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Air Distribution Systems, IAQ Procedure Addressed
Requirements to ensure that air distribution systems are capable of delivering
outdoor air to occupied spaces are contained in the American Society of Heating,
Refrigerating and Air-Conditioning Engineers' (ASHRAE) ventilation standard
in an addendum newly approved for publication.
ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality,
sets minimum ventilation rates and other requirements for commercial and institutional
buildings.
Addendum 62v addresses balancing, plenum mixing systems, exhaust duct location
and documentation. In systems with air-conditioning units, heat pumps or fan-coil
units mounted in a ceiling or floor plenum, ventilation air is often supplied
to the plenum to mix with recirculated air, according to Andrew Persily, chair
of the Standard 62 committee.
If outdoor air is poorly distributed in such systems, units that are far from
the outdoor air supply to the plenum may not receive the intended levels of
outdoor air. In any air distribution system, required airflow can only be assured
by measuring actual airflow and adjusting dampers and orifices to account for
the effect of as-installed duct pressure drops.
"The systems must be designed so that they can be balanced," Persily
said. "Designers must consider and make provision for system balancing."
The IAQ procedure is one of two approaches in the standard used in designing
ventilation systems. The procedure, which permits innovative, energy-conserving
practices, allows engineers to use whatever amount of outdoor air deemed necessary
if they can show that the levels of indoor air contaminants are held below recommended
levels, according to Persily.
The standard now requires that the procedure be used when credit is taken for
controls that remove contaminants or for other design techniques that can be
reliably demonstrated to result in indoor contaminant concentrations equal to
or lower than those achieved using the ventilation rate procedure.
The ventilation rate procedure is a prescriptive approach in which outdoor air
intake rates are determined based on space type/application, occupancy level
and floor area.
Published addenda to ASHRAE standards are available for free at ASHRAE online,
www.ashrae.org
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