Honeywell AUTOMATIC CONTROL, Engineering Manual

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INDOOR AIR QUALITY FUNDAMENTALS
161 ENGINEERING MANUAL OF AUTOMATION CONTROL
This standard also requires detailed documentation of design
assumptions and intent to permit the operating personnel to
maintain the system as designed to assure continued IAQ during
occupancy. The standard is currently in the normal review
process. Many changes are being proposed, including
incorporating code language, i.e., shall.
VENTILATION RATE DESIGN PROCEDURE
The Ventilation Rate Procedure is deemed to provide
acceptable IAQ by providing an indirect solution to the control
of IAQ.
Acceptable outdoor air is generally defined in terms of sulfur
dioxide, carbon monoxide, ozone, nitrogen dioxide, lead, and
particulate levels. If levels are too high the air “should” be
treated (BOCA says high levels shall be treated). Exceptions
allow recirculating air during rush hour traffic if treatment is
not possible. Minimum outdoor air airflow per person are listed
for each type of occupancy, for example 15 cfm for classrooms,
30 cfm for operating rooms, and 60 cfm for smoking lounges.
These rates are set on the assumption that they will maintain
CO2 levels below 1000 ppm and that this is a valid indicator of
acceptable IAQ. Recirculation of air to reduce quantities of
outdoor air requires that air cleaning equipment be used to lower
the contaminant levels to those required for outdoor air and
that the Indoor Air Quality Procedure must be used. The air
cleaning equipment must be designed to reduce particulates
and where necessary and feasible, gaseous contaminants.
Spaces with variable occupancy served by a common supply
may have their fraction of the outdoor air requirement adjusted
to match the occupancy. This requires additional sensors and
controls. This can done with CO2 sensors, occupancy sensors,
VAV flow readings, and measurement of the outdoor air airflow.
Table 6.1b in ASHRAE Standard 62-1989 provides minimum
ventilation requirements based on per person or square foot
and non-smoking. The table has many references and is
excerpted here as Table 4 with the recommendation that the
latest ASHRAE publication on Ventilation For Acceptable
Indoor Air Quality be consulted before using any of the data.
Table 4. Minimum Ventilation Requirements
INDOOR AIR QUALITY DESIGN PROCEDURE
The Indoor Air Quality Procedure provides a direct
solution by restricting the concentration of all known
contaminants of concern to specified acceptable levels. This
procedure lists acceptable levels for CO2, chlordane, ozone,
and radon gas and references many other potential
contaminants without defined limits. The use of CO2 as an
indicator of IAQ is inferred. The limit of 1000 ppm is
recommended to satisfy comfort and odor criteria. The use
of subjective evaluation implies an occupied building which
further implies changes after occupancy.
The requirements for recirculation are similar to the
Ventilation Rate Procedure.
Maintenance of acceptable IAQ is complicated by the use of
variable air volume systems. Some designers choose to utilize
dual duct constant volume HVAC systems to simplify IAQ
control. Use of direct drive variable geometry fans and
innovative outdoor air economizer designs allow designers to
provide both IAQ and energy efficiency.
Designers frequently use constant volume terminal boxes
in their designs. One adaptation of this technology applies a
constant volume terminal box to supply 100 percent
conditioned outdoor air to a conference room when a motion
sensor indicates that the room is occupied. This air is the
ventilation component. The conference room is also served
by a fan powered variable air volume box which cools and
heats the room with centrally supplied primary air and
recirculated air. Other designers apply constant volume
terminal boxes in the outdoor air intakes of smaller VAV fan
systems to ensure minimum outdoor air at all times. This may
supply too much outdoor air at light loads depending upon the
percentage of the load represented by occupants.
Prescriptive Requirements Simple System Requirements
People R p
cfm/person
Building R b
cfm/ft2
Outside Air Rsb
cfm/ft2
Supply Air R ss
cfm/ft2
Office Building
General office space 6.0 0.07 0.13 0.13
High density open office space 6.0 0.07 0.13 0.16
Reception areas 7.0 0.07 0.18 0.26
Telecommunications/data entry 7.0 0.07 0.49 1.2
Conference rooms 5.0 0.07 0.32 1.0
Main entry lobbies 7.0 0.06 0.12 0.12
Excerpt of Table 6.1b (I-P units) from ASHRAE Standard 62-1989