United States
Environmental Protection
Agency
Air and Energy
Engineering Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-88/097 Jan. 1989
&EPA Project Summary
Indoor Air Quality Model
Version 1.0
Leslie E. Sparks
The manual describes a micro-
computer program written to
estimate the impact of various
sources on indoor air quality in a
multiroom building. The model treats
each room as a well-mixed chamber
that contains pollutant sources and
sinks. The model allows analysis of
the impact of interroom air flows,
HVAC (heating, ventilating, and air
conditioning) systems, and air
cleaners on indoor air quality. The
model is written for the IBM-PC and
compatible family of computers.
The model is designed for ease of
use and is menu driven. Data entry is
handled with a fill-in-the-form
interface. Default values for inter-
room air movement and other input
data are provided.
The predictions from the model
have been compared with pre-
dictions from other models and with
experimental data. The model pre-
dictions are in excellent agreement
with both.
The model is completely docu-
mented, including a brief discussion
of the theory on which the model is
based. Most of the report is devoted
to user instructions and demon-
strations of how the model can be
used.
The model is quite useful and
allows rapid analysis of the impact of
air pollution sources and mitigation
measures on indoor air quality.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
introduction
Indoor air quality is determined by the
interactions of sources, sinks, and air
movement between rooms and between
the building and the outdoors. Sources
may be located in rooms, in the HVAC
system, or outside the building. Sinks
may be located in the same locations.
Sinks may also act as sources when the
pollutant concentrations drop below a
given value.
Air movement in a building consists of:
1. Natural air movement between rooms.
2. Air movement driven by a forces air
(HVAC) system.
3. Air movement between the building
and the outdoors.
The pollutant concentration in a room is
calculated by a mass balance of the
various pollutants flows. For the single
room shown in Figure 1:
Amount in - Amount out + Amount
produced - Amount removed = Amount
accumulated
The analysis can be extended to
multiple rooms by writing a system of
equations for each room. The amount of
air entering a room from all sources (the
HVAC system, outdoors, and other
rooms) must equal the amount of air
leaving the room.
The type of mixing between the
pollutant and the room air must be
specified before the mass balance
equations can be used in a model.
Because mixing is complex, the exact
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Amount in
Accumulated
Produced
Figure 1. Single room mass flows
mixing cannot be specified; simplifying
assumptions must be made. Plug-flow
and well-mixed mixing are two common
mixing possibilities.
In the plug-flow mixing model, the
pollutant concentration varies from point
to point along the air flow path. In the
well-mixed model, the pollutant
concentration is the same for every point
in the room.
The current model uses the well-
mixed model. This model was selected
because data from the EPA test house
showed that pollutant concentrations
within a room do not vary significantly
with position in the room.
Once the mixing is defined, the various
mass balances discussed above can be
used to write a set of linear differential
equations. These equations can be
solved using many techniques. The
model used a midpoint method that is
stable and accurate for reasonable time
step sizes. When the room volumes are
of about the same size, large time steps
can be used with little difficulty (unless
the source and sink terms exhibit short
term time behavior). However, when the
room volumes differ by orders of
magnitude, as is possible when an HVAC
system is included in the model, small
time steps (10 sec or less) are needed to
avoid numerical instabilities.
The User Interface
The IAQ model uses a menu-driven,
fill-in-the-form, data-input user in-
terface. This interface is easy to use and
is self prompting. The user interface
allows the user to change the input
parameters quickly and easily and allows
rapid analysis of several conditions.
The master menu shown in Figure 2
controls the operation of the program.
The model can be configured for
various personal computers. It can run on
a computer with a monochrome adapter,
a color graphics adapter (CGA), or an
enhanced graphics adapter (EGA). When
the model is run with a monochrome
adapter, all graphics are disabled.
Data entry is handled with a fill-in-
the-form interface. Figure 3 is an
example form used in the model.
Removed
Amount out
Indoor Air Model Control Menu
un indoor air model
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Figure 2.
Master menu for AEERL IAQ
model
The form shown in the example is
used to enter the number of rooms in the
building and the total ventilation rate
The most complicated form used in
the model is the room definition form,
which is used to obtain data on individual
rooms in the building. Figure 4 is an
example of this form.
Figure 4 shows the overall room
definition screen. The options available
from this screen are:
Select room number,
Define room size and initial concen-
tration (definition),
Define sources,
Define sinks, and
Define interconnections with outside,
HVAC system, and other rooms.
The various options are selected by
moving the highlight bar across the top
of the screen, using the left and right
arrow keys.
The results of the model calculations
are displayed as plots of concentration
versus time for the various rooms. The
plots require that a graphics adapter and
a monitor be installed on the computer.
Source Terms
A wide range of source terms are
available in the model including random
on/off sources (cigarettes), sources that
are on for a specified period of time
(heaters), steady-state sources (moth
crystals), and sources with high initial
emission rates followed by low steady-
state (floor wax). The IAQ model
accommodates all these possibilities in
an idealized fashion. Each source in the
model is discussed below.
Cigarette Smoking
Cigarette smoking is modeled as a
random event with from 1 to n cigarettes
smoked per hour. The cigarette is turned
on at some random time during the hour.
A second cigarette is not allowed on until
the first cigarette is smoked. Multiple
smokers are accommodated in the
model; however, all smokers smoke at
the same time.
Unvented Kerosene and Gas
Heaters
Unvented kerosene and gas heaters
are common sources of indoor air
pollution. These heaters are modeled as
steady-state on/off heaters. The on and
off times are part of the data input to the
program. Up to three on/off cycles per
day are allowed.
Building Definition
==::r==z===::=:=zsr
Item
Value
:? Number of rooms Max =10
Total ventilation rate air changes/hr
7
0
Figure 3. Example form used in model
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7
2
3
4
room number definition
—— [Status of room J]
Building vol 150 m2 C
Vol 150m3 Wall 77 m*
Sources selected
k-heater
sources
sinks
interconnections
done
•) 0.0 mg/m3
inkO
[Air tlows\
Air Hows
Air from hvac
Air to hvac
Air from outside
Air to outside
Case 1 Case 2
0.0 00
00 0.0
1500 00
1500 00
^_ [Interconnections] ______
: Roomtt Air out to Air in from
• [Air Balances] —————
Case 1 Case 2
Air entering 1500 00
Air leaving 1500 00
Balance 00 00
I
Pollutant being modeled Paniculate
Figure 4. Room definition screen
10,000
Oj'
5
1000
WO
10
0 1
001
0 1
10
Time (hr)
Figure 5. Floor wax emission factor
Moth Crystal Cakes
Moth crystal cakes can be an important
source of volatile organic compound
(VOC) emissions indoors. Moth crystal
cakes are long-term steady-state
sources. The emissions from moth
crystals are a function of the temperature
nd the surface area of the cakes.
Floor Wax
Flow wax is an example of a "wet"
source of VOC emissions. Wet sources
have an initial very high emission factor
followed by a low-level, steady-state
emission factor. The emission factor for
floor wax is based on work conducted by
EPA and is shown in Figure 5.
Other
The "other" source is provided as a
user defined steady-state source. The
source cannot be turned off.
Sinks
It is generally recognized that walls
and furnishings can serve as collectors
(sinks) of indoor air pollutants.
Unfortunately, the data on sinks are
limited. The model allows investigation of
the behavior of sinks by providing a
single sink that is a function of the
surface area of the walls in the room.
This sink can be a pure sink (i.e.,
pollutants trapped by the sink are not
reemitted) or a reemitting sink.
Small chamber and test house studies
are planned to provide fundamental data
on sink behavior. The results of these
studies will be incorporated into the
model as soon as they are available.
The Air-Handling System
The airflows generated by an air-
handling system are generally larger than
natural airflows. Thus, when an HVAC
system is on, the building's airflows are
dominated by the HVAC system. Airflow
patterns in a building with the air-
handling system on may be significantly
different from those in the same building
with the air-handling system off. For
example, many houses have a single
return vent for the air-handling system.
When the air-handling system is on,
airflow is dominated by the flow to the
return vent. When the air-handling
system is off, airflow is less directed.
The on/off behavior of the air-handling
system is modeled by allowing two dif-
ferent airflow patterns to exist in the
building: one pattern is active when the
air-handling system is on; and the
other, when the air-handling system is
off. The model switches between these
two patterns depending on the state of
the air-handling system. The state of
the air-handling system (on or off) is
determined by a random number gen-
erator that ensures that the air-handling
system is on for a specified fraction of
each hour. The air-handling system
may switch from on to off and back
several times in an hour. This random
switching appears to provide a qualitative
description of actual air-handling
system behavior. Experiments are
planned to determine how well the model
fits actual air handler behavior.
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EPA Test House Data
Comparison with Model
Predictions
The model was used to estimate the
p-dichlorobenzene concentrations from
moth crystals in the EPA test house. The
emission factors for p-dichlorobenzene
were determined in small chamber
studies. The comparison between the
model predictions and the measured
concentrations are shown in Figure 6.
Hardware Requirements
The model requires an IBM-PC or
compatible computer with at least 512
k-bytes of RAM, one floppy drive, and a
monochrome or color monitor. The model
provides graphics on a computer with a
color graphics adapter (CGA) or an
enhanced graphics adapter (EGA).
Graphics are not provided on a
monochrome display adapter.
wo
= - Closet
10-
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c.
o
I
a
Corner bedroom
Closet
Corner bedroom
Master bedroom
Den
f- Master bedroom & den
0.01-
12
—I—
24
36 48 60
Time (hr)
Figure 6. Final mode results with measured flows
72
84
96
The EPA author, Leslie E. Sparks, is with the Air and Energy Engineering
Research Laboratory, Research Triangle Park, NC 27711..
The complete report consists of two parts, entitled, "Indoor Air Quality Model
Version 1.0,"
Paper Copy (Order No. PB 89-133 615/AS; Cost: $21.95)
Diskette (Order No. PB 89-133 6071 AS; Cost: $50.00, price of Diskette includes
paper copy)
The above items will be available only from: (subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA author can be contacted at:
Air and Energy Engineering Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
S. OFFICIAL MA/L"
Official Business
Penalty for Private Use $300
EPA/600/S8-88/097
0000 3 29
60604
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