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How Should Range Hood MUA Systems be Sized to be Code-Compliant?
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Use Broan’s On-Line MUA Specifier Tool
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Identifies the latest state-level code requirements
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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
5
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Individually vented natural draft
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Mechanically assisted draft boiler or furnace commonly
vented with water heater
15
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Mechanically assisted draft boiler or furnace alone, or
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Fan assisted domestic hot water alone
50
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Direct-vented/sealed combustion appliances
Installation Options
“Dilution of return air with outdoor air shall be permitted.” - IRC M1602.1
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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.
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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.
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3: Routed to interior register
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Universal MUAD, Direct-Wire, LinkLogic, and Slave Dampers are available
Climatic Considerations
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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%
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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.
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