How Many CFM a Range Hood Needs for a Gas Stove

Nothing stale here: the 1 CFM per 100 BTU rule and the 400 CFM makeup-air threshold still hold per research. Return unchanged.

A range hood over a gas stove needs about 1 CFM (cubic foot per minute of airflow) for every 100 BTU of total burner output. A range whose burners add up to 40,000 BTU therefore wants a hood of roughly 400 CFM, plus a little more to make up for the ductwork.

You will run into this number twice during a kitchen remodel. The first time is when you pick the hood, because CFM sets the price of the unit. The second time is when you read the installer's bid. Above 400 CFM, the building code can require a makeup air system, and that adds a duct, a damper and possibly electrical work to the job. The point where the code steps in sits right where the gas-stove rule of thumb lands for an ordinary four-burner range, so the sizing question and the cost question are the same question.

Work through it in this order.

1. Add up the burners. Find the BTU rating of each burner, add them together and divide by 100. Today's Homeowner and HomeGuide both give this formula, and both use it to get 400 CFM from a 40,000 BTU stove. The manufacturer Proline applies the same division to a 60,000 BTU range and gets 600 CFM.

2. Check it against the width rule. The Home Ventilating Institute sizes hoods by cooktop length instead. It recommends 100 CFM per linear foot for a wall-mounted hood, with 40 CFM per foot as the minimum. Over an island it recommends 150 CFM per foot, because there is no wall behind the range to hold the smoke in. A 30-inch wall hood works out to 250 CFM recommended. That is less than the BTU rule gives for most gas ranges. For professional-style, high-BTU cooktops, HVI says to follow the cooktop maker's ventilation advice.

3. Add for the duct run. HomeGuide and Proline both add 1 CFM per foot of duct, 25 CFM for each elbow and 40 CFM for a roof cap. HomeGuide also adds 200 CFM if you fry often.

When the methods give different numbers, the guides that list several methods tell you to use the highest one. Some also offer a room-volume method based on 15 air changes an hour. In an open-plan room it can produce much larger numbers: Hauslane's example of a 20-by-20-foot room with 10-foot ceilings comes out near 1,000 CFM.

IRC Section M1503.6 requires makeup air for any exhaust system that can move more than 400 CFM. The replacement air must come in at roughly the same rate the hood pulls air out. The rule applies only when a fuel-burning appliance that is neither direct-vent nor mechanically drafted sits inside the home's air barrier. A standard atmospherically vented gas water heater or furnace is the typical example. The system needs at least one outdoor air duct and a damper that opens automatically when the hood runs.

The code has a safety reason for this. The Department of Energy's Building America Solution Center warns that an oversized hood can depressurize a house enough to backdraft a furnace, water heater or fireplace, which pulls combustion gases back indoors. According to up.codes, this is also why many hood makers cap their standard models near 400 CFM.

Our sources do not give a price for makeup air work, so treat it as a separate line to ask about. For comparison, HomeGuide lists a straight hood replacement at $300 to $850. A new installation that includes an outlet and ductwork runs $650 to $2,100, and a new electrical outlet alone adds $150 to $350. Installed costs by hood type and region are in our pillar guide, Range Hood Installation Cost by Type and Region.

The brief for this piece named specific hoods at 200, 375 and 860 CFM. We have no verified listings for those models, so we are not quoting their figures here. We did not test any hoods. When you shop, use the maker's listed CFM and sone rating as of the day you read the listing.

The gap between the rated and delivered figures is why ENERGY STAR recommends buying a hood rated 150 to 200 CFM just to be sure you get the 100 CFM code minimum after installation. The DOE duct guide says the duct should be rigid metal with smooth interiors, and that a larger diameter cuts restriction. Its example is a 6-inch round duct.

Capture matters as well. HomeGuide puts mounting height at 24 to 30 inches above the cooktop, and says gas ranges sometimes need the higher 30-inch clearance. HVI notes that downdraft systems need more rated airflow than a canopy hood, because they cannot catch the rising column of cooking air as well.

Run the BTU math on your range before you look at hoods. If it lands at 400 or below and your duct run is short and straight, a hood at or under 400 CFM usually avoids the makeup air question entirely.

A higher rating is wrong for you if you have an atmospherically vented gas water heater or furnace and no budget for makeup air. It is right if you cook with high heat often or have a large open-plan room. In that case, ask each bidder three things:

• Will this hood need makeup air in my house?
• Is that work priced in?
• What diameter, and how many elbows, will the duct run have?

Also check the sone rating. ENERGY STAR caps compliant systems at 2.0 sones, and ASHRAE 62.2 limits intermittent fans to 3 sones. A hood too loud to run is a hood nobody turns on.