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Broan Automatic Make-Up Air Damper Product Guide – 04-17-13 6
1. Definitions & Key Terms
Within the scope of this document, several important terms are used. To give product installers
and users a clear understanding, several key terms are defined below.
Available static pressure: for the purpose of this guide, available static pressure represents the
amount of suction that is available to draw in outdoor make-up air when the central air handler is
running. Manual D, the industry standard for sizing residential duct systems, notes that an
available static pressure between -0.2 and -0.35 in w.g. is typical at the return plenum. When
estimating the flow through a make-up air damper that is connected to a return trunk, provide
the available static pressure at the operating speed of the blower that will be used when the
make-up air damper is engaged.
Natural draft vented appliances: a category of combustion appliances, including some water
heaters, wood burning stoves, and fireplaces, which rely upon the buoyancy of the hot
combustion exhaust gas to force it upward through a flue pipe and out of the house. Such
appliances do not use a fan to assist in exhausting combustion gasses outdoors.
Combustion air: The air provided to fuel-burning equipment including air for fuel combustion,
draft hood dilution and ventilation of the equipment enclosure.
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Depressurization: a condition of lower pressure in one zone with respect to another. For
example, a kitchen with a very large range hood flow rate (~ 1000 cfm) may be depressurized
with respect to outdoors when the range hood is on. This is due to the fact that the range hood
is pulling air out of the kitchen space at a rate greater than the rate at which fresh outdoor air
replaces the exhaust air. Pressure levels in homes are typically measured in units called
Pascals (1 Pa = 0.004 in w.g.), and are measured “with respect to” another zone such as
outdoors or a different part of the home.
Design depressurization limit: the designer-selected maximum depressurization of the home
with respect to the outdoors, selected by the designer and assuming indoor spaces freely
communicate with each other. Recommendations from the Building Performance Institute for
maximum acceptable depressurization in a home depend on the types of combustion
appliances that are installed indoors, as follows. Lower values are more conservative.
• Orphan natural draft water heater (including outside chimneys): 2 Pa
• Natural draft boiler or furnace commonly vented with water heater: 3 Pa
• Natural draft boiler or furnace with vent damper commonly vented with water heater;
Individually vented natural draft boiler, furnace, or domestic water heater; or
Mechanically assisted draft boiler or furnace commonly vented with water heater: 5 Pa
• Mechanically assisted draft boiler or furnace alone, or fan assisted domestic hot water alone:
15 Pa
• Chimney-top draft inducer (Exhaust-type or equivalent);
High static pressure flame retention head oil burner; or
Direct-vented appliances/Sealed combustion appliances: 50 Pa
Design exhaust rate: the flow rate that must be offset by make-up air. Depending on friction
losses and fittings along the length of the exhaust duct, the design exhaust rate may be less
than the exhaust appliance’s nominal rated flow.
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Sourced or adapted from the 2012 International Residential Code.
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