ASHRAE Journal - February 2014 - 32

COLUMN ENGINEER'S NOTEBOOK

the 0.08 in.w.g. [20 Pa] maximum
FIGURE 5 Other transfer air options.
across the door), transfer from the
return air plenum is not possible. If
A
B
the plenum pressure is 0.05 in.w.g.
(12 Pa), the pressure drop through
the transfer grille and duct, including entry losses into the duct, must
be less than 0.03 in.w.g. (7.5 Pa) to
keep the total differential pressure
below 0.08 in.w.g. (20 Pa). With
perforated face grilles, this can be
achieved with a duct velocity of less
than about 325 fpm (1.65 m/s).
3. Ducted transfer to the adjacent
space (Figure 5a). This option results
FIGURE 6 Transfer air plus cooling supply options.
in the same space temperature as
Option 1. As with the undercut, the
A
B
grilles and ducts must be sized for a
total pressure drop of less than 0.08
in.w.g. (20 Pa). With perforated face
grilles, this can be achieved with a
duct velocity less than about 375 fpm
(1.9 m/s).
4. Ducted transfer to ceiling return air plenum with integrated
supply air (Figure 6a). When the
space will be too uncomfortable
5. Ducted transfer to ceiling return air plenum with
without some cooling (or heating if the room is on the
building perimeter), conditioned air must be supplied separate supply air (Figure 6b). If the design in Figure 6a is
along with transfer air. This typically can be air from felt to be too confusing, conditioned air may be supplied
an adjacent VAV zone; there is little benefit to provid- by a separate diffuser. This option is more expensive than
ing a separate zone for the restroom since most of the Option 4 but easier to understand and to balance.
air supplied to the room is neutral transfer air which
buffers any temperature excursions. The supply air Conclusions
Traditional restroom exhaust and makeup air systems
rate should be sized just for the cooling or heating
load, 40 cfm (20 L/s) in this example, with the remain- can be both expensive and energy inefficient. The tips in
der of the makeup air supplied through the transfer this month's column show how to reduce first costs and
opening. If the adjacent zone is VAV, the makeup air minimize energy use.
must be sized for the airflow required when the VAV
References
zone airflow is at its minimum.
The design shown in Figure 6a is a bit unusual and often 1. ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air Quality.
2. ASHRAE Standard 90.1-2013, Energy Standard for Buildings Except
generates a clarification request from mechanical conLow-Rise Residential Buildings.
tractors: the conditioned supply air from the adjacent
3. 2013 Building Energy Efficiency Standards for Residential and
Nonresidential Buildings, Title 24, Part 6 CEC-400-2012-004-CMF.
zone is ducted into a standard wye fitting with one end
4. "Discussion of the Use of Transfer Grilles to Facilitate Return Air Flow
open at the return air plenum. The negative pressure
in Central Return Systems." http://tinyurl.com/o4sghcf; and "Discusin the restroom ensures that the supply air will not go
sion of the Use of Transfer Grilles to Facilitate Return Air Flow in Central Return Systems." http://tinyurl.com/ovabs66 are two examples.
backwards into the plenum.
32

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