Why a big range hood needs makeup air in Florida.

What Makeup Air Actually Is

Makeup air is replacement outdoor air, brought into a house deliberately to balance the air a powerful exhaust fan throws out. When a range hood pulls hundreds of cubic feet of air per minute out of a kitchen, that air has to be replaced from somewhere — and a makeup-air system gives it a controlled path instead of letting the house find one on its own.

In a leaky old house, replacement air seeps in through gaps around windows, doors, and the sill plate, and nobody notices. A modern Florida house is built and air-sealed to keep conditioned air in and humidity out, so those gaps are largely gone. The exhaust still has to be balanced, but the house has no easy way to do it — which is the whole reason this provision exists.

Exhaust without a plan finds the easiest opening

Air follows the path of least resistance. If the tightest opening into the house happens to be the flue of a natural-draft water heater, the hood will draw replacement air down that flue, dragging combustion byproducts with it. That reversed flow is the hazard the code is written to prevent.

Balanced, not just supplied

The goal is pressure balance. Makeup air is sized to roughly match the exhaust so the kitchen — and the rooms connected to it — stay close to neutral pressure while the hood runs. Under-supply it and the house still depressurizes; the system has to be deliberate, ducted, and damped, not a cracked window.

The 400 CFM Rule, Stated Plainly

The number every contractor quotes is 400 CFM: once a kitchen exhaust system is capable of moving more than that, the makeup-air provision is on the table. CFM measures airflow volume, and 400 is roughly the point at which a residential hood can shift enough air to depressurize a tight home.

The word that does the work is capable. The rule looks at the hood's rated maximum, not the speed you usually run it on. A hood marketed at "up to 600 CFM" is a 600-CFM appliance for code purposes even if you live on the low setting, because an inspector cannot police how you use the dial.

Why this threshold lands hard in Florida

Two regional facts stack up. First, Florida homes run sealed and air-conditioned almost year-round, so envelope leakage is low and there is little incidental makeup air. Second, plenty of Florida houses still heat water with an atmospherically vented natural-draft gas appliance. A tight envelope plus a natural-draft flue is precisely the combination the rule targets.

What 400 CFM does to indoor pressure

Depressurization is measured in pascals. Natural-draft combustion appliances are sensitive: a sustained drop of about 10 Pa is enough to overcome the buoyancy that carries flue gas up the vent and pull it back down instead. A large hood in a tight kitchen can reach that depression easily, which is why the threshold is set where it is.

What the Code Actually Says

Florida builds from national model codes with state amendments. One- and two-family dwellings follow the FBC-Residential, which mirrors the IRC; other occupancies follow the FBC-Mechanical, which mirrors the IMC. Both carry the same makeup-air logic, and both were updated in the Eighth Edition (2023).

The residential provision: IRC M1503.6

In the 2021 IRC — the basis for the current FBC — the makeup-air language lives in Section M1503.6 (it was numbered M1503.4 in earlier editions). It states that where a fuel-burning appliance that is neither direct-vent nor mechanical-draft sits inside the dwelling's air barrier, each exhaust system capable of exceeding 400 CFM must be provided makeup air at a rate approximately equal to the exhaust rate.

The commercial-side mirror: IMC 505.4 / FBC-M Chapter 5

The mechanical code carries the parallel rule in Chapter 5. The makeup-air duct must include at least one outdoor-air duct with an automatic damper, and that damper has to open and operate simultaneously with the hood. You cannot satisfy it with a manual vent someone has to remember to open.

Rate

Makeup air supplied at a volume approximately equal to the exhaust air rate — not a token trickle.

Control

A means of closure (damper) that is automatically controlled to start and run with the exhaust system.

Source

At least one dedicated outdoor-air duct, so the replacement air is conditioned-house air balanced by outside air, not flue gas.

Do You Actually Need It?

This is where most homeowners get the wrong answer. The 400 CFM number alone does not decide it — the deciding question is what burns fuel inside your home's air barrier. If nothing vents by natural draft, a big hood is not a code-mandated makeup-air case.

The single test that settles it

Walk the house and classify every combustion appliance. The exemption hinges on draft type, not fuel type: a sealed, direct-vent gas appliance pulls its own combustion air from outside and pushes exhaust out under power, so a depressurized house cannot reverse it.

The common Florida configurations

Most Florida kitchens land in one of three buckets, and the makeup-air answer follows directly from the appliance inventory.

• Electric range, electric or heat-pump water heater — exempt; hood size is a capture-and-comfort decision only.
• Gas cooktop, tankless gas water heater (direct-vent) — usually exempt, because modern tankless units are sealed; verify the model is direct-vent.
• Gas cooktop, older tank gas water heater (natural draft) — the classic trigger; a hood over 400 CFM requires makeup air.

Because the answer turns on the water heater as often as the cooktop, planning the hood and the mechanical permit together is the only way to avoid a failed inspection — something we fold into every whole-kitchen remodel from the start.

Sizing the Hood for a Gas Cooktop

Choosing hood capacity comes before the makeup-air question, because the size you pick is what decides whether you cross 400 CFM. For a gas cooktop, the Home Ventilating Institute rule of thumb is 1 CFM per 100 BTU of total burner output — divide the cooktop's BTU rating by 100 to get a sensible minimum.

Working the BTU math

Add up every burner's rated BTU and divide by 100. A modest four-burner gas cooktop can total in the mid-tens of thousands of BTU, which already pushes the recommended airflow toward — and sometimes past — the 400 CFM line. A pro-style cooktop with high-output burners blows past it comfortably.

Cross-check against room volume

A second method sizes by air changes: kitchen volume times a turnover rate. Use whichever of the two methods (burner BTU or room volume) gives the higher number, and treat that as your floor, not your target.

Do not over-buy capacity for its own sake. A hood matched to the cooktop captures grease and moisture without needlessly pushing you across the threshold, and a minimum-grade vented hood under ASHRAE 62.2 still moves at least 100 CFM — enough for everyday cooking in a smaller kitchen.

Ducted vs Recirculating in Florida

A ducted hood vents grease, heat, moisture, and combustion byproducts to the outdoors; a recirculating hood filters the air through charcoal and blows it back into the room. Only the ducted type removes humidity and cooking byproducts from the house — a real distinction in a climate already fighting moisture.

Why ducted wins in a humid climate

Florida kitchens generate steam on top of the state's ambient humidity. A recirculating hood leaves that moisture indoors, where it loads the air conditioner and feeds the mold risk that drives so many Florida material choices. A ducted hood ejects it.

The makeup-air twist most people miss

Here is the counterintuitive part: a recirculating hood does not exhaust air, so it never triggers the makeup-air rule regardless of its fan rating. A ducted hood does. The trade is real — ducted hoods perform far better but carry the pressure-balance obligation when paired with a natural-draft appliance.

• Ducted hood — removes moisture and byproducts; can trigger makeup air over 400 CFM with a natural-draft appliance present.
• Recirculating hood — never triggers makeup air, but returns heat and humidity to the kitchen and only filters odor and grease.
• Downdraft systems — duct under or through the slab; in slab-on-grade Florida construction this routing is set during the remodel, not after.

For most Florida homes the right answer is a properly ducted hood sized to the cooktop, with makeup air added if the appliance test calls for it — a sequence that fits naturally into an open-concept kitchen build where ductwork and pressure are planned alongside the layout.

How to Comply on a Remodel

Compliance is a short, ordered sequence, and getting the order right keeps the makeup-air detail from becoming an expensive afterthought. The deciding inputs — hood rating and water-heater draft type — are both known before a single duct is cut.

None of these steps is optional once the trigger is met, and a missed makeup-air detail is a classic Florida inspection failure on otherwise-finished kitchens. We handle the mechanical permit and the damper interlock as part of permit handling, and the broader sequence is laid out in our Florida kitchen remodeling guide so the hood, the water heater, and the permit get reconciled before demolition starts.