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Make-Up Air Fact Sheet
November 1, 2012
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.
What is MUA?*
• Make-up air (MUA): Outdoor air and transfer air intended to replace exhaust air
• Outdoor air (OA): air that enters a building through a ventilation system, through intentional
openings for natural ventilation, or by infiltration
• Transfer air: Air moved from one indoor space to another
MUA is NOT intended for combustion air for space or water heating appliances. Combustion air must be
provided separately.
*Definitions adapted from ASHRAE 62.1
Where does MUA come from?
• Intentional openings (like an outdoor air duct with a damper)
• Infiltration through naturally occurring cracks and gaps in the building envelope
• Can be introduced into the same space as the exhaust appliance or into a separate indoor space*
• 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?
• Meet code requirements
• Maintain good indoor air quality by avoiding excessive depressurization during operation of
exhaust appliances
• 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
• International Residential Code (IRC), 2009 & 2012 versions
o Section 1503.4: required for Range Hoods > 400 CFM
o 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”
• International Mechanical Code (IMC), 2009 & 2012 versions
o Section 505.2: same requirements as Section 1503.4 of the IRC
• 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
• Large range hoods (those over 300 CFM)
• 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
• 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
Broan-NuTone, LLC
262.673.4340
www.broan-nutone.com
1
How Should Range Hood MUA Systems be Sized to be Code-Compliant?
• Use Broan’s On-Line MUA Specifier Tool
o Identifies the latest state-level code requirements
o Less familiar Tool entries include:
 Available Static Pressure at Intersection of OA Duct and Return Trunk: the negative pressure
available to draw outdoor MUA into the return trunk of a central duct system.
 Design Depressurization Limit (DDL): the designer-selected maximum depressurization of the
home with respect to outdoors (measured in Pascals – Pa – or inches water gauge – in w.g.).
Industry recommended values depend on the type of combustion equipment in the dwelling,
and are as follows:
DDL (Pa)*
Combustion Appliance Description
Individually vented natural draft
Mechanically assisted draft boiler or furnace commonly
vented with water heater
5
•
•
15
•
•
50
• Direct-vented/sealed combustion appliances
Mechanically assisted draft boiler or furnace alone, or
Fan assisted domestic hot water alone
Installation Options
“Dilution of return air with outdoor air shall be permitted.” - IRC M1602.1
1
•
•
•
•
2
3
1: Trigger operation of central fan when the MUAD opens. This increases flow through the
MUAD based on the available static pressure in the return trunk.
2: No connection between operation of central fan and the MUAD. Flow through the MUAD
depends primarily on the home’s design depressurization limit and secondarily on whether
the central fan’s run time overlaps with the MUAD operation.
3: Routed to interior register
Universal MUAD, Direct-Wire, LinkLogic, and Slave Dampers are available
Climatic Considerations
• When tied into the return trunk of gas-fired forced air furnace, check furnace manufacturer
warranty limitations that may constrain OA intake. The following general guidance is not
intended to replace manufacturer guidance, but is offered as a rule of thumb:
IECC Climate Zone Max Recommended OA MUA Flow as % of Total Furnace Air Handler Flow
1
No limit (but keep within reason)
2
40%
3
30%
4
25%
5
20%
6
15%
• If located in an especially hot and humid climate, consider triggering the central AC to run
automatically when the MUAD is open and the thermostat is set to “Cool” mode.
Broan-NuTone, LLC
262.673.4340
www.broan-nutone.com
2
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