DCContrarian, a frequent contributor to GBA’s Q&A forum, now has a question of his own: How much air should a kitchen exhaust fan move?
“So how many cubic feet per minute should the fan that vents a cooktop be?” he asks in this recent post. “I’ve done an exhaustive search of the GBA archives, and there are lots of articles for sizing the makeup air to the vent fan, but nothing I’ve been able to find that helps you decide how big the fan itself should be.”
Most of what he’s been able to find online “lacks any sort of analytic rigor,” he continues. The rules of thumb he has found—1 cfm for every 10 Btus of output for a gas range, or 10 cfm for every inch of width for an electric range—suggest the capacity of the range hood should be about 300 cfm.
But what about a number that’s science-based? That’s the issue for this Q&A Spotlight.
The IRC addresses makeup air, not fan capacity
As Armando Cobo points out, a section of the International Residential Code (IRC) requires makeup air be provided for exhaust fans rated at more than 400 cfm. Each exhaust fan of that size must be “mechanically or passively” provided with makeup air at about the same rate that indoor air is being expelled.
“That says nothing about how big the exahust system should be,” DCContrarian says, “just the makeup air.” Plus, that code provision would not apply to an all-electric house—just to those with a fuel-burning appliance.
Where does that leave us? Picking a number out of the blue and letting GBA readers lob rotten tomatoes at it, suggests Malcolm Taylor.
“I’m pretty sure you just pick a size and we reply it’s either too…
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