ASHRAE Journal - April 2015 - 53

COLUMN ENGINEER'S NOTEBOOK

purpose whenever possible; see Taylor5 and a subsequent
letter to the editor6 for further discussion. Obviously, you
may place return air grilles in a corridor to return that
corridor's own supply air, and it is permissible to have a
minor imbalance of air in a corridor to maintain proper
room pressure relationships in health care or laboratory occupancies. Finally, this prohibition does not apply
within an individual residence or in individual tenant
office spaces of 1,000 ft2 (93 m2) or less in area.

FIGURE 1 Simplified office building incorrectly using the corridor as a return air passage.

10 ft

15 ft

Sales

25 ft

Conference
Room

Support

Corridor

15 ft

8 ft

Separate Ventilation
As stated in 2012 IMC ¶403.2.1, ventilation air cannot
be recirculated from one dwelling unit* to another or
to dissimilar occupancies. This prohibits, for example,
the use of a common recirculating VAV system in a
multi-family apartment building, because it would
recirculate return air from one apartment to another. I
am not aware of any such designs and have always seen
individual air-supply systems (usually fan-coil units
or small air-handling units) on a dwelling-by-dwelling
basis in multi-family housing. However, the second half
of 403.2.1.1 is where I sometimes see violations-there
is a prohibition against recirculating air to dissimilar
occupancies. This means that a large building complex
with more than one occupancy classification cannot use
a common recirculating ventilation system across the
occupancy boundary.
For example, a large urban office complex may have
portions of the building classified as B (Business) occupancy while an attached conference center may be
classified as A3 (Assembly) and a street-level retail section may be classified as M (Mercantile). While it may
be tempting and expeditious to serve the retail section
from the same air-handling unit that serves an adjacent
business office zone, this clause in the code prohibits
recirculation of air from one occupancy to a dissimilar
one†-meaning that for all practical purposes, separate
air-handling systems are required within each formal
occupancy category. Alternatively, a system that does
not rely on recirculation, such as a dedicated outdoor
air system (DOAS) paired with hydronic or refrigerantbased local terminal units could be applied across the
occupancy boundary.
* A dwelling unit under the IBC is an individual independent living facility that includes permanent
provisions for living, sleeping, eating, cooking and sanitation. Therefore, some hotel rooms or college
dormitory rooms may not qualify as a "dwelling unit."
† There appears to be no equivalent prohibition in the UMC.

Office
12 ft

AHU

Computer
Room

12 ft

30 ft

8 ft.

The rationale behind this code requirement is that an
occupancy may have environmental air that contains
odors or contaminants that are unique to that specific
occupancy (such as a restaurant within an office complex, or offices within an industrial plant) and the IMC,
therefore, wants to discourage recirculation from one
occupancy to a dissimilar one. This could potentially
be a very large retrofit cost if done incorrectly and not
caught until the building is ready to open-it is not as
simple as adding a few missed fire dampers-so make
sure this is designed correctly from the outset.

Respect the Interior Stairway Enclosure
Paragraph 1022.5 of the IBC forbids routing of unrelated
utilities through an interior stairway enclosure. Ductwork
and piping penetrations through interior enclosed exit
stairways are prohibited except for ductwork necessary
for independent ventilation or pressurization, sprinkler
piping, and standpipes. It is important to realize there
cannot be penetrations by unrelated ductwork, nor ductwork that shares service with other parts of the building,
even if protected by fire and/or smoke dampers. So it is
not permissible to deliver supply air into a stairwell off a
system serving adjacent spaces, as the code emphasizes
that interior exit stairway ventilation systems shall be
independent of other building ventilation systems.
One must avoid the temptation to route other services
through the stairway enclosure, including but not limited to domestic water, waste, hydronic reheat piping
looped all around a floor, condenser water piping from
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