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Make-Up Air Fact Sheet
November 1, 2012
What is MUA?*
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Make-up air (MUA): Outdoor air and transfer air intended to replace exhaust air
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Outdoor air (OA): air that enters a building through a ventilation system, through intentional
openings for natural ventilation, or by infiltration
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Transfer air: Air moved from one indoor space to another
*Definitions adapted from ASHRAE 62.1
Where does MUA come from?
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Intentional openings (like an outdoor air duct with a damper)
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Infiltration through naturally occurring cracks and gaps in the building envelope
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Can be introduced into the same space as the exhaust appliance or into a separate indoor space*
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Can be introduced into the return trunk of a centrally ducted heating and/or cooling system (see
IRC M1602.1)
*The IRC does not specifically address where MUA comes from, but the International Mechanical Code Section 403.4, Section
403.2.2, Table 403.3.1.2 footnote “e”, and ASHRAE 62.1’s definitions of MUA, OA, and transfer air support this explanation.
Also see IRC G2439.4, which permits MUA to be provided through a louvered door – meaning that MUA can be transferred
from other indoor spaces.
Why provide MUA?
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Meet code requirements
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Maintain good indoor air quality by avoiding excessive depressurization during operation of
exhaust appliances
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If unchecked, excessive depressurization can reduce flow rate of exhaust appliances and lead to
improper venting of combustion appliances
MUA Requirements in Codes and Standards
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International Residential Code (IRC), 2009 & 2012 versions
o
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Section 1503.4: required for Range Hoods > 400 CFM
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Must be “approximately equal to the exhaust air rate”
o
Must be “equipped with a means of closure that is automatically controlled to start and operate
simultaneously with the exhaust system”
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International Mechanical Code (IMC), 2009 & 2012 versions
o
o
Section 505.2: same requirements as Section 1503.4 of the IRC
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ASHRAE 62.2 (62.2): Section 6.4 of the 2007 and 2010 versions; required when the maximum
flow of the two largest-volume exhaust appliances exceeds 15 CFM/100 sqft of habitable area
(i.e., 300 cfm for 2000 sqft)
MUA is Recommended in Dwellings with One or More of the Following Conditions
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Large range hoods (those over 300 CFM)
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Newer energy efficient, “tight” or “green” dwellings that are well air-sealed – OA will have more
trouble finding a way in to replace exhausted air, as opposed to older draftier homes
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Dwellings with natural vented combustion appliances (i.e., a water heater or natural draft
fireplace), which are more susceptible to improper venting if depressurization occurs in the
dwelling
MUA is NOT intended for combustion air for space or water heating appliances. Combustion air must be
provided separately.
Navigating the codes and standards requirements for make-up air (MUA) can be a real
challenge for builders and contractors. Broan-NuTone and BEST have the products and
the design expertise to customize code-compliant solutions for the industry.