ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDOOR AIR QUALITY FUNDAMENTALS
162
ventilation fraction, and resets the outdoor airflow to the
minimum outdoor air that will satisfy the critical space or
spaces. See the latest version of ASHRAE Standard 62 for
additional information on critical zone reset application.
An alternate method maintains a constant supply fan mixing
box static pressure as the supply fan load varies. Since the entire
Ventilation Rate Procedure is based on a series of assumptions,
this lower cost, static pressure approach may be as effective
and is certainly more cost effective.
When the Air Quality Procedure is used, CO
2
monitoring
and control has been used to reduce outdoor air to conserve
energy. However, CO
2
is only one of the possible contaminant
in indoor air. It was intended as an index of odor quality on the
assumption that if odor was low all other contaminants would
also be low. This is not necessarily so. Some designers use VOC
Sensors in lieu of or in addition to CO
2
sensing and control.
The standard lists CO
2
, Chlordane, Ozone, and Radon levels.
Table 3 references other contaminants which may be of concern.
The engineer is left to decide which ones are to be monitored.
CODES AND STANDARDS
A host of regulatory agencies, code bodies, and advisory
agencies are involved in the maintenance of Air Quality. Some
are concerned with Outdoor Air Quality while others are
concerned only with IAQ. These agencies and their jurisdictions
are listed in Table 5.
M10482
GENERAL
EXHAUST
AIR CLEANER
LOCATION
VENTILATION AIR
INFILTRATION
LOCAL
OCCUPIED
SPACE
EXFILTRATION
RETURN
AIR
ALTERNATE AIR CLEANER LOCATIONS
OUTDOOR AIR
MAKEUP AIR
SUPPLY
AIR
ENERGY
RECOVERY UNIT
Fig. 3. Possible Location for Filtration in an HVAC System.
Recirculation Requirements
Recirculation of air in HVAC systems is regulated by
buildings codes and other rules as well as ASHRAE 62-1989.
The BOCA mechanical building code, for example, requires
filtration to remove particulates above a certain level for
recirculation.
Figure 3 shows locations where filtration might be appropriate
in different system configurations. The filter shown in the space
is located in a local makeup air unit if one is used. The filter
shown in the outdoor airstream is utilized when outdoor air
cannot meet the requirements of the building code, ASHRAE
recommendations, or flow sensors or coils are located in the
outside air.
Each of the filter locations shown might represent multiple
particulate, gas phase, and/or ozone generators.
Ventilation Optimization
The Ventilation Rate and Air Quality Procedures both
increase outdoor air consumption and increase operating cost
unless techniques are used to minimize outdoor air when the
spaces are not fully occupied. The two procedures require
different approaches.
One approach utilized for the Ventilation Rate Procedure
monitors the load, or airflow, of all the VAV terminal boxes
and the outdoor air flow of the supply fan, calculates the required
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