ASHRAE Journal - February 2014 - 28

COLUMN EnginEEr's notEbook
Steven T. Taylor

Restroom Exhaust Systems
BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

Designing restroom exhaust and makeup air systems should be a trivial matter, but
many designs are expensive and inefficient. This month's column outlines some tips
to design a system that performs effectively yet has low first costs and energy costs.
Figure 1 is an example of a poor design of a restroom
exhaust and makeup air system for a typical nonresidential men's restroom. The design is summarized as
follows:
* The exhaust rate is based on 10 air changes per hour;
* Air is exhausted above each water closet and urinal
bank; and
* Makeup air is sized at 90% of the exhaust rate and
supplied by a constant volume reheat box connected to
the building's cooling system.
A much less expensive and more energy-efficient
design is shown in Figure 2. Its design characteristics are
discussed below.

Exhaust Rate Sizing
In the example design in Figure 1, the exhaust rate was
based on air changes per hour (ACH), a rate proportional to the volume of the restroom. Using ACH to size
any ventilation system is fundamentally flawed:*
* It implies that the rate of air needed to dilute odors
in the room is proportional to the volume of the room.
In fact, the ventilation rate is proportional to the source
strength of odors which has no relationship to the volume of the room. Would there be more odors to dilute
if the entry vestibule were larger or if the ceiling were
higher? Clearly not.
* It requires larger ventilation rates for rooms with
larger volumes. Under steady-state conditions, the
required ventilation rate is the same for two rooms with
identical source strength regardless of volume. Under
non-steady-state conditions, the ventilation rate is
smaller, not larger, for rooms with larger volumes since
pollutants have more volume in which to mix.
*this is true of hospitals and laboratories as well, but ACH has been used for so long there is
bureaucratic momentum to retain it. Airflow rates per unit area or some other metric that comes
closer to scaling with the source strength of contaminants should be used.

ashraE JourNal

ashrae.org

FEBRUARY 2014

Furthermore, ACH is inconvenient from a design
perspective since rates must change as the design progresses if the size of the room or ceiling heights change.
A more rational metric for determining ventilation
rates in restrooms is airflow per plumbing fixture (e.g.,
urinal or water closet) as prescribed by ASHRAE Standard
62.11 and the model building codes that are based on
Standard 62.1 requirements. Standard 62.1-2013 requires
an exhaust rate of 70 cfm (35 L/s) per water closet/urinal
where long periods of heavy use are expected to occur,
e.g., restrooms in theatres, schools, and sports facilities,
and 50 cfm (25 L/s) in other applications, such as office
buildings. For simplicity, and to be conservative, the
higher rate is often used for all applications.
In our example restroom, the exhaust rate at 70 cfm (35
L/s) per plumbing fixture would be 420 cfm (210 L/s) (Figure
(
2), down from 560 cfm (280 L/s) based on 10 ACH (Figure 1).
(Figure

Number of Exhaust Grilles
The design in Figure 1 includes four exhaust grilles, one
over each water closet and one over the bank of urinals.
It may seem to make sense to exhaust above each fixture
since that is where odors are emitted, but it actually does
not improve ventilation effectiveness because the ceiling
grilles cannot capture the odors before they diffuse into
the space.
Figure 3 shows a computational fluid dynamics simulation of a typical exhaust grille. Note the velocity vectors
are only high near the grille; 2 ft or 3 ft (0.6 m or 0.9 m)
away from the grille face, the velocity vectors are zero
which means that odors produced closer to the floor
level will not be captured by the grille.** Thus, there is no
value to placing an exhaust grille over each toilet; using
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.

ASHRAE Journal - February 2014

Table of Contents for the Digital Edition of ASHRAE Journal - February 2014