ASHRAE Journal - March 2015 - 44

COLUMN ENGINEER'S NOTEBOOK
Steven T. Taylor

Return Air Systems
BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

Most HVAC systems at least partially recirculate air to increase cooling or heating
capacity to conditioned spaces while avoiding the energy and first cost impact of
conditioning outdoor air. These systems generally take one of the following forms:
* Return air is conveyed entirely in ductwork from
the conditioned space back to the air-handling unit;
* Return air is conveyed entirely using architectural
plenums such as ceiling cavities, drywall shafts, and
mechanical rooms; or
* A combination of ductwork and architectural plenums.
Using architectural plenums is prohibited in some
applications. For instance, most model mechanical
codes do not allow conveying air in plenums exposed
to materials that do not meet certain flame spread and
smoke generation limits, such as wood beams or trusses.
Most health-care codes also prohibit the use of architectural plenums for critical medical spaces because of
concern about asepsis. But for most commercial and
residential applications, architectural plenums can be
used.

Benefits of Architectural Return Air Plenums
The benefits of using architectural plenums vs. ductwork include:
* Reduced HVAC system costs of about $3 to $5 per
square foot ($32 to $54 per square meter), about 10% to
20% of the total HVAC system cost.
* Reduced costs to other trades to accommodate the
congestion caused by the added return air ductwork,
such as raising the floor-to-floor height or adding additional offsets in plumbing and sprinkler piping.
* Reduced fan energy costs of about 20% to 30% due
to the much lower pressure drop of the plenum return
system.
* Reduced fan energy in systems with outdoor air
economizers due to the ability to use non-powered relief
Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a member of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.
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MARCH 2015

or relief fans in lieu of less efficient return fans, which
are generally required when return air is fully ducted.1,2
* Little or no balancing costs for the return air system
and, for VAV systems, balance is better maintained as
supply airflow rate varies as loads vary. Ducted return
systems can only be balanced at one condition, generally
at design airflow rates, and are inherently unbalanced
at other conditions, possibly leading to overly positive or
negative space pressurization.
* Reduced noise transfer between rooms. Sheet metal
ducts, if unlined, are very adept at channeling crosstalk
from room to room,* much more so than a large ceiling
plenum where noise can dissipate.

Disadvantages of Architectural Return Air Plenums
On the other hand, using architectural plenums has
some potential disadvantages:
* There is the possibility of indoor air quality problems in humid climates if the architectural plenum is
negatively pressurized to the outdoors.3 Humid outdoor air can be drawn into the architectural plenum
and cooled below the dew point, causing condensation
and subsequent mold and mildew problems within the
structure. This potential problem can be avoided by simply not allowing the architectural plenums to become
negatively pressurized relative to outdoors. For example,
a return air plenum can be easily designed and controlled to be positively pressured. First, the building can
be controlled to be pressurized to about 0.05 in. w.c.
(12.5 Pa),4 and ceiling return air grilles can be selected
to have a pressure drop of only about 0.02 in. w.c. (5 Pa).
The ceiling plenum will thus be positive 0.03 in.w.c. (7.5
*The author experienced this firsthand with a home that was custom built for the previous owner
who required that the furnace be fully ducted to each room with unlined sheet metal ducts. My
teenage children entertained themselves for hours spying on each other by listening through the
return air grilles. The first modification my wife and I made to the house was to blank off the return
air grille to our bedroom...
ASHRAE Journal - March 2015

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