What make-up air is, in plain terms
A house is a closed box with small leaks. When the hood exhausts air, the pressure inside drops slightly and outdoor air pushes in through whatever openings exist to take its place. Broan's application guide defines make-up air as the outdoor and transfer air that replaces the exhausted air, and describes its main purpose as letting exhaust appliances work as designed.
At 250 or 300 CFM the leaks in most homes keep up. Broan's guide notes that hoods over 300 CFM are often considered large, and gives the example of a kitchen hood around 1,000 CFM depressurizing the house whenever it runs, because it removes air faster than the leaks can replace it.
Why newer houses feel it first
Air sealing, tight windows and spray foam are good for energy bills and bad for a big blower. The tighter the shell, the less air slips in on its own and the more of the replacement air has to come through a deliberate opening. The same large hood that works well in a 1960s ranch may struggle in a new build.
Source: Broan — Automatic Make-Up Air Damper Application Guide — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing
The 400 CFM line in the building code
Planning make-up air for a range hood starts with its rating. Broan's guide summarizes the building code requirement: range hoods rated over 400 CFM need some of their make-up air supplied through a system that is automatically controlled to start and operate at the same time as the exhaust. Local codes can be stricter, and the local building department has the final word.
The code also accepts that part of the replacement air arrives through ordinary leakage. Sizing tools such as Broan's make-up air specifier estimate how much the envelope supplies and how much the damper must add, and produce a sheet an installer can hand to the inspector.
A second limit in the island manual
Vent-A-Hood's island instructions cite ASHRAE's residential ventilation standard 62.2, which limits total exhaust fans to 15 CFM per 100 square feet of occupiable space unless a back-drafting test is done or make-up air is provided, and advise asking a local HVAC engineer to evaluate it.
Source: Broan — Automatic Make-Up Air Damper Application Guide — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing
What goes wrong without it
The first sign is usually the hood itself. A blower working against a depressurized house moves far less than its rating, gets louder, and lets smoke roll out past the canopy. Doors to the outside may be hard to open while it runs, and the fireplace or a bathroom fan duct may whistle as air is pulled in backward.
The serious risk is combustion. Broan's hood manuals warn that sufficient air is needed for the flue of fuel-burning equipment to exhaust properly and prevent backdrafting. A natural-draft water heater or furnace relies on warm gas rising up its flue; a strong hood can reverse that flow and draw the gases into the house.
- The hood sounds strong and captures poorly.
- Exterior doors pull shut or resist opening while it runs.
- A whistle at windows, the fireplace or other exhaust grilles.
- Soot or a smell of exhaust near a gas water heater or furnace.
- A carbon monoxide alarm sounding while the hood is on high.
If the carbon monoxide alarm sounds
Treat it as an emergency, not as a ventilation question. NFPA's advice is to move at once to fresh air outdoors or by an open window or door, make sure everyone in the home is accounted for, and call for help from there, staying until emergency personnel arrive. Have the gas appliances and the make-up air arrangement checked before the hood is used on high again.
Source: Broan — BXT1 Series Range Hood Installation Instructions — Sufficient air is needed for proper combustion and exhausting of gases through the flue of fuel-burning equipment to prevent backdrafting
How make-up air is brought in
The usual arrangement is an insulated duct from an outdoor intake to a motorized damper that opens when the hood starts. Broan's damper, for example, can be wired directly to compatible hoods or triggered by a pressure switch in the exhaust duct. The fresh air is delivered either into the return side of the heating and cooling system or through a register near the kitchen.
Broan's guide sets the rules for the intake: it must draw directly from outdoors, never from a garage, attic or crawl space; sit at least 10 feet from flues, plumbing stacks and exhaust outlets and at least a foot above the ground; and carry a screen with openings between a quarter and a half inch.
- An outdoor intake with a screen, placed away from flues and exhaust outlets.
- An insulated duct from the intake into the house.
- A motorized damper that opens when the hood runs and closes when it stops.
- A signal from the hood, or a pressure switch in its duct, that tells the damper when to open.
- A delivery point: the return side of the central system or a room register.
Cold and humid outdoor air
Air brought in on a January evening is cold, and air brought in during a Gulf Coast summer is wet. Broan's guide gives limits on how much outdoor air a furnace's return can take by climate zone, and suggests controls that run the air conditioning when the damper opens in humid climates. Those decisions belong to a heating and cooling contractor who knows the local climate.
Source: Broan — Automatic Make-Up Air Damper Application Guide — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing
Living with a make-up air system
Once installed, it asks for little. Broan's maintenance list is short: keep the outdoor screen free of debris, keep snow, leaves and plants away from the intake, clean or replace any filter in the fresh-air path, and have the damper checked during routine heating and cooling service.
If the hood starts to feel weaker than it did, look at the intake before blaming the blower. A screen matted with cottonwood fluff or leaves quietly turns a balanced system back into a depressurized one.
Source: Broan — Automatic Make-Up Air Damper Application Guide — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing
