Kitchen Hood Makeup Air

Why this matters

The gas range comes out, an induction cooktop goes in, and while the kitchen is open the customer upgrades to a 600 cfm chimney hood because the cooktop deserves it. The hood works beautifully. It also pulls 600 cubic feet of air out of the house every minute it runs on high, and that air has to come back in from somewhere. In a house that was air-sealed two years ago, one of the places it comes back in is the water heater's vent, down the chimney, carrying the water heater's flue gas into the basement. The conversion took one combustion appliance out of the kitchen and made the one left in the basement dangerous.

This card covers the residential code's makeup air requirement for big hoods, why induction remodels so often trigger it, and what a big hood does to the combustion appliances that stay. The test procedure for the appliances left behind belongs to Combustion Safety After a Partial Conversion; the venting changes from removing an appliance belong to What Removing a Gas Appliance Does to the Ones Left.

What the code requires

The International Residential Code requires that kitchen exhaust hood systems capable of exhausting more than 400 cfm be provided with makeup air at a rate approximately equal to the exhaust rate, through a makeup air path with a damper that opens automatically when the hood runs. That requirement is M1503.6 in the 2018 and 2021 editions and M1503.4 in 2015 and earlier, and it is binding only as your jurisdiction has adopted it. Several states have amended it, changing the threshold or adding exceptions, so read the adopted text rather than the model code.

Three words in it do most of the work:

• "Capable of." The trigger is the hood's maximum rated capacity, not how the customer plans to use it. A 600 cfm hood that will "mostly run on low" is a 600 cfm hood.
• "Approximately equal to the exhaust rate." The makeup air is sized to the hood's flow, not to the excess over 400.
• Automatic. A window the customer is supposed to crack does not meet it; the damper has to open with the hood, by interlock.

The code sets a floor for kitchen exhaust too: local kitchen exhaust in the IRC is 100 cfm intermittent at minimum. Almost any hood sold clears that. The makeup air question starts at four times that.

Why induction remodels so often trigger it

Induction itself needs less exhaust than gas, not more. A gas burner adds combustion products, nitrogen dioxide, carbon monoxide and a lot of water vapor, to the cooking plume, and a large share of the reason to exhaust a gas range is the burner, not the food. Induction adds none of that.

But the cooktop swap is rarely the only change. The kitchen is torn up, the cabinets are coming out, and the customer has seen hoods in showrooms. A pro-style chimney hood rated 600 to 1,200 cfm is a common upgrade at exactly the moment the range goes electric. Nobody on the job thinks of it as mechanical work, so nobody checks the makeup air rule, and the cabinet installer hangs it.

The honest advice to the customer is that a well-placed hood at 300 to 400 cfm captures an induction cooktop's plume well, and that capture depends more on geometry than on flow: a hood at least as wide as the cooktop, deep enough to cover the front burners, and mounted at the height the manufacturer specifies. A big hood mounted high and narrow captures less than a modest one mounted right. If the customer still wants the big hood, the makeup air comes with it.

What a big hood does to the house

An exhaust fan does not delete air; it moves it out, and the house's pressure drops until an equal flow leaks back in through every crack, flue and opening. How far the pressure drops depends on how leaky the house is.

A blower door test reports leakage as CFM50, the airflow that holds the house at 50 pascals (Pa) below outdoors. At lower pressures, flow through the house's leaks scales with pressure raised to a flow exponent, commonly around 0.65 for a house. Rearranged, the depressurization a given exhaust flow produces is:

pressure (Pa) = 50 x (exhaust cfm / CFM50) ^ (1 / 0.65)

That is an estimate. It treats the house as one zone with typical leaks, ignores wind and stack effect, and assumes the hood actually moves its rated flow. Use it to decide whether to worry, then measure.

What matters is the comparison with what a natural-draft appliance can tolerate. A natural-draft (Category I) water heater or furnace moves its flue gas by buoyancy alone, and a draft of a few pascals is all it has. When the house depressurizes past that, the vent reverses and the appliance spills. BPI's combustion safety standard sets worst-case depressurization limits by appliance type, as low as 2 Pa for an orphaned natural-draft water heater (BPI-1200; confirm against the edition your program uses). Mechanical-draft appliances tolerate more, and sealed-combustion, direct-vent appliances, which take combustion air from outdoors and vent under pressure, are largely immune.

Worked example: the basement water heater

The house: 2,400 square feet, air-sealed two years ago, a blower door result of 1,500 CFM50. The furnace was replaced by a heat pump last year, leaving a natural-draft gas water heater alone in a masonry chimney in the basement. The kitchen is being remodeled: induction, and a 600 cfm chimney hood.

Estimate with the hood on high and no makeup air. 600 over 1,500 is 0.40. Raised to 1 over 0.65 (about 1.54), 0.40 becomes about 0.244, and 50 times 0.244 is about 12.2 Pa of depressurization. That is several times the 2 Pa limit for an orphaned natural-draft water heater.

The 400 cfm alternative. If the customer chose a hood capped at 400 cfm, below the code trigger: 400 over 1,500 is about 0.267, which becomes about 0.131, and 50 times 0.131 is about 6.5 Pa. Still more than three times the limit. A hood that needs no makeup air under the code can still backdraft the water heater in a tight house. The code threshold is a mechanical rule, not a combustion safety test.

The makeup air path. A passive opening relies on the house depressurizing to pull air through it. Treating a damper and short duct as a sharp-edged opening, with a discharge coefficient of about 0.6, moving 600 cfm at only 5 Pa of driving pressure takes about 250 square inches of free area, roughly an 18-inch round duct, before any duct friction. And 5 Pa is already past the water heater's limit. So in a tight house, makeup air for a big hood is in practice a powered, interlocked supply fan, sized to the hood, with a motorized damper.

Tempering. In January, with 10 F outdoors and a 70 F house, bringing in 600 cfm takes 1.08 times 600 times 60, about 38,900 Btu/h, to warm, at standard air density. That is more than the heat pump's spare capacity on a cold evening. Whether your jurisdiction requires makeup air to be heated is set by your edition and local amendments, but the customer will feel 10 F air arriving in the kitchen either way. Options are a duct heater on the makeup air, introducing it near the hood so it is exhausted before it mixes, or limiting how long the hood runs on high in winter.

What the shop recommended. Two things together: a powered, interlocked, tempered makeup air system sized to the hood, and moving the water heater conversion up in the plan, from next year to this one. With the water heater replaced by a heat pump water heater, there is no combustion appliance left to backdraft. The makeup air is still code-required for a 600 cfm hood, but it stops being a life-safety system.

The measured result. Until the water heater conversion, the installer measured the basement's pressure against outdoors with the hood on high, all interior doors in their worst-case positions and the makeup air disabled: minus 11 Pa, close to the 12.2 estimate. With the makeup air enabled and balanced, the same test read minus 1.5 Pa, under the 2 Pa limit. The water heater was fired under those conditions with a personal CO monitor on, and drafted within the time the test procedure allows. That held the house safe for the months until the water heater came out.

What changes the answer

A recirculating hood. A hood that filters and returns air to the kitchen exhausts nothing, so it does not trigger makeup air and does not depressurize the house. It also removes grease and some odor but none of the moisture or heat, which is a real trade-off for a heavy cook.

No combustion appliances left. In an all-electric house, depressurization is a comfort and moisture question, not a CO question. Makeup air may still be required by the code, but nothing in the house will spill.

Other exhausts at the same time. Bath fans, a vented dryer, a central exhaust fan and a fireplace chimney all add to the hood. A worst-case test runs them together. A heat pump dryer that is ventless removes one of them, which is a quiet benefit of a full conversion.

A leaky house. At 4,000 CFM50, the same 600 cfm hood produces far less depressurization. The arithmetic is the same; the answer is not. Measure before recommending makeup air on the strength of the code alone, and before skipping it on the strength of a leaky-looking house.

How to verify you got this right

Before the hood is ordered, get its maximum rated cfm from the spec sheet, not the sales copy, and compare it to your adopted code's threshold. If any combustion appliance remains, run a worst-case depressurization and spillage test with the hood on high and every other exhaust running, per the procedure in Combustion Safety After a Partial Conversion. After installation, confirm the makeup air damper opens and the fan starts every time the hood starts, at every speed setting, and record the measured pressure with the makeup air on.

References

• International Residential Code, M1503.6 (2018 and 2021 editions; M1503.4 in 2015 and earlier), as adopted and amended by your jurisdiction.
• BPI-1200, Standard Practice for Basic Analysis of Buildings, worst-case depressurization limits by appliance type.
• Hood and makeup air manufacturer installation instructions (rated cfm, mounting height, interlock wiring).
• See related: Combustion Safety After a Partial Conversion; What Removing a Gas Appliance Does to the Ones Left; Induction Cooking Conversion Reference.