Guide to using exhaust fans for removing or keeping out indoor contaminants
Home exhaust fan ventilation strategies
Best methods for cleaning & filtering indoor air
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Exhaust-only ventilation is the most common approach,
due to its simplicity and use of familiar components such
as bathroom fans. However, unless houses are built very
tight, there is little control over where fresh air enters the building. Also, depressurization can be a problem, particularly
with high-capacity fans. In addition to the increased
potential for backdrafting, a depressurized house tends
to draw more soil gases, including radon if it is present.
And in hot, humid climates, moist air infiltrating through
exterior walls can condense on interior finishes such
as the back face of vinyl wallpaper that is chilled by air
conditioning.
The simplest and least expensive
central ventilation system consists of an automatic
timer wired to one centrally located bathroom or laundry
fan so it cycles on and off for a portion of every hour or
for the 8 to 12 hours per day when most people are home,
typically mornings and evenings.
See the figure at left (click the image for details) about a single port exhaust house ventilation system.
The simplest ventilation system uses a single, centrally located exhaust fan that runs on a
timer or continuously at a low speed. The fan may also serve as a bathroom or laundry
fan, but a dedicated fan is optimal.
Passive air inlets are sometimes installed but will only
work properly in very tight homes.
Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by
Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above.
Exhaust fan control switch: Since the house ventilation exhaust fan is doing double duty as a bath or laundry
fan, it must have a manual override switch for intermittent
use. In larger homes, two fans at separate locations
can be used. Another upgrade is to use a dedicated fan in a
central location, such as a hallway ceiling, which will provide
better distribution of both exhaust and supply air.
Exhaust fan noise: For the house ventilation system to work well, it is important to use a
quiet exhaust fan of one sone or less and choose a central location.
Also, the door to the bathroom with the exhaust fan
must be undercut by 3/4 to 1inch, along with doors to all 4 bedrooms and other rooms that require ventilation. An
alternative is to connect the rooms with through-the-wall
transfer grilles.
The biggest drawback to exhaust-only ventilation is
that there is little control over distribution of the incoming
air. Makeup air will come via the path of least resistance.
In a leaky house, this might be a window or drop ceiling
in the bathroom with the exhaust fan, leaving the rest of
the house un served by the ventilation system. For this reason,
single-port exhaust-only ventilation works well only
in relatively small, tight houses.
Passive air inlets. Some contractors install passive air
inlets in an effort to direct makeup air into bedrooms
and main living areas. For these to work properly, however,
the house must be extremely tight and doors must
be left open or be cut at least an inch above the carpet.
If a house is too leaky or rooms are cut off from household
airflows, the inlets will function like other random
holes in the building shell, leaking air inward or outward,
depending on the wind, stack effect, and imbalances
in the HVAC system.
The inlets typically require at
least 10 Pascals of negative pressure to operate. They
do not eliminate depressurization as sometimes thought.
In fact, they require it to work properly.
Multiport Exhaust House Venting Systems
This type of system uses a more
powerful exhaust fan that is remotely mounted, typically
in the attic or basement. See the figure at left for details of a multi-port whole house exhaust fan vent system).
A multiport exhaust system improves air distribution by picking up air from bathrooms and main living areas. These are often used in conjunction with passive air inlets. Exhaust-only systems are best used in homes with electric heating or sealed-combustion appliances where backdrafting is not a concern.
Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by
Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above.
The multiport house exhaust fan system is ducted to exhaust grilles in bathrooms,
laundries, and other wet areas, and sometimes to a centrally
located pickup point in the main living space. A room with
no outside walls would also benefit from a pickup point.
Systems typically run on a low background speed with
timer switches in bathrooms for higher-powered spot ventilation.
If installed correctly, these systems are very quiet
and provide good distribution of ventilation.
Multiport exhaust systems may incorporate passive air
inlets (see description above) that install either in windows
or through the wall, providing some control over supply
air. The inlets, typically three or four for a small house, go
in bedrooms, main living areas, and other occupied rooms,
such as dens or home offices. Inlets should be placed high
on the wall away from beds, chairs, or other places where
drafts might cause discomfort. Placement near a window
is preferred.
Because these systems use more powerful fans that
depressurize the house, they should not be used in houses
with fireplaces or atmospherically vented combustion appliances.
They are also not recommended in hot climates,
since hot, moist exterior air may be drawn into walls and
condense behind interior surfaces chilled from air conditioning.
This variation on exhaust-only ventilation passes the exhaust air
through a heat-pump water heater, reclaiming heat from the outgoing air stream. Some systems can be reversed
in summer, functioning as a supply ventilation system
while cooling and dehumidifying the incoming air. A packaged heat-pump ventilating system is available from
Therma-Stor.
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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