Do I need makeup air for a high-CFM range hood in a new Arizona home?

The code is specific about when makeup air kicks in. Under IRC Section M1503.4, "Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or naturally provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not less than one damper." The pro-style hoods that go over big gas ranges in most luxury Arizona kitchens are rated well above 400 CFM (cubic feet per minute, the measure of how much air the hood pulls). That puts them over the line, so makeup air is required and your plans have to show it.

Here is what that 400 CFM threshold means, why it matters even more in a tight desert home, and how builders supply makeup air.

The 400 CFM rule, straight from the code

The trigger is simple: a range hood that can move more than 400 CFM of air must be paired with a makeup air system that brings in roughly the same amount of outside air. Makeup air is fresh air drawn into the house to replace the air the hood blows outside. The rule lives in Section M1503.4 of the International Residential Code, the model code Arizona cities build to, and the language is exact: a hood "capable of exhausting in excess of 400 cubic feet per minute" must be "mechanically or naturally provided with makeup air at a rate approximately equal to the exhaust air rate."

The word "capable" matters. The code looks at the hood's rated capacity, not how you usually run it. A hood with a 600 CFM blower needs makeup air even if you cook on the low setting most nights. Many high-end kitchens go far past 400. A 36-inch or 48-inch pro hood over a commercial-style range is commonly rated at 600, 900, or 1,200 CFM and up. Once the number on the spec sheet clears 400, makeup air is on the plans.

The code also requires at least one damper in the makeup air path. A damper is a movable flap that opens when the hood runs and closes when it does not, so you are not leaving an open hole to the outside that bleeds conditioned air all summer. In Arizona, where you pay to cool that air for half the year, a tight automatic damper is not just a code item. It protects your power bill.

Why a tight Arizona home makes makeup air matter more

Makeup air matters more in a new Arizona home because today's homes are built tight, and a powerful hood can depressurize a sealed house fast. A modern custom home has a sealed envelope, low-leakage windows, and often spray foam or dense insulation. That tightness is good for cooling bills, but it means the house does not have many gaps for replacement air to sneak through when a big hood is running.

When a 900 CFM hood fires up in a tight house with no makeup air, it pulls hundreds of cubic feet of air out every minute and the house has to replace it from somewhere. The U.S. Department of Energy explains how exhaust systems behave: they "work by depressurizing your home," and "make-up air infiltrates through leaks in the building shell and through intentional, passive vents." In a tight home there are few leaks, so the pressure drops.

That negative pressure causes two real problems. First, the hood loses performance, because it is fighting the house to get air. Second, and more serious, the low pressure can pull combustion gases back down a flue. The Department of Energy warns that exhaust systems can draw in "flue gases from a fireplace or fossil-fuel-fired water heater and furnace" and can "cause backdrafting in combustion appliances." DOE's Building America program flags the same risk for atmospherically vented gas appliances. Backdrafting can pull carbon monoxide into living space, which is why the makeup air rule is a safety rule, not just a comfort one. A gas water heater, a gas furnace, or a vented fireplace plus a strong hood is the exact mix the code is guarding against.

How builders supply makeup air

Builders supply makeup air with a dedicated outside-air duct, a damper that opens with the hood, and often a small amount of tempering so the incoming desert air does not wreck comfort. The most common setup is a passive or motorized makeup air duct that runs from an outside hood or vent to a register near the kitchen. When the range hood switches on, an electrical interlock opens the damper so fresh air flows in at about the same rate the hood exhausts. When the hood shuts off, the damper closes.

There is an Arizona wrinkle worth planning for. Outside air in Phoenix can hit 110-plus degrees in summer and drop near freezing on a January morning, and dumping that raw air straight into the kitchen is uncomfortable. Higher-end systems add a makeup air unit (MAU) that gently tempers the incoming air, warming it in winter and sometimes pre-cooling it in summer, so the kitchen stays livable while the hood runs. A tempered MAU costs more than a simple passive duct, and on a luxury custom build it is often worth it.

A few practical notes for your plan set:

• Match the duct to the hood. The makeup air capacity should roughly equal the hood's rated CFM. Oversize the hood and you oversize the makeup air, the duct, and the unit.
• Place the supply away from the range. You want fresh air entering the room, not blowing across the burners and killing the flame or scattering heat.
• Interlock the damper. The damper must open automatically with the hood, not by hand, so the system actually works when you cook.
• Mind the gas appliances. If you have a gas furnace, gas water heater, or vented fireplace, the makeup air and combustion air both need to be right so nothing backdrafts.
• Size the electric circuit if you temper. A heated or cooled makeup air unit is itself an electrical load. On an all-electric custom home it is one more circuit your panel has to carry, which folds into your service-size decision.

One more thing to plan early: the location of the outside intake. The makeup air opening should pull from a clean spot, away from a dryer vent, a flue, a trash area, or a pool deck where dust blows. Set the intake well, and the fresh air stays fresh. Place it poorly, and you pull odors or fine desert dust straight into the kitchen every time the hood runs. Your builder marks this on the elevations during design.

The takeaway for an Arizona custom home is clear. Pick your hood, check its rated CFM, and if it can exhaust more than 400 CFM, your builder must design makeup air with at least one damper under IRC M1503.4. Confirm the hood rating and the makeup air detail with your builder during design, because changing it after the kitchen ductwork is roughed in is a painful fix. For how a tight envelope and big mechanical loads fit together, see our Arizona HVAC system page and the conditioned space glossary entry.

How Jematell Homes helps

Desert-smart design and systems are standard on every home we build. If you are planning a custom home in Scottsdale, Rio Verde, or the greater Phoenix metro, we are happy to walk through your project.

Sources

• ICC 2021 International Residential Code, Section M1503.4 (Makeup air required)
• U.S. Department of Energy, Energy Saver: Whole-House Ventilation
• DOE Building America Solution Center (PNNL): Combustion Appliances