United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/125 December 1992
<8rEPA Project Summary
HVAC Systems in the Current
Stock of U.S. K-12 Schools
Jerald D. Parker
This report summarizes information
on the types of heating, ventilating, and
air-conditioning (HVAC) systems com-
monly found in U. S. school buildings
and the effect that the operation of
these systems has on indoor radon lev-
els. The report describes the ability of
various HVAC systems to pressurize
and ventilate classroom spaces, how
they operate, and how they are con-
trolled. 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.
Previous studies of school buildings
have shown that radon levels can be
reduced by depressurization and venti-
lation of soil under slabs, but this
method is not readily applicable to all
such buildings. The HVAC systems
have also been shown to have an im-
pact on radon levels through pressur-
ization and ventilation of classroom
spaces.
Not all HVAC systems can provide
pressurization since some have no pro-
vision for the makeup air to replace the
exfiltration losses always created by
positive room pressure. The level of
pressure attainable in a space depends
upon fan characteristics, duct design,
room leakiness, and the method of con-
trol of fans and dampers. Return fans
and relief dampers play an important
role in some systems, and exhaust fans
always work against maintaining posi-
tive room pressures.
There appear to be no well defined
trends in the types of HVAC systems
involved in current school building con-
struction and modification. Some sys-
tems using reheat and/or mixing have
been prohibited or their use discour-
aged by local codes and regulations
because they waste energy. Capital
costs appear to vary more with locale
and quality of construction than with
the type of system installed.
The unit ventilator (UV) has been the
most popular system in U.S. schools
but its noise and operating limitations
have reduced its popularity in recent
years relative to central systems. UVs
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 an
excellent choice for relatively low oper-
ating 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 pres-
sure levels in classrooms. This is par-
ticularly true for U.S. school buildings,
many of which are not tightly con-
structed (i.e., they have high passive
rates of outdoor air exchange through
the building envelope).
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Printed on Recycled Paper
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Introduction
The U.S. Environmental Protection
Agency (EPA) has studied ways to re-
duce radon levels in schools since 1987.
Some of these studies are described in
the proceedings of the 1991 International
Symposium on Radon and Radon Reduc-
tion Technology. Radon mitigation re-
search 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 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
alternative approach to radon mitigation,
particularly in schools with moderately el-
evated radon levels (4 to 20 pCi/L). One
recent study concludes that one of the
most significant factors contributing to el-
evaled levels of radon in schools and in-
fluencing the mitigation approach is the
design and operation of the HVAC sys-
tem, and that complexities of large build-
ing HVAC systems, present problems not
previously encountered in house mitiga-
tion.
A report on an EPA school evaluation
program (SEP), involving site studies in
26 schools in 8 regions in the U.S., states
that 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 school 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.
Researchers at EPA desired to better
understand the various types of HVAC
systems that exist in kindergarten through
twelfth grade (K-12) schools throughout
the U. S. This report fills the need for a
reference document that identifies the vari-
ous HVAC systems that one should ex-
pect to find in U. S. schools, the ability of
these systems to pressurize and ventilate,
strategies used to control pressurization
and ventilation, and modifications (and their
effects on pressurization and ventilation
control) that might have been made by
owners to conserve energy.
HVAC System Prevalence/
Characteristics
Data regarding types and numbers of
HVAC systems presently installed in U. S.
schools was not readily available. Data
from a 1979 report give the distribution of
U. S. school building heating and air-con-
ditioning (HAC) systems. These data are
described and shown graphically in the
full report. Most of the school buildings
described by the 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.
Calls and inquiries to school adminis-
trators and staff and to consulting engi-
neers revealed no newer quantitative data
but did show clearly that a very wide vari-
ety of HVAC systems are in use in U. S.
schools. While most school systems oper-
ate within guidelines of state and federal
regulation, they are generally free to se-
lect 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 eco-
nomics and wage scales that upon geog-
raphy or even climate, although cooling
systems do appear to be more common
in the warmer southern states than in
states with cooler or milder summers. They
are also more common in schools used
year-round.
Air conditioning (cooling) very often has
more elaborate ventilation and control sys-
tems than where only heating is utilized.
Some types of heating systems, such as
radiant systems, have no controlled venti-
lation 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. There is a
boom in construction and refurbishing of
schools 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 construc-
tion 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 con-
struction projects involve overhauls of ex-
isting buildings, most of which were built
during the 1950s and 1960s and which
generally were of low-cost construction.
Many school buildings have undergone
modification, some with only quick fixes
attempting to reduce energy consumption.
Many school buildings have HVAC sys-
tems that need significant repairs. Thus it
is a good time to be promoting good de-
sign for improvement of indoor air quality.
Regarding future trends, the require-
ments 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, and ASHRAE Standard 62-
1989, Ventilation for Acceptable Indoor
Air Quality. These standards and the de-
sire of school boards to have both low
initial and operating costs in their build-
ings will probably cause certain types of
systems (e.g., reheat and dual-duct, con-
stant flow) to no longer be built. No single
type of system seems to be the obvious
best choice for all schools. The following
is from the 1991 Applications Handbook:
"No trends in educational facility design
as related to heating, ventilating, and air-
conditioning-systems are evident, In
smaller single-building facilities, central-
ized systems are often applied. These sys-
tems include unit ventilator, rooftop and
single and multizone-type units. Central
station equipment, especially variable vol-
ume systems, continues to have wide ap-
plication in larger facilities; water-to-air heat
pumps have also been used."
The single most significant factor con-
sidered 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 sys-
tems, for example ones with only radiant
heating or schools with exhaust only ven-
tilation, cannot (without modification) pres-
surize the space for reduction of radon
levels. Modification for radon abatement
would require the addition of an air distri-
bution system, properly designed to be
compatible with the existing comfort sys-
tem or to totally replace it.
Some existing school HVAC systems
have air circulation but no controlled pro-
vision for outdoor air. Some systems with
outdoor air have been modified to mini-
mize (or eliminate) outdoor airto save on
energy costs, and in some systems out-
door air dampers no longer operate prop-
erly due to poor maintenance. Every one
of the 26 schools in the EPA SEP study
was reported to have at least one ventila-
tion 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 leak-
age 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
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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 al-
ways accompanied by some dilution due
to the required introduction of outdoor air.
After a brief introduction of pertinent
HVAC terms, the full report describes the
basic central air system and uses it as the
basis for comments on the different types
of systems existing in schools.
'U.S. Government Printing Office: 1993 — 750-071/60173
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J.D. Parker is with Oklahoma Christian University, Oklahoma City, OK 73136.
Timothy M. Dyess is the EPA Project Officer (see below).
The complete report, entitled "HVAC Systems in the Current Stock of U.S. K-12
Schools," (Order No. PB92-218338/AS; Cost: $19.50 subject to change) willbe
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Off her can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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