Supply-Only Fresh Air Ventilation System Design InspectAPedia® -
Guide to using supply-only air ventilation systems for improving indoor air quality and removing or keeping out indoor contaminants
Home ventilation strategies
Best methods for cleaning & filtering indoor air
InspectAPedia offers impartial, unbiased advice without conflicts of interest.
We will block advertisements which we discover or readers inform us are associated with bad business practices,
false-advertising, or junk science. our contact info is at
InspectAPedia.com/Contact.htm.
While not widely used, supply-only systems have distinct
advantages over exhaust systems. The incoming air is
easily filtered and can be directed to bedrooms and main
living areas. The slight positive pressure helps guard
against radon, backdrafting, and other problems associated
with negative pressures.
In cold climates, however, delivering nontempered air
can lead to uncomfortable drafts. Also, forcing moist, interior
air out through gaps in the building shell could contribute
to condensation problems in building cavities and
between prime and storm windows. In airtight homes in very cold climates, supply-only systems have reportedly
iced up keyholes to entry doors as exfiltrating warm, moist
air came in contact with the cold metal. Adding a single
exhaust fan wired to operate whenever the supply fan
switches on can alleviate these problems.
Below we discuss two types of supply-only building ventilation approaches, followed by a discussion of VENTILATION, BALANCED designs.
Forced-Air Supply Ventilation Piggybacks on Existing HVAC System
A multiport forced air home ventilation system piggybacks on the
ductwork of a central heating or cooling system by running
an intake duct from outside into the return ductwork.
The screened intake has a motorized damper that is
timed to open when ventilation is needed, blending fresh
air into the HVAC system and slightly pressurizing the
house.
A special controller is needed to control the damper
and fan, activating the damper whenever ventilation is
needed and activating the fan whenever the air handler has
not run for a set period of time, typically 20 minutes.
Several manufacturers, including Lipidex Corporation,
Tjernlund, and Honeywell, make controllers for this application.
One manufacturer, American Aldes, offers a packaged
system for warm, humid climates: the DHV-100V, which dehumidifies and filters incoming air and integrates
with the home’s central air-conditioning system.
This approach is relatively inexpensive since it uses
existing ductwork, and it provides good distribution of
fresh air. The chief drawback is that ventilation is required
at regular intervals, often when the HVAC system is not operating.
At those times, the controller will switch on the
air handler, which is typically noisy and inefficient, making
this the most expensive system to operate.
Also, delivering
untempered outside air through the duct system can
cause discomfort in very hot or cold weather. And if the
return ducts are not well sealed, they can pull contaminants
from attic or basement spaces into the ventilation
system.
The operating costs can be cut in half by replacing the
standard air-handler blower with an efficient, variable-
speed fan with an integrated control motor (ICM). The
fan would work on high speed for heating and cooling
and continuous low speed for ventilation-only, cutting
operating costs in half.
However, the damper adjustment that provides the right amount of ventilation air at 500 cfm Laboratory (LBNL) as the optimal system for tract homes
will produce too little when run at 100 cfm. Either the based on first cost, operating costs, air distribution, and the
setting has to be changed seasonally or a compromise level potential health and safety benefits of positive pressures.
found. It is also easy to filter and, if necessary, to dehumidify the
incoming air (Figure 7-4 below).
Multiport Supply House Ventilation System to Improve Indoor Air Quality
Although the least common type of ventilation system, the multiport supply system was
identified in a recent study by Lawrence Berkeley National Laboratory (LBNL) as the optimal system for tract homes
based on first cost, operating costs, air distribution, and the potential health and safety benefits of positive pressures.
It is also easy to filter and, if necessary, to dehumidify the
incoming air.
See our figure at left for details about a multi-port fresh air supply whole house ventilation system.
Though not widely used, supply-only ventilation has several advantages: incoming air can be easily filtered and distributed to the rooms where it is needed, and positive pressures help guard against backdrafting and radon entry.
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.
While few packaged systems are currently available,
one manufacturer, Tamarack Technologies, offers a unit
with a replaceable filter and an efficient variable-speed fan
that provides 90 cfm of ventilation through one 3-inch and
three 2-inch ducts.
Since these systems pressurize the house, the LBNL
study recommends that, in cold climates, the supply fan be
balanced by a single-port exhaust fan, which could also
serve as a bathroom fan. In this type of system, a central
fan, typically in the attic or basement, draws in outside air
through a filter and delivers it through ducts to bedrooms
and main living areas. The supply ductwork should be in
conditioned space or insulated and sealed airtight. Supply
grilles should be placed high on the wall away from beds,
chairs, or other places where drafts could cause discomfort.
Since these systems pressurize the house, the LBNL
study recommends that, in cold climates, the supply fan be
balanced by a single-port exhaust fan, which could also
serve as a bathroom fan. In this type of system, a central
fan, typically in the attic or basement, draws in outside air
through a filter and delivers it through ducts to bedrooms
and main living areas. The supply ductwork should be in
conditioned space or insulated and sealed airtight.
Supply
grilles should be placed high on the wall away from beds,
chairs, or other places where drafts could cause discomfort.
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.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Mobile View
