Sizing Make Up Air Supply Grilles for Low Velocity
Corbett Lunsford
In modern air-sealed homes, any big one-way exhaust fans should be at least considered for their potential depressurization of the whole home. In MANY MANY cases, the building code requirement of make up air only for kitchen fans bigger than 400 cfm MAKES NO SENSE and is stupid and dangerous.
If you’re bringing in make up air, you don’t want it shooting through your home at high velocity- in my experience keeping it under 300 feet per minute (3.4 mph) is a good way to slow it down enough. Here’s a simple trick to size your make up air system for a home kitchen exhaust hood:
The toe-kick area under your kitchen cabinet is 4” high. However many 100’s of CFM you’re bringing in, you’ll need the same amount of feet of length for this toe kick grille.
For example:
300 cfm kitchen exhaust, 300 cfm make up air, 3 ft of length (36 in long x 4 in high)
400 cfm kitchen exhaust, 400 cfm make up air, 4 ft of length (48 in long x 4 in high)
500 cfm kitchen exhaust, 500 cfm make up air, 5 ft of length (60 in long x 4 in high)
600 cfm kitchen exhaust, 600 cfm make up air, 6 ft of length (72 in long x 4 in high)