ASHRAE Journal - April 2015 - 52

COLUMN ENGINEER'S NOTEBOOK
Stephen W. Duda

Part 2

Overlooked Code Requirements
BY STEPHEN W. DUDA, P.E., BEAP, HBDP, HFDP, FELLOW ASHRAE

In my most recent Engineer's Notebook column four months ago,1 I gave a
review of three important safety-oriented code requirements that tend to
be overlooked in mechanical design. Reaction to that column was favorable,
and I am still admittedly on my code soapbox, so I offer several more code
requirements similarly overlooked. These are also critical safety- or servicerelated features applicable to building mechanical systems: code requirements that are frequently overlooked by engineers, design-build specialists,
contractors, and even code officials. These are all real examples from actual
facilities upon which I have performed property condition assessments, peer
reviews I performed of designs by others, or retrofit of designs by others.
Don't Use Corridors as an Air Duct
This is a classic blunder that I still see from time to
time; not often in large commercial or institutional construction with reputable engineering and proper code
enforcement, but sometimes in light commercial construction in areas with lax code enforcement. It is not
appropriate to use an egress corridor as an air plenum,
including as a path for return air.
The 2012 International Building Code2 (IBC) defines a
corridor as "an enclosed exit access component that defines
and provides a path of egress travel." The 2012 International
Mechanical Code3 (IMC) in paragraph 601.2 states clearly
that corridors shall not serve as supply, return, exhaust,
relief or ventilation air ducts, and essentially the same clause
has appeared in previous editions and in other model codes
such as the Uniform Mechanical Code4 (UMC).
Picture an office building (Figure 1) with a corridor
down the middle, flanked by enclosed offices on both
sides of the corridor. If an air-handling unit is located
in a room adjacent to the corridor, for example, it is
unacceptable to place a return grille serving that airhandling unit directly into the corridor wall, and use
transfer openings from each office into the corridor
proper. This has the effect of turning the corridor into a
return air "duct" in violation of this code clause.
The consequence of using this arrangement could be
dire in a fire. Were a fire to break out in, say, a trash can
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in the conference room, the suction of return air by the
air-handling unit in the corridor would tend to draw
smoke from the conference room out into the corridor-
the very corridor that is needed for safe egress by building occupants. A responsible engineer should strive to
keep the corridor clear of smoke, and use of the corridor
as a return air path would actually encourage smoke to
migrate there.
To clarify, it may be proper and acceptable under some
model codes cited (e.g., IMC 601.2.1) to use the space
above a corridor ceiling as a return air path if the corridor is not required to be rated. If transfer openings are
placed from the ceiling cavity above each office to the
ceiling cavity above the corridor, and the air-handling
unit draws its return air out of the ceiling cavity (not the
corridor itself), then the integrity of the egress path is
maintained. Smoke in the trash can fire example would
then tend to be drawn into the ceiling cavity but not the
corridor itself. If the architect has designed the corridor
without a ceiling, then the return air will need to be
ducted so that the corridor is not violated.
Paragraph 601.2 goes on to grant a few exceptions or
clarifications. For example, it is acceptable to use a corridor as a source of makeup or transfer air for adjacent
toilet rooms, although I still advise using the corridor
ceiling cavity rather than the corridor itself for this
Stephen W. Duda, P.E., is senior mechanical engineer at Ross & Baruzzini, Inc. in St. Louis.
ASHRAE Journal - April 2015

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