Guides / Range hoods
Makeup air for a range hood
Based on the IRC, section M1503.6. Your town can change it, so the building department has the final say.
Short answer
If your hood can pull more than 400 CFM and the house has a gas or oil furnace or water heater that vents up a flue, code wants makeup air. That's a duct to the outside with a damper that opens when the hood runs. At 400 CFM or less, you don't need it.
Do you need it?
| Hood | In the house | Makeup air |
|---|---|---|
| 400 CFM or less | Anything | No |
| Over 400 CFM | Gas or oil furnace, water heater or boiler that vents up a flue | Yes |
| Over 400 CFM | All electric, or only sealed or power-vented units | Not under the IRC since 2018. Many towns still want it. |
| Recirculating, no duct out | Anything | No. It pulls nothing out of the house. |
CFM is the hood's rated max, not the speed you plan to run it at. Before 2018, the IRC wanted makeup air over 400 CFM no matter what was in the house, and some towns still do.
Why it matters
Every bit of air the hood blows out has to come back in from somewhere. In a tight house, the easy way in can be down the water heater or furnace flue. That pulls exhaust, carbon monoxide included, back into the house instead of up the chimney.
Even with no flue to worry about, a big hood in a tight house just doesn't pull its rating. There's no air to replace what it moves, so it gets loud and the smoke hangs around.
How to tell if you have a flue appliance
Look at the top of your water heater and furnace. A metal pipe running up into a chimney or through the roof means it vents up a flue. Plastic vent pipes out the side of the house mean it's sealed or power-vented, and the 2018 rule doesn't count it. A gas fireplace or wood stove counts too, unless it's sealed.
What makeup air is
A duct from outside into the house, near the kitchen, with a motorized damper. The damper is wired to the hood, so it opens when the hood turns on and closes when it shuts off. It should bring in about as much air as the hood pulls out.
In a cold climate, that air comes in cold, so some setups heat it. That adds cost, and it's a job for an HVAC contractor. Opening a window works in a pinch, but it doesn't pass code.
Staying at 400 CFM
Plenty of kitchens don't need more than 400. Here's what the calculator gives for a few common cooktops.
| Cooktop | Hood | Makeup air |
|---|---|---|
| 30 in electric or induction, on a wall | 250 CFM | No |
| 36 in electric or induction, on a wall | 300 CFM | No |
| Gas, 40,000 BTU total | 400 CFM | No |
| Gas, 60,000 BTU total | 600 CFM | Maybe |
| Gas, 90,000 BTU total | 900 CFM | Maybe |
Gas is total burner BTU ÷ 100. Electric and induction are 100 CFM per foot of width. Maybe means it depends on the flue question.
Electric and induction almost never need more than 400. Big gas ranges are where it comes up. If you're over, a 400 CFM hood that sits low and a little wider than the cooktop often catches more than a bigger one hung high.
Before you buy
Call the building department and ask what they want over 400 CFM. It's a quick call, and it's cheaper than adding makeup air after the hood is in.