Range hood makeup air — a hood over 400 CFM can backdraft a water-heater flue
A hood capable of more than 400 CFM (IRC M1503.4) can depressurize a tight house and backdraft combustion appliances — makeup air prevents it.
What this diagram shows
Two house cross-sections. The left red one has no makeup air: a 600 CFM hood exhausts out the roof, the house goes negative (shown by a minus sign), and replacement air is pulled back down the water-heater flue, spilling combustion gas into the house. The right green one adds an interlocked makeup-air damper that supplies fresh air, so the house stays balanced and the flue vents normally.
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Range Hood CFM Calculator
Free range hood CFM calculator: size by width or BTU, see it vs the code minimum, and learn if you need makeup air (the 400 CFM rule). No signup.
