc.
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not have provision for exhaust or outdoor air and the corresponding
dampers, and would have no need for a preheat coil. Most small
systems do not have a return air fan and many provide only heating
or cooling and no humidification. In typical small systems the air
flows sequentially (in series) through the heating unit and the
cooling coil.
In most small air handling systems and in many large ones the
air flow rate is constant regardless of the thermal load. The
variable air volume system compensates for variations in heating or
cooling load by regulating the volume of air supplied to each zone.
These systems are sometimes simply called variable-volume systems
and more often VAV systems. Energy conservation as well as improved
controls and equipment have made VAV an increasingly popular
option.
PRESSURIZATION WITH A CENTRAL SYSTEM
In these more simple systems, illustrated in Figure 2, the
air flow can be assumed to start in the room, flow through the
return ducts, through the air handler, then through the supply
ducts and back to the room. Initially the room pressure might be
assumed to be atmospheric and the pressure profile for the air
might be assumed as shown. In this case the pressure drop through
the return duct and filter and up to the fan is seen to be below
atmospheric pressure and the pressure drop to be about one half the
pressure drop from the fan to the room. The single fan furnishes
exactly the pressure required to make up the losses in both the
supply and the return systems, and the flow rate of air to the room
is exactly equal to the return flow rate from the room. Any
infiltration into the room through cracks and other openings will
be exactly matched by exfiltration from the room through other
cracks and openings.
The pressure variation shown in Figure 2 is the total
pressure, the sum of the static pressure and the velocity pressure.
The static pressure and the velocity pressure in a duct system may
tradeoff, increasing or decreasing in the flow direction as the
velocity of the air flow changes, but the total pressure can only
increase when energy is added to the stream at the fan. In a room
where the air velocity is relatively low the static pressure and
the total pressure are for all practical purposes the same. Static
pressure difference determines whether or not air flows in a
particular direction through a crack or opening such as might exist
in a duct or in a floor slab or wall.
It has already been emphasized that neither room
pressurization nor air dilution can occur unless there is an intake
of outdoor air somewhere in the system. With a single fan, such as
shown in Figure 2, the intake must be at some point where the
static pressure is below atmospheric, since that is only way that
air can be induced into the system.
9
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0.4
Element
Return grille
Return duct
Filter
Heat anfl cool coils
Supply ducts
Diffusera
Total
pressure
Iobb. in. H^O f Pascalb|
0.04
0.03
0.08
0.23
0.14
0.03
Fan total pressure-in.H.0 0.50
10. D
20.0
20.0
57.5
35.0
7.5
150.0
0.17 '
0.03-
-0.04
Distance ¦
-0.12
-0.2 —v
Figure 2 - Example of a Pressure Profile in a Simple HVAC System
Reprinted by permission of John Wiley & Sons, Inc.
From Keating, Ventilating and Air Conditioning, Copyright ° 1977,
1932, 1986 by John Wiley & Sons, Inc.
10
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If there are leaks in the return duct of the system where the
duct static pressure is below atmospheric, air will enter the
system. If that part of the system is located in the subslab radon
may be drawn in with that air. In many cases duct leakage works to
the detriment of heating or cooling efficiency. Unless that air
comes from the space being conditioned air must be exfiltrated at
the same rate it enters and at some location where the static
pressure is above atmospheric, either from the supply ducts or from
the room being conditioned by the system. If the rate of air loss
in the supply duct is less than that entering, then the space being
served will rise slightly in pressure over what it would otherwise
be and the excess air leaked into the air returns will be
exfiltrated to the surroundings. Any system will always operate so
as to be in balance, with the rate of air intake always equal to
the rate at which air is lost from the entire system.
For the same reason, any system that is exhausting air to the
outdoors must have outdoor air furnished to it at an equal rate. If
there are no forced supply systems or fans providing that air then
the space pressure will drop below atmospheric. The space pressure
will be at a level so that leakage in is equal the rate at which
air is being exhausted. If a space is to be pressurized relative to
the surroundings then the rate of forced air supply must be greater
than any rate of exhaust so as to provide the air that will be lost
by leakage.
Deliberate, controlled pressurization of a space having a
central air system and a single fan requires skill in duct design
and a knowledge of the fan's characteristics and of the room
leakiness. The control techniques required for zoned pressurization
were presented by S.A. Anderson (13). The room leakage or room
porosity, a term used in the predictive equations, is a function of
room construction. It is probably intuitive that tight rooms are
more easily pressurized than rooms with a great deal of leakage. It
would be expected in most cases that a room operating at a positive
pressure level relative to its surrounding could have its pressure
level increased by simply making the room tighter, assuming no
change in the HVAC system.
Modification of existing HVAC systems serving rooms which do
not have any level of positive room pressurization would require
the introduction of additional makeup air, most likely at some
location in the air return system where the static pressure is
below atmospheric. Even moderate levels of room pressurization are
difficult to maintain where exhaust rates are fairly high This has
been a continuing problem in operating clean rooms, for example,
where makeup air requirements for pressurization might be 25 to 50
times that required for typical office space. For such applications
Brown (14) presented a discussion of energy-saving opportunities
for makeup air systems. The techniques discussed there have some
general application to schoolrooms, especially where there might be
ah expenditure of significant energy in maintaining desired room
11
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pressures for radon reduction. In some cases excessive rates
outdoor air needed to provide pressurization might freeze the
heating coils in cold weather.
With regard to radon gas, which comes in primarily from the
soil, the room pressurization required is relative to that in the
subslab or walls in contact with the slab or soil. For rooms
directly above unoccupied basements the pressurization of
importance might be that of the room relative to that of the
basement space. Upper floors may be relatively free of radon
contamination. Pressurization levels needed to prevent radon entry
into a space are not high, being in the range of .004 - .016 inches
of water column (1-4 Pascals).
Outdoor air pressures are variable with changes in the
barometer and, for a particular wall, with wind direction and
velocity. Stack effects in a building may increase or decrease the
indoor pressure relative to that outside and further complicate
one's ability to predict with precision the room pressure levels
that might be attained in a particular situation.
If one has poor control of the room pressurization,
excessively high pressures may inadvertantly occur This can lead to
such undesirable behavior as doors standing ajar or hard to open
and slamming doors or whistling air through the space around a
door. Static pressures greater than 0.2 inches of water column
(50.0 Pascals) must be avoided at any time to prevent problems with
opening doors. Dirkes (15) and Holness (16) have discussed the
procedures and the difficulty of maintaining balanced air pressures
in buildings of the large industrial type. These discussions may be
pertinent to school shop areas and gymnasiums.
Raising the room pressure will increase the static discharge
pressure on the supply fan and this will in turn reduce the air
flow, assuming constant fan rpm. This reduced air supply from the
fan combined with increased exfiltration losses may leave some
systems short of necessary air flow to meet the comfort demands of
the system. In some cases the fan may even be backed up to the
point where it cannot operate at a stable condition. In some cases
fans will need their rpm increased, in other cases fans may need to
be replaced.
PRESSURIZATION CONTROL
Ideally the relative air pressure should be controlled by a
pressure differential sensor that control fan speed or dampers to
maintain the desirf *1 pressure differential under all operating
conditions. A typical control block diagram for damper control only
with fixed fan speed is shown in Figure 3. A schematic for
pressure control using both damper and fan speed control for a
typical VAV system from Haines (17) is also shown.
12
-------
OUTSIDE
AIR
SNUBBER
-"^=3
T . —• FROM SUPPLY FAN
RETURN AiR
y nc
\
ulier—
SP
— STATIC PRESSURE CONTROLLER
I—O— SENSOR IN CONTROLLEO SPACE
SENSOR IN REFERENCE LOCATION
TO AIR HANDLING
'sysiem
Figure 3 - Pressure Differential Fan Control
From Haines, Control Systems for Heating, Ventilating and
Air Conditioning, Fourth Edition (17)
With permission, Copyright ® 1987 by Van Nostrand Reinhold Co., Inu.
13
-------
The importance of the flow transmitter selection for return
fan control in VAV systems is discussed by Smith (18). Location of
both indoor and outdoor sensors is critical to satisfactory
operation. Building pressurization control is discussed from a
practical standpoint in the Trane Applications Engineering Manual,
Trane (19). The difficulty seems not to be in just maintaining a
suitable pressure level in a space but rather in doing this
economically while maintaining thermal comfort within the space
being pressurized.
In many schools there are separate buildings, connected by
corridors which may become uncomfortable "wind tunnels" if the
pressure differences between buildings is not controlled. It may be
desirable in some cases, such as with laboratories, to maintain
some reasonable pressure difference to control fumes and odors. The
pressure control problem can be particularly critical for variable
air volume (VAV) systems or where economizer cycles (with large
changes in outdoor air) are utilized. Atkinson (20) discusses the
control of such systems and the experience with a situation in a
Western U.S. university laboratory and classroom building. He
concluded that control could be accomplished in such cases if the
duct pressure control system in each building can control
accurately, laboratories are always controlled to remain at
negative pressure relative to classrooms, and by fine tuning
relatively balanced mechanical system airflows by measuring the
differential pressure across the corridor and making up outdoor air
or relieving the classroom building to maintain the building
pressure relationships at an acceptable level.
The similar problem created in multizone buildings with
variable thermal loads in each zone, a building stack effect and
the need for odor and fume or smoke control for some zones is
discussed by Bentsen (21). He concluded that flow tracking or
control of air flow rates should be used in zones with high leakage
and zones lacking containment, for example where there are large
openings between zones. Static pressure control seemed, to be best
suited for tightly sealed, contained zones, for example where doors
are always closed between zones.
A secondary benefit from introduction of outdoor air for
pressurization is the dilution of the polluting gas in the
conditioned space. In many practical situations rooms have a large
number of cracks and openings to the outside and the exfiltration
caused by pressurization requires the introduction of significant
amounts of outdoor air. Thus pressurization would rarely occur
independently of ventilation. On the other hand ventilation can
occur in a space without room pressurization, depending upon the
relative rates of outdoor, infiltration and exhaust air. There is
a practical limit to what ventilation can accomplish, a limit which
is dependent upon the source strength of the polluting source. This
is illustrated in Figure 4, which shows the diminishing effect of
increasing air ventilation rates. Piersol and Riley (23) in their
14
-------
12
50% reduction in
radon concentration
results from a twofold
increase (0.25 to 0.5
ach) in ventilation.
75% reduction results
from a fourfold increase
in ventilation
/
90% reduction results
from an eightfold
increase in ventilation
I III L_
0 0.25 0.5
1.0
2.0
3.0
Tight
house
Average
house
Leaky
house
Ventilation rate, air changes per hour (ach)
Figure 4 - The Diminishing Effect of Ventilation Rate (22)
15
-------
study of ventilation and air quality state that "Pollutant source
strength, not ventilation, is the predominant parameter in
determining indoor pollutant levels." This study did not consider
the effect of pressurization in reducing source strength, such as
might occur with radon entering from the subslab.
AIR DISTRIBUTION; RETURN, RELIEF, AND EXHAUST COMPONENTS
Return fans, such as the one shown in Figure 1, are necessary
when the pressure drops in the return ducts are excessively high
due to long duct runs or inadequate space is available for a
normally sized return duct. Simply removing the return fans from a
system might result in increased room pressure levels but without
controls the pressures might vary unacceptably. The removal of
return fans might also reduce room outdoor air flow rates to
unacceptable levels. Return fans are commonly used to permit the
conditioned space to be held closer to atmospheric pressure, a goal
which appears to be in opposition to what is desired in radon
abatement.
Designers have widely varying opinions about the use of return
fans, some never using them and others employing them frequently in
their designs. Thus one might expect to find their use in one case
and not see them in a very similar type of system designed by
another firm. Compensation for this difference would be in the
duct design, with larger return ducts used in the systems without
return fans.
Care must be taken in the use of return iins since they can
create negative pressures in the conditioned space and may create
disturbing fluctuations in the flow. An additional complication
occurs if the supply fan operates on a variable flow basis (as in
VAV systems) since then the return fan must "track" the supply fan
properly to maintain both the proper rate of makeup air and the
desired space pressure. Kettler (24) discussed some of the field
problems associated with return fans on VAV systems and suggested
that major problems in surge and building pressurization
difficulties could be solved by controlling the supply fan from
duct static pressure and the return fan from the building
pressurization. using low-limit control. In new designs relief
fans should be used instead of return fans so that the problems
created by return fans are eliminated.
Alcorn and Huber (25) state that many stability problems in
control of supply and return fans in VAV systems are the result of
the fans being coupled in the control scheme and due to
deficiencies in the control algorithms being used. Their paper
discusses how some of the problems may be corrected.
Care must be taken in both design and operation of new or
modified systems to assure that no part of the duct system is
16
-------
damaged by the consequences of too high or too low a static
pressure within the duct.
Relief fans are used to control the static pressure in return
ducts. They differ from return fans in that they are not in series
with the total air return flow and they remove some air from the
duct. They are similar to return fans in that their operation tends
to reduce the pressure in the room. Fans which remove air directly
from the conditioned space are referred to as exhaust fans.
The operation of relief and exhaust fans in an existing system
may be necessary in order to assure that there is adequate incoming
outdoor air and to assure .continuous thermal comfort and
satisfactory air quality but they tend to lower room static
pressure levels, and consequently will work against radon control.
THE ECONOMIZER CYCLE
It is often possible to use cool outdoor air to either totally
or partially meet the cooling load in one or more zones. This is
especially the case during the spring and fall seasons and at night
in northern latitudes and at high altitudes. Some buildings have
interior zones that require cooling year around and the economizer
can be used to meet that need during the cooler part of the year.
This results in significant savings in many cases and the extra
equipment required to do this is called an economizer. The
economizer consists of the controls necessary to sense the outdoor
and indoor (or return) air conditions and to operate the intake,
return and relief dampers to provide the optimum amount of fresh
air. for the existing conditions.
Figure 5 shows two of the possible arrangements, using only
temperature sensing to control the dampers. In (a) when the
outdoor temperature is at the winter design condition the outside
and relief dampers are set to provide the required minimum outside
air to meet ventilation requirements, and the return dampers are at
a maximum open position. For warmer outdoor temperatures the mixed
air thermostat (Tl) opens the outside air damper to provide the
desired mixed air temperature and the return and relief dampers
adjust accordingly. At some outdoor temperature, usually between 5 0
and 60 F (10 and 16 C) the damper system will provide 100% outside
air. At some higher temperature, usually slightly above 70 F (21 C)
the high limit thermostat will close the damper system back down to
minimum outside air. An interlock with the supply fan assures that
the outside air damper will close when the fan is off. Outdoor
humidistats are widely used in place of thermostats to limit the
use of outdoor air if the outdoor air enthalpies are too high for
maintaining comfort.
The system shown in (b) operates in a manner similar to the
system in (a) except that there is a separate damper to provide the
minimum outside air whenever the fan is running and which is not
17
-------
R€LIEF
DAMPER
OUTSIOC
AIR
HIGH
Limit
low
LIMIT
FROM
SUPPLY FAN
TO AIR
HANDLING
SYSTEM
(a) No fixed minimum
outside air
RELIEF
DAMPER
FROM
»—~- SUPPLY
FAN
(b) Fixed minimum
outside air
Figure 5 - The Control of An Economizer Using
Temperature Sensing
From Haines, Control Systems for Heating, Ventilating and
Air Conditioning, Fourth Edition (17)
With permission, Copyright ° 1987 by Van Nostrand Reinhold Co., Inc.
18
-------
affected by the air temperatures.
In both cases, where room pressurization control is desired,
there must be sufficient outdoor air at minimum setting of the
damper system to provide positive room pressure and there must not
be too high a room pressure when the economizer damper system is
providing high rates of outdoor air. Static pressure control of the
relief system may need to be considered.
TYPES OF HVAC CENTRAL SYSTEMS
SINGLE ZONE
A single zone system can be used where all of the space or
spaces have heating and cooling requirements sufficiently similar
that comfort conditions can be maintained by a single controlling
device or thermostat. Very large spaces and large or multi~story
buildings usually require more than a single zone to maintain
comfort in all spaces.
In a single zone constant-air volume (CAV) system the
distribution of the air to the rooms is fixed by the design of the
ductwork, and can be modified only to some degree by the adjustment
of dampers within the duct system or at diffuser outlets. In a zone
with multiple rooms and with limited air returns with restricted
air flow between rooms a variation in pressure between rooms is
highly probable with the possibility of one or more rooms being
below while others are above atmospheric pressure. As with any
system, pressurization of all rooms could only be attained with
total rate of system intake of outdoor air exceeding that of the
exhaust.
MULTIZONE
In all-air, multizone systems the discharge area of the air
handler is divided so that several zones may be served, with
separate temperature control in each zone. Hot and cold decks
within the air handler provide heating or cooling as required in
each zone. Dampers in the air handler are controlled by the zone
thermostats to supply the proper air temperature and flow to each
zone to meet that zones' individual load. A schematic of a typical
multizone system is shown in Figure 6. The typical control system
for a multizone system, shown in Figure 7, is the same as that for
a constant volume dual-duct single fan system. Normally there is no
provision for automatic control of either room or duct static
pressure as these are set by system design and component
adjustment.
If room pressurization and control is to be added to a
constant volume multizone system it might be most easily
accomplished by addition of controlled relief dampers in the return
duct of each zone and a corresponding reduction in return and
19
-------
Ma xiinum
outdoor
air
Minimum
outdoor
air
Figure 6 - Schematic of a Simple Multizone System (12)
Reprinted by permission of John Wiley & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston and Parker, Copyright ° 1977,
1982, 1988 by John Wiley & Sons, Inc.
20
-------
Figure 7 - A Typical Control Scheme for a Multizone System
Reprinted by permission of ASHRAE from the 1991 ASHRAE
HVAC Applications Handbook (33)
-------
central exhaust flow. With this modification for pressure control
there would be concern that the existing fan and duct system could
provide sufficient air flow to all zones to meet comfort demands.
The second concern is that additional quantities of hot and cold
air would be used to maintain comfort during both occupied and
unoccupied periods and operating costs could be increased
significantly. These concerns would be particularly valid if the
building or parts of it have high leakage rates.
With supply and return ducts for each zone it is possible for
the zones to operate at different static pressures although
normally the engineer would attempt to design for each space to be
near atmospheric pressure. In a multi-story building, where it
might be necessary to pressurize only the ground floor rooms,
careful duct design or modification might be necessary. For a
positive pressure to exist in every zone the makeup air rate must
exceed the overall exhaust air rate for the air handler.
For energy savings it is common practice to close outdoor air
dampers during warm-up and unoccupied cycles. If room
pressurization is considered to merit the additional operating cost
involved, this control feature must be modified to allow the
minimum makeup air rate required to obtain the desired level of
pressurization.
Some multizone systems may have been modified to be VAV
systems to conserve energy, particularly where cooling is the major
load. These modifications could be carried out in several different
ways. Wendes (26) discusses ways to accomplish this modification.
VARIABLE AIR VOLUME (VAV) SYSTEMS
The variable air volume system has been mentioned previously.
These systems, which compensate for variations in heating or
cooling load by regulating the volume of air supplied to each zone,
are sometimes simply called VAV systems. Energy conservation as
well as improved controls and equipment have made VAV an
increasingly popular option.
In the VAV system each space supplied by a controlled outlet
is a separate zone with its own thermostat. A VAV schematic is
shown in Figure 8. Although some heating may be done with a
variable volume system, it is primarily a cooling system and should
be applied only where cooling is required a major part of the year.
The best candidate buildings are those with large internal loads.
A secondary heating system, such as baseboard heat, should be
provided for boundary zones. During the heating cycle tempered
fresh air is supplied to these boundary zones to maintain air
quality. For zone pressurization a minimum rate of supply airflow
would be required, depending upon zone leakiness and rate of return
air flow.
22
-------
Return fan
NJ
U>
0=3-2
Return
air
VAV
uniti
U 4, rt
CooU Supply fan
Figure 2-19. Variable air-volume system.
Figure 8 - A Simple VAV System
Reprinted by permission of John Wiley & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston and Parker, Copyright ® 1977,
1982m 1988 by John Wiley & Sons, Inc.
-------
The variable air volume concept has been applied to a variety
of HVAC systems including both single and dual duct, with and
without reheat, and with both all-air and air-water installations.
It offers significant savings in operating costs, due primarily to
reduced fan power costs at part load. In some cases it permits
closer control and, especially where there is load diversity,
reduced first costs. For cool weather conditions the economizer
cycle is easily adapted to VAV systems. In addition the duct
systems are virtually self-balancing. Haines (27), Wendes (26), and
others have described the double-duct, double fan VAV system to be
one of the most energy efficient HVAC systems available.
The main concern with using VAV systems to control radon
levels is the maintenance of suitable static pressures and rates of
dilution in each zone under all conditions of operation, especially
at the very low air flow rates that normally occur when there are
low cooling loads in one or more zones. It has been mentioned
previously that Wheeler (6) expresses concern over the effect of
ASHRAE Standard 62-1989 on school classroom ventilation rates. With
VAV systems the new standard will require ventilation rates that
will prevent large turndown ratios. IN such cases concern with
reduction in the pressurization level will be lessened. However,
this will tend to overcool the classroom space when cooling loads
are low and will require reheat to maintain thermal comfort.
Wheeler suggests that the number of occupants in a classroom be
estimated realistically when computing required outdoor air. He
also suggests the use of a fan powered VAV system to produce a
continuous high rate of room air exchange and the employment of
demand controlled ventilation (DCV) to save energy.
An additional concern arises in VAV systems when return or
relief fans are used since they must successfully track the
variable flow supply fan. These concerns have already been
discussed in some of the previous sections. Alcorn and Huber (25)
have discussed the need for taking special care in the design of
the automatic control sequence. Their method involves decoupling
the control of the supply and return fans and selecting suitable
scan times in the direct digital control system.
Wendes (26) discusses the maintaining of building pressures
when using VAV system with no fan controls (fan bypassing to obtain
VAV) . If no economizer is used, the outdoor air damper can be set
to bring in sufficient air to cover losses and still maintain
pressurization. The proper amount of bypass and return air must be
maintained to prevent over or under pressurization. If an
economizer is used there may be too much pressurization when
additional outside air is brought in for free cooling. Relief air
to the outside, equal to the additional outside air brought in,
must be provided.
24
-------
FROM SUPPLY
FAN STARTER
I
I
Figure 9 - Air Handling System for VAV with Return Fan Control
Using Flow Measurement
From Haines, Control Systems for Heating, Ventilating and
Air Conditioning, Fourth Edition (17)
With permission, Copyright © 1987 by Van Nostrand Reinhold Co., Inc.
-------
FROM SUPPLY
FAN STARTER
t
RETURN FAN
WITH VOLUME
CONTROL
f~ OUTSIDE AIR |
RELIEF AIR j j
\/\Htp'\/\
NC r-L, NC
OM
~
\
tf NO FI1-TEB
\
r
zzz—t:
CC Ul
5 5
X Z
VI
NO
Tt
OA
CC CO
u u
V2
NC
J
"V
sp
ALTERNATE
SENSING
POINT
7
LLT
HEATING
COIL
LOW TE.V1?
SAFETY
COOLING SUPPLY FAN
COIL WITH VOLUME
CONTROL
Figure 10 - Air Handling System for VAV With Return Fan Control
Using Plenum Static Pressure
From Haines, Control Systems for Heating, Ventilating and
Air Conditioning, Fourth Edition (17)
With permission, Copyright © 1987 by Van Nostrand Reinhold Co., Inc.
-------
Haines (17) describes the control of VAV systems having return
fans. His schematic for control using flow measurement is shown in
Figure 9 and for control using plenum pressure is shown in Figure
10. He states that flow measurement is more accurate but more
expensive. Plenum pressure control is simpler and less expensive.
Holding plenum pressure constant provides constant pressure across
the outside air dampers and thus a constant flow of outside air,
but at least 10 percent outside air is necessary to maintain
control.
REHEAT
The reheat system, a variation of the single-zone system,
permits close temperature control in several zones having unequal
loading. A simple schematic of a reheat system is shown in Figure
11. A heating device (hot water or steam coil or electric
resistors) is placed in the duct system leading to each zone and is
controlled by the zone thermostat. Preconditioned air coming to the
heater is heated to maintain comfort in that particular zone. The
air flow to each zone may be constant or variable.
In the cooling season the central unit should provide cold air
at a temperature just sufficient to meet the requirements of the
zone with the largest cooling load. The other zones are furnished
sufficient heat via their reheat coils to provide comfort in those
zones. In the heating season the heat is added to the recirculated
and makeup air which in some cases has been preheated at the
central air handler. Reheat is often used where humidity control
is desired since the air can be cooled to a very low, and therefore
very dry, condition before being reheated to the desired level of
temperature and low relative humidity. Reheat systems are not
commonly found in classrooms in primary and secondary schools in
the U.S. and their use in other situations has been limited by
codes and general concern over energy conservation.
Reheat systems do not present any special features that
distinguish them from the basic central system as regards room
pressurization and air dilution. As with the basic central system
these desired capabilities depend entirely upon the fan, duct and
room characteristics. Modification of these components to
accomplish the desired levels of radon reduction should generally
create no new problems in maintaining comfort, providing that
satisfactory air flow is maintained across coil and/or heater
surfaces.
The zone heater is usually controlled by the thermostat to
provide the necessary comfort regardless of the air flow rate to
the zone. Since pressurization may require increasing the air flow
rates to some or all zones there may not be sufficient fan power to
provide the air flow rates necessary. Simply increasing air flow
rate from the fan may not provide even pressurization to each zone.
Fans may need to be replaced or their speed modified. In some cases
27
-------
Terminal
reheat unils
in each zone
Figure 11 -A Simple Reheat System (12)
Reprinted by permission of John Wiley & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston and Parker, Copyright ° 1977,
1982, 1988 by John Wiley & Sons, Inc.
-------
adjustment of supply duct dampers or a combination of damper
adjustment and addition of relief dampers might be required.
Additional intake of outdoor air will be required in any case. As
with any system the introduction of higher levels of outside air,
except during economizer operation, will tend to lower the heating
or cooling efficiency of the unit. Since reheat systems have not
been considered energy efficient some of these systems have been or
should be modified into VAV systems. Wendes (2 6) discusses some of
the steps involved in such a modification.
The 1991 ASHRAE Applications Handbook (11) states, "For energy
conservation, reheat systems have been restricted by ASHRAE
Standards 90A-80 and 90.1-1989 and usually cannot be justified for
schools, unless recovered energy is used for reheat.
"Dual-conduit and dual-duct systems are recommended if they
are designed to minimize air quantities to those required and
incorporate adequate energy conservation features to make them as
economical as other systems."
The next section will discuss the dual-duct system and its
special characteristics.
DUAL DUCT
Dual duct systems that are capable of meeting the energy
conservation requirements of the latest ASHRAE standards have been
recommended for use in some school systems (11). In the dual-duct
system central equipment supplies warm air through one duct run and
cold air through another. A simple schematic is shown in Figure 12.
The temperature in a zone is controlled by a mixing box that mixes
the warm and cold air to maintain the thermostat set-point. For
dual-duct systems to work satisfactorily some form of control is
required to maintain a constant overall flow rate of air as the
thermal load changes. According to Int-Hout (28) the primary
disadvantage with dual-duct systems has been the complexity of the
necessary control systems and the lack of an affordable zone
controller. The availability of affordable microprocessor-based
controls has permitted dual-duct systems to now be considered a
desirable alternative. Energy codes which prohibit the use of
dual-duct systems unless energy savings can be proved in advance
can now be more easily satisfied with the new controls technology.
This type of system can provide great flexibility in
satisfying highly variable sensible heat loads between zones. It
is capable of furnishing heat to one zone at the same time cooling
is being furnished to another. An economizer with all outdoor air
can be used when the outdoor temperature is low enough to handle
the cooling load. Because of these features these systems are very
common in office buildings, hotels, hospitals, and large
laboratories. They are less common in school buildings.
29
-------
Figure. 12 - Schematic of a Simple Dual Duct System (12)
Reprinted by permission of John Wil&y & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston arid Parker, Copyright ° 197 7,
1982, 1986 by John Wiley & Sons, Inc.
-------
Many of these systems have been converted from constant
volume to variable volume (VAV) to conserve energy and almost all
new dual-duct systems proposed are of the VAV type. In VAV systems
two supply fans are usually used, one for the hot duct and one for
the cold, with each controlled by static pressure downstream in
each duct. If a return fan is used, and it most often is, it is a
single fan, controlled by room static pressure. Successful
application and advantages of these systems in California is
described by Linford (29), using 100 percent shutoff air valves
(dampers), pressure-dependent controls, and 100 percent
recirculation during heating. Outside air is furnished to perimeter
zones by mixing returns with the interior zones on the economizer
cycle. This system would require minimum outside air during heating
in colder climates than experienced in Northern California.
The control of such a system is shown in Figure 13 from ASHRAE
(11) . The advantages of the properly designed dual-duct VAV system
is further described by Kettler (30). Haines (27), a well-known
writer in the controls area, states "It is not unreasonable to
argue that two-fan double-duct with VAV may be the best possible
solution to many air-conditioning problems, in terms of both energy
conservation and quality of control.11 Wendes (26) makes a similar
statement regarding the energy efficiency of these systems. A
problem common to all VAV systems might occur in attempting to use
dual duct VAV systems for room pressurization since there would
have to be minimum flow rates established to maintain desired
pressure levels. In the case of classrooms, new outdoor air
requirements f.:er occupant might give higher minimum flow rates than
that necessary for pressurization however. Again the need for
reheat at low cooling loads might cause excessive energy
requirements and eliminate this type of system from consideration.
Higher first costs may also be important in eliminating these
systems from consideration.
AIR INDUCTION
Air induction systems are not common in school buildings but
need mentioning as a possible consideration. A simple diagram of an
air-water induction unit and a typical control schematic, Haines
(17) are shown in Figure 14. Centrally conditioned primary air is
supplied to the plenum at high pressure. The plenum may be
acoustically treated to reduce noises generated in the duct system
(often high velocity) and in the unit itself. A balancing damper
can adjust the rate of primary air over some limited range. The
primary air flows through the induction nozzles, drawing in
secondary air from the room and over the coil. The coil either
heats or cools the secondary air, which is mixed with the primary
air and the mixed air is then discharged into the room. Induction
units are usually installed under a window or overhead. In the
heating mode floor mounted units can operate by free convection to
provide heat with no primary air during unoccupied hours. In such
cases the pressurization and dilution effect of the primary air is
31
-------
CCCUMJ
supper
f*AM
Figure 13 - Control of a 2-Fan Dual-Duct VAV System
Reprinted by permission of ASHRAE from the 1991 ASHRAE
HVAC Applications Handbook (3 3)
-------
Miiad
mon siP^y
fam rrAjrrcH
Figure 14 - Simple Diagram of an Induction Coil
and Typical Schematic of an Induction System
From Haines, Control Systems for Heating, Ventilating and
Air Conditioning, Fourth Edition (17)
With permission, Copyright ® 1987 by Van Nostrand Rejnhold Co., Inc.
33
-------
not in effect.
Since induction units with no air returns give the room
positive pressurization as well as dilution when operating with
primary air a decision must be made with regard to their use for
radon control is one of scheduling the hours of operation with
primary air. With proper data one could determine a schedule that
would permit the turning off of primary air for some periods, but
which would still reduce levels prior to occupancy. For maximum
effect when operating with primary air the unit should be set for
highest primary air flow consistent with acceptable noise level and
considering the effect of changing damper setting on the remaining
units operating from the same central fan. As with most systems
having a positive space pressure the tightening of the room itself
will raise pressure levels. Assuming that the air delivery system
has been properly designed and constructed it would normally not be
productive to attempt to increase room pressurization levels by
increasing fan speeds or si'tes for moving additional primary air
through the induction units .
UNITARY AND OTHER SYSTEMS
FAN COIL SYSTEMS
A typical fan-coil unit, a terminal unit used in both
air-water and water-only systems, is shown in Figure 15. The fan
section circulates room air continuously across a coil, which is
furnished with either hot or cold water. In some fan-coils there is
a separate heating unit which may be electric, steam or water
heated. Primary (makeup) air may be ¦ furnished directly to the room
by a separate central system. The primary air is normally only
tempered to room temperature by the central system during the
heating season but is cooled and dehumidified during the cooling
season. During unoccupied periods of the heating season the primary
air may be shut off to conserve energy. This conditioning of air in
a central system does not appear to be common in school buildings.
As with the induction system there is normally no air return
and the space operates with positive static pressure during the
time that primary air is being furnished. The characteristics of
the air delivery system is totally independent of the fan-coil unit
and is not controlled by the zone thermostat. Room primary air
delivery and static pressure must be controlled by central fan
operation and any individual room dampers that might be present.
Room pressurization levels can be increased by improving the
tightness of the room.
Fan coil terminals or fan coil units have been described
previously as devices used to provide heating or cooling in
all-water systems as well as in the air-water systems. In all-water
systems no primary air is provided from a central source. The Type
1 fan coil unit has no provision for outside air, recalculating 100
34
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•f-
I
I
-t-
I
! 1—1 I
4 1—J
I I I '
•t
1
I
¦t
I 1
IT
r
1. Finned tube coil
2. Fan scrolls
3. Filter
4. Fan motor
5. Auxiliary condensate pan
6. Coil connections
7. Return air opening
8. Discharge air opening
9. Water control valve
Figure 15 - A Simple Fan Coil Unit
Reprinted by permission of ASHRAE from the 1987
ASHRAE HVAC Systems and Applications Handbook (33)
35
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percent room air, and is used for heating only in all-water
systems. Type 2 fan coils are designed to introduce up to about 2 5
percent outside air to control air quality and may have cooling as
well as heating coils. Type 1 units, with no provision for outside
air, would be a poor choice for classrooms, which require
ventilation, and would not be capable of providing room
pressurization for radon abatement.
A type 3 fan coil unit has provision for 100 percent outside
air and is usually referred to as a unit ventilator.
UNIT VENTILATORS
The unit ventilator is perhaps the most popular type of
heating system in U.S. classrooms, and the classroom is its primary
application. The heating unit ventilator heats, ventilates and
cools a space by introducing outdoor air but has no refrigerated,,
cool water provided to its coil, depending entirely upon the
outdoor air for any space cooling that might be required. An
air-conditionincr unit ventilator has refrigerated water provided to
its coils to provide air cooling in addition to its ventilating and
heating capabilities. The arrangements of components in typical
floor-mounted unit ventilators are shown in Figure 15 (31). The
units may be either blow through or draw through depending upon the
location of the coil or coils relative to the fan.
The fan in a typical unit ventilator runs continuously, with
the discharge air temperature being used to control the heating,
cooling and ventilating functions. Whenever the outdoor air
temperature is below the indoor temperature the heating unit
ventilator increases the intake of outdoor air to provide space
cooling. The air-conditioning unit ventilator can provide cooling
even when outdoor temperatures are too high for ventilative
cooling. A room thermostat controls the functions to provide the
desired comfort level. During heating seasons the room thermostat
is typically set down for unoccupied periods to save energy and
during the warmer seasons the unit is turned off.
The unit ventilator can provide positive pressurization to the
classroom since it brings makeup air into a space having no return
system. Pressurization would not take place of course when the unit
fan is turned off, during the warmup stage, or because the unit has
been modified to turn the fan off when the indoor setpoint is
reached. Pressurization would also not occur when the outdoor air
dampers are closed, as they might typically be during the warmup
stage or as a freeze protection measure when outdoor temperatures
are below a setpoint. In the EPA SEP study of 2 6 schools Brennan
(3) found that teachers frequently turned unit ventilators off
because of the noise.
36
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0UTSJ06 AW
OAMPEfl
outs** Am
oowNtcnof#"
DISCHARGE GWU.E
M ' * t
ooo
o
ooo
0
COIL LOCATION
on BLOW THROUOH
uwtrs
FAN
COIL LOCATION
-ON DRAW THROUGH
UNITS
return air
DAMPER
RETURN AIR
GRTU_£
Coil
—xam'//yjy/yss/y,
(a)coil-flow arrangement
(b)separate heating-cooling
coils
Figure 16 - Typical Component Arrangement
in a Unit Ventilator. Fig. 16 (a) reprinted by permission of
ASHRAE from the 19S8 ASHRAE Equipment Handbook (31)
Fig. 16 (b) reprinted by permission of John Wiley & Sons, Inc.
From Heating, Ventilating and Air Conditioning, Analysis
and Design (12) by McQuiston and Parker, Copyright ° 1977,
1982, 1988 by John Wiley & Sons, Inc.
-------
Since the unit ventilator is providing air only to its
classroom, pressurization of the room may be nullified if the
classroom door is open and the hallway, is not maintained at a
positive pressure.
The most likely modifications of this type of system for
energy savings would probably be in the lowering (or raising) of
thermostats and reduced hours of operation. Control of the fan
cycle and minimum outdoor air can be easily modified to provide
room pressurization over any period of time deemed desirable for
radon control. As with any system the cost of conditioning any
quantity of outside air beyond that required for comfort may be
significant. Pressure sensing and damper or fan control to maintain
room pressure would likely be prohibitive since it would have to be
applied to each classroom unit.
EXHAUST ONLY
Exhaust fans are frequently used in conjunction with HVAC
systems to balance air flows in a zone and they are also used as
the sole method of maintaining comfort in spaces with thermal gains
(cooling load). Exhaust fans always tend to reduce the static
pressure within a conditioned space. When used alone they always
create a negative static pressure relative to atmospheric, and in
most cases relative to subslab pressures. Where use would permit it
(as regards noise levels and air turbulence and velocity and code
requirements) exhaust only fans could be replaced with intake fans
to reverse the direction of air flow to provide a positive internal
pressure and exfiltration from instead of infiltration into the
space. Filters are usually desirable in such situations to prevent
the drawing in of dust and other even larger objects that might
more easily be ingested by intake fans than by leaking cracks or
openings with relative low air velocities. As with other HVAC
systems the level of pressurization obtainable will depend upon the
characteristic, number and location of the fans, the duct layout
(if any) and the leakiness of the space. It is practically
impossible to make reliable quantitative predictions of pressure
levels in advance of installation.
Whereas exhaust increases depressurization of the interior
space and thus the potential for radon entry, it also increases the
entry of outdoor air which will dilute the radon that has entered
the space. However exhaust and intake fans are almost never
operated continuously, and are usually off during unoccupied
periods and during periods of cool weather and in some cases where
there is high outdoor humidity. Of course when the fans are off,
the building internal static pressure will be approximately that of
the outdoors, assuming there is no significant stack or wind
effect. Some level of continuous pressurization could be
accomplished by operating one variable speed fan or sequencing
several small fans using static pressure difference control. In
most cases some form of supplementary heat would be required to
38
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supply the load created whenever introduction of cold outside air
would lower internal temperatures to an unacceptable level.
RADIANT AND FREE CONVECTIVE SYSTEMS
Some HVAC terminal units are natural (free) convective heating
and/or cooling devices in which the heating or cooling is delivered
without the use of fans or blowers. Thermal radiation provides an
important fraction of the total heat furnished from the units.
The term radiator is usually applied to units made up of
sectional cast-iron columns. The term convector describes a unit
where the heating element is surrounded by an enclosure with an air
inlet opening below and an air outlet opening above the heating
element.The term baseboard heater refers to heat distribution units
designed for installation along the bottom of walls, in place of
the conventional baseboards. The term finned-tube heaters refers to
heat-distributing units fabricated from metallic tubing with
attached metallic fins. They may be bare or have an enclosure. In
warm climates, where cooling is more significant than heating,
natural convection downward is more desirable than the upward
convection provided by these devices. Valance terminals consist of
finned tubing housed in an insulated enclosure which is placed on
the wall near the ceiling. Heating or cooling is accomplished by
circulating a fluid such as water, a brine or a refrigerant through
the tubing.
For these systems a supplementary system is necessary in order
to provide fresh air for humidity or air quality control or to
pressurize or depressurize the space. It is that supplementary air
delivery system, which may or may not exist, that must be evaluated
in the radon problem. Prescurization will require that the
supplementary system introduce additional outside air compared to
the conditions where no pressurization is required. One must be
sure that the terminal heating-cooling system is capable of
handling the additional thermal loads imposed by that makeup air.
The extra cost of the fan power as well as the extra thermal loads
will make careful scheduling of fan operation during unoccupied
hours very important.
UNITARY SYSTEMS
With unitary systems the components are factory'assembled into
an integrated package or packages. Unitary systems can be split
systems with an outdoor section being separate from the indoor
section or they can be packaged units as for example a rooftop
unit. Both built-up and unitary systems may have external duct work
and plumbing or tubing installed on site. Where central units may
be made up of an almost endless combination of components unitary
systems come in a limited number of discrete sizes, optimized for
a particular set of conditions. The advantage of unitary systems is
in the quality control of manufacture and quite often in the fact
39
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that they tend to have lower first costs. In some cases, such as
for rooftop units, the entire package sits outdoors and does not
take up valuable indoor space. Maintenance may be more difficult in
bad weather and performance may not be as good as carefully
designed and built central systems which operate in a more
comfortable indoor environment.
Rooftop Units
Because they usually have lower first costs than most indoor
systems rooftop units are very popular in new construction and
retrofits. They are usually ducted to distribute air throughout all
or part of a building and they may be single or multizone. They do
not create any special air flow situations not available or present
in central systems and therefore their use in radon control is
described in the appropriate section describing the particular
airflow system. An interesting quote is taken from the 1991 ASHRAE
Applications Handbook (11):
"Life expectancy may be less than for indoor equipment.
However, some school districts have successfully operated rooftop
units for over 15 years, with maintenance costs significantly less
than schools with other systems."
Packaged Terminal Units
Packaged terminal units incorporate a complete heating and/or
cooling system in one unit, usually to keep a relatively small area
comfortable. They are usually mounted on the floor near the outside
wall and are frequently seen in motels and hotels since they give
good economics in such situations. There is no ducting or piping
required, by fitting into the wall they take up little room space,
and they can provide either heating or cooling independent of what
any other unit in the complex is furnishing. They are also popular
in some building renovation projects since their installation is
less disruptive than in systems where there is extensive ducting
and piping.
In a typical package terminal unit outside air can be
introduced at the discretion of the operator, and since there is
usually no return the unit pressurizes the space it serves. The
level of pressurization depends upon the fan setting and
characteristic and the leakiness of the room.
Most package terminal units with cooling have high compressor
and fan noises which make them undesirable for use in classrooms.
Like fan-coil units they also require the servicing of many filters
and fans and the actual servicing must take place within the
classroom itself. This same feature means that only a small
fraction of the entire building would be without heating or cooling
at any one time due to equipment failure.
40
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Small size units, which usually only provide cooling, are
considered to be appliances and are referred to as room air
conditioners.
SPECIALIZED SYSTEMS
KITCHENS
Kitchens represent a unique situation because of the need to
prevent kitchen odors, fumes and smoke from escaping to other parts
of the school building. Kitchens are usually kept at a static
pressure below that of the dining rooms but should not be at lower
pressure than surrounding areas that might lead to contamination.
Local makeup air is usually provided at a point near the range and
other exhaust hoods to supply less conditioned air to be exhausted
to reduce energy costs. Because the rate of air removal is so high
in a properly designed kitchen, dilution by the forced makeup air
and the air drawn in from other spaces probably keeps radon levels
low. Maintaining kitchen static pressures consistently above
subslab pressure is probably not practical. Generally the best
abatement technique would likely be the sealing off of any cracks
or other paths through which soil gases might enter the kitchen and
operating the kitchen at a static pressure no lower than necessary
to provide satisfactory odor control in the dining areas.
GYMNASIUMS
Gymnasiums are characterized by the large space and high
ceiling in the playing area and for many cases by the anticipated
large internal loads from crowds. In addition there are usually
relatively large areas for dressing which include several shower
stalls. Many gymnasiums are separate from the remainder of the
school facilities and more often than not have their own
heating/cooling/ventilating system. Often the heating system
consists of fan type unit heaters or infrared heaters and/or
perimeter heaters, none of which have the capability of introducing
outside air. Most gymnasiums have either forced or natural exhaust
systems for maintaining comfort in warm weather and for removal of
crowd pollutants. The typical air change rate is four to six air
changes per hour. Increasingly gymnasiums are placed in
multipurpose use and the heating-cooling systems are designed with
that in mind. Cooling of gymnasiums is becoming more common in all
parts of the country.
Generally, significant pressure levels are difficult to
maintain in gymnasiums because of the large building volume and
extensive leakage of the typical type of construction.
Dressing rooms usually have significant levels of exhaust air,
which may or may not come from the main room of the gym. This
exhaust air which is primarily to remove humidity and odors will in
most cases serve to dilute the levels of radon in these areas
41
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during the hours of operation. Pressurization of these dressing
areas would not be desirable if the main playing (crowd) area were
not also pressurized to a slightly higher level since the humidity
and odors would otherwise flow into that playing area. For the
typical gymnasium that means that dilution would likely be the best
approach, assuming that reasonable steps have been taken to seal
off any passages through which soil gas might enter the building.
Controlled air intake systems, contrasted to the more common
exhaust fan systems, would work more favorably toward increasing
the static pressure level in the main gym arena.
LABORATORIES
Laboratories, particularly chemistry laboratories, are usually
designed to minimize the likelihood of dangerous or poisonous gases
being present in the laboratory or in adjoining spaces. Fume hoods
are usually provided for the more critical situations and require
the drawing in of makeup air either locally or from the laboratory
space. As a result the laboratory would normally be at a pressure
below that of the surrounding rooms at least during operation of
the exhaust system or the fume hood. Any pressurization of the
laboratory to keep its static pressure above subslab pressure would
need to be additionally equipped with controls to assure that the
laboratory pressure is always below the pressure in the adjoining
spaces. This most likely would be an undesirable complexity and
makes dilution the best HVAC candidate for maintaining safe radon
levels in laboratories.
Location of laboratories on upper floors, where possible, will
decrease or even eliminate concern about radon there. Since it is
not desirable to have positive pressures in laboratoriesthey might
be given priority in location on upper floors in future
construction or renovation.
SWIMMING POOLS
Swimming pools create a very large source of humidity that
must be continually removed from the space in which the pool is
located. Good design practice attempts to create a flow of air
from other spaces to the pool by maintaining a static pressure in
the pool area that is below that of any of the surrounding spaces.
This decreased pressure must be maintained constantly since the
pool is evaporating constantly. This makes it impractical to try to
operate the pool area at pressures less than might exist in the
subfloor around the pool. Fortunately the pool itself usually
covers a very large part to the floor space in the room where the
pool is located. There should be no concern for radon gas leakage
into the area occupied by the pool since a high hydrostatic head
will exist beneath the pool. Large exhaust and consequent makeup
rates for the pool area will tend to dilute radon level in that
area. Care must be taken that surrounding room pressures are not
dropped to below atmospheric as those spaces furnish some of the
42
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makeup air to the pool area. This balance can be obtained by a
suitably controlled fresh air intake and exhaust fans.
RESTROOMS AND CUSTODIAL CLOSETS
All properly constructed and maintained restrooms and
custodial closets have exhaust fans for odor removal. These fans
are intended to maintain pressures in the rooms that are below the
pressures in the surrounding spaces. It would be difficult to
maintain positive (above atmospheric) pressures in these types of
rooms and at the same time maintain them at pressures below that of
the surrounding rooms. Some dilution takes place in restrooms and
custodial closets. Because of this exhaust action, and since these
rooms are not occupied for long periods of time by individuals,
they are not a significant part of the radon abatement problem.
Good sealing of the floors and walls to prevent gas leakage seems
to be the most practical step.
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APPENDIX A
197 9 HVAC Survey Results
A 1979 survey, Nonresidential Building Energy Consumption
(NBEC) , conducted by DOE, provided Royal and Tsai (32) with
information on the types of HVAC facilities in educational
buildings in the U.S. This information has been reproduced
graphically in Figures A-l and A-2.
Among the heating systems at that time central radiators
(C-RAD), central forced air (C-FA), and unitary forced air
(U-FA) predominated in over 73 percent of the buildings.
Tb^ forced air systems could be expected to already have some
outdoor air and pressurization capability or can be modified t.o
have that capability. Information on the central radiators does not
show whether ventilation air is or is not furnished. If all or some
of the central radiators have outdoor air ventilation capability
the 73 percent total is still less than the 83 percent found to
have that capability in the AEERL study (4). one would expect that
most recent construction provides for outdoor air capability in
keeping with building codes and/or standards and the percent of
forced air systems probably continues to increase.
Among the air-conditioning systems, window units and packaged
units together were more than double the number of central systems.
Room pressurization and control of outdoor air quantities would
likely be more readily carried out with the central systems. Air
conditioning was found in less than 32 percent of the buildings.
The present trend is probably fewer
window units and an increase in the number of rooftops, a unitary
type of system with outdoor air capability.
Although many of these buildings and their systems still
exist/ many have been modified and updated and recent construction
probably has not followed the trends of previous years. It is
doubtful that these data are applicable to existing schools as
evidenced by the AEERL study (4).
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AIR CONDITIONING SYSTEMS
, figure A-J - Distribution of U.
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Other Systems
U-FA U-NFA C-FA C-RAD C-OTH OTHER
HEATING SYSTEMS
U-FA UNITARY FORCED AIR
U-NFA UNITARY NON-FORCED AIR
C-FA CENTRAL FORCED AIR
C-RAD CENTRAL RADIANT
C-OTH CENTRAL OTHER
Figure A-2 - Distribution of U. S. School Heating Systems in 1979.
46
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REFERENCES
1. EPA, "Session X, Radon in Schools and Large Buildings", In:
Proceedings: The 1991 International symposium on Radon and
Radon Reduction Technology, Volume 4, EPA-600/9-9l-037d (NTIS
PB92—115369), pp. P10-3 thru P10-60, November 1991.
2. Leovic, K.W., A.B. Craig, and D. Saum, "Characteristics of
Schools with Elevated Radon Levels", In: Proceedings: The
1988 Symposium on Radon and Radon Reduction Technology, Volume
1, EPA—600/9-89—006a (NTIS PB89-167498), pp. 10-37 thru 10-47,
March 1989.
3. Brennan, T., "Radon Control and IAQ Concerns in
Underventilated Buildings: School Studies", Fourth National
Conference on Indoor Air Pollution, University of Tulsa,
Tulsa, OK, 1991
4. Leovic, K.W., A.B. Craig, and D.B. Harris, "Update on Radon
Mitigation Research in Schools", 1991 American Association of
Radon Scientists and Technologists Meeting, Rockville, MD,
October 9-12, 1991.
5. ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air
Quality, 1989
6. Wheeler, A.E., "Energy conservation and-acceptable indoor air
quality in the classroom", ASHRAE Journal, April 1992.
7. The Education Writers Association,"Wolves at the Schoolhouse
Door", Washington, DC, 1989
8. Hansen Associates, "Schoolhouse in the Red - Cutting Our
Losses", American Association of School Administrators,
January 1992
9. Cutter Information Corp.,Indoor Air Quality Update, July 1991
10. ASHRAE Standard 90.1-1989, Energy Efficient Design of New
Buildings Except Low-Rise Residential Buildings. 1989
11. ASHRAE Applications Handbook, ASHRAE, Atlanta, GA, .1991
12. McQuiston, F.C. and J.D. Parker, Heating. Ventilating and Air
Conditioning. Analysis and Design, Third Edition, Wiley, 1988
13. Anderson, S.A., "Control Techniques for Zoned Pressurization",
ASHRAE paper NT-87-04-1, 1987
14. Brown, W.K., "Makeup Air Systems Energy-Saving Opportunities",
ASHRAE paper SL-90-5-1, 1990
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15. Dirkes, J., "Balancing Air Pressure: An Open Door Could
Blow It", Air Conditioning, Heating & Refrigeration News,
October 30, 1989
16. Holness, G.V.R., "Pressurization Control: Facts and
Fallacies", Heating/Piping/Air Conditioning, February 1989
17. Haines, R.W., Control Systems for Heating, Ventilating and Air
Conditioning, Van Nostrand-Reinhold, 4th Edition, 1987
18. Smith, R.B., "Importance of Flow Transmitter Selection for
Return Fan Control in VAV Systems", ASHRAE paper AT-90-17-4,
1990
19. The Trane Company,"Building Pressurization Control", Trane
Applications Engineering Manual AM-CON 17 (882), The Trane
Company, LaCrosse, WI, 1982
20. Atkinson, G.V., "Controlling Space Static Pressure Between
Connected Buildings", ASHRAE paper NT-87-04-3, 1987
21. Bentsen, L.J., "Zoned Airflow Control", ASHRAE paper
NT-87-04-2, 1987
22. EPA, "Radon Reduction Techniques for Detached Houses", EPA-
625/5-86/019, Research Triangle Park, NC, 1986
23. Piersol, P.G. and M.A. Riley, "Ventilation and Air Quality
Monitoring in R-2000 Homes", ASHRAE paper NT-87-07-1, 1987
24. Kettler, J.P., "Field Problems Associated with Return Fans
VAV Systems", ASHRAE paper DA-88-18-3, 1988
25. Alcorn, L.H. and P. J. Huber, "Decoupling Supply and Return
Fans for Increased Stability of VAV Systems", ASHRAE paper
DA-88-18-4, 1988
26. Wendes, H.C., Variable Air Volume Manual, Fairmont
Press-Prentice Hall, 1991
27. Haines, R.W., "Double-Duct Systems", ASHRAE paper NT-87-16-1,
1987
28. Int-Hout, D.,"Stand-Alone Microprocessor Control of Dual-Duct
Terminals", ASHRAE paper NT-87-16-2, 1987
29. Linford, R.G., "Dual-Duct Variable Air Volume - Design/Build
viewpoint", ASHRAE paper NT-87-16-4, 1987
30. Kettler, J.P. "Efficient Design and Control of Dual-Duct
Variable Volume Systems", ASHRAE paper NT-87-16-3, 1987
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31. ASHRAE Equipment Handbook, ASHRAE, Atlanta, GA, 1988
32. Royal, G.C. and S.S. Tsai, "Building Stock Energy Analysis:
Elementary and Secondary Schools", DOE'CE 30751-T2, E 1.99,
DE86 004525, March 1985
33. ASHRAE HVAC Systems and Applications Handbook, ASHRAE,
Atlanta, GA, 1987
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BIBLIOGRAPHY
ASHRAE HVAC Systems and Applications Handbook, ASHRAE, Atlanta
GA, 1S87
Bogucz, E. A., "Valance Heating and Cooling", Handbook of
HVAC Design edited by Nils R. Grimm and Robert C. Rosaler,
McGraw-Hill, 1990
Khashab, A.M., Heating, Ventilating, and Air-Conditioning System
Estimating Manual, McGraw-Hill, 1977
Leovic, K.W., A.B. Craig, and D. Saum, "The Influences of HVAC
Design and Operation on Radon Mitigation of Existing School
Buildings", ASHRAE IAQ Proceedings, The Human Equation:
Health and Comfort, NTIS PB89-218-762, 1989
Leovic, K.W., D.B. Harris, T.M. Dyess, B.E. Pyle, T. Borak, and D.
W. Saum, "HVAC System Complications and Controls for Radon
Reduction in School Buildings", In: Proceedings: The 1991
International Symposium on Radon and Radon Reduction
Technology, Volume 2, EPA-600/9-91-037b (NTIS PB92-115369) ,
pp. 10-85 thru 10-104, November 1991
Turner, W.A., K.W. Leovic, and A. B. Craig, "The Effects of HVAC
System Design and Operation on Radon Entry into School
Buildings", In: Proceedings: The 1990 International Symposium
on Radon and Radon Reduction Technology, Volume 2, EPA-600/9-
91-026b (NTIS PB91-234450), pp. 9-35 thru 9-44, July 1991
Johnson Controls Corporation, "Building Pressure Control for
Variable Air Volume Systems", Technical Support Group
Engineering Report 4-47, Section II, July 1988
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