Four Hundred CFM in a 110-Degree City: Why Phoenix Makeup-Air Dampers Get Taped Shut and the Wolf Hood Stops Pulling

There is a small door on the side of a lot of north Phoenix houses that is supposed to open every time the hood runs hard. In a city where Sky Harbor logged 31 consecutive days at or above 110 degrees in July 2023 and 54 days at or above 110 across that year, homeowners find that door, work out what it does, and shut it. Then the hood stops pulling, and the blower gets blamed.

What people here assume is wrong with it

The complaint is nearly always phrased as motor failure: it is loud, it has lost suction, the searing smoke still crosses the island, so the blower must be dying. Sometimes it is. But a blower that sounds strong and clears nothing is describing a system that cannot get air, and a fan is only as good as what the room can give it.

The second assumption is that the answer is more fan. Owners ask for a higher-CFM blower on a duct and an air supply that already could not feed the one they have, which usually makes the noise worse and the clearing no better.

What is actually happening

A kitchen exhaust fan can only remove as much air as the house can replace. Push a large hood hard in a reasonably tight modern envelope and the kitchen goes negative: the hood strains against the pressure difference, actual airflow collapses well below what the fan is rated for, doors get heavy, and smoke that should be captured rolls out from under the canopy instead.

That is precisely the problem the makeup-air provision exists to solve. The residential code Phoenix adopts and amends carries the familiar requirement — M1503.6 in the IRC text — that where an exhaust system is capable of moving more than 400 CFM and a fuel-burning appliance that is neither direct-vent nor mechanically drafted sits inside the dwelling’s air barrier, makeup air must be provided at approximately the exhaust rate, through at least one outdoor air duct with a gravity or automatically operated damper, discharged into the same or a communicating room. Check the adopted edition and the city’s amendment sheet before relying on a section number for a permit, and note that the later wording makes the trigger conditional — it does not attach to every house.

Where it does attach, the damper’s whole job is to let outside air into the kitchen while the hood runs. In Deer Valley, Moon Valley, Paradise Valley Village and Laveen, that is outside air in the summer described above. So the damper gets taped, blocked, disconnected from its interlock or never wired to one in the first place. The comfort problem is solved and the ventilation problem is created.

The evidence, in the order we look at it

Find out whether the hood is being starved before touching the motor. Run the fan on high and open an exterior door or a window: if the smoke suddenly clears and the noise character changes, the system is air-starved and the fault is makeup air, not the blower. That one test reorders the whole call.

Then trace the makeup-air path physically — the exterior intake, the duct, the damper, and whatever is supposed to open it. Look for tape, foam, a screwed-shut louver, a disconnected actuator or an interlock that was never connected to the hood control. Check the intake screen for dust, which in Laveen’s field-edge subdivisions blinds a screen faster than anyone expects.

Only then work the exhaust side: the roof or wall termination and its own damper, the duct run for length, elbows, crushed flex or a transition to something smaller than the hood collar, the filters and baffles, and finally the blower and speed control. In the older stock — houses of the 1970s and 1980s vintage, where PHOENIX magazine puts the typical Paradise Valley Oasis home around 1,400 square feet and the neighbourhood’s mall opened in 1978 — that duct usually climbs through the attic to a roof jack because the range moved to a wall it was never on, and the run is longer and more tortured than anyone planned. Those houses were laid out for a compact range under a shallow recirculating hood, not for a pro-style rangetop and a proper duct.

The fix that holds

Restore the air balance, then size the fan to what the system can actually deliver. In practice that means reinstating the makeup-air path and its interlock so the damper opens with the hood; correcting the duct so it is the right diameter for its length with the fewest elbows achievable; sealing and re-terminating properly; and choosing a speed the household will genuinely use rather than a maximum setting nobody can tolerate.

Where the reason the damper was defeated is thermal — and in this climate it usually is — the answer is to address the makeup air as a design problem rather than to keep sabotaging it. Tempering, placement of the discharge away from where people stand, and a hood control that only opens the damper at high speed all keep the system honest. What does not work is a bigger blower on a sealed kitchen. That is more noise per unit of smoke removed.

Where to stop and call

Stop when the fix involves the damper interlock, gas appliances in the same air barrier, or a new roof or wall penetration. Backdrafting an atmospherically vented appliance is the risk the code provision is written around, and it is not a trial-and-error area.

An $89 diagnostic visit establishes whether you have a blower problem or an air-supply problem, which is the fork that decides everything after it. If it is the appliance: a convection fan or motor runs $300 to $660, a control board or touch panel $470 to $860, an igniter or spark module $190 to $440, burner valve or orifice service $250 to $610, oven thermostat or sensor calibration $190 to $440, and door hinge, glass or seal work $280 to $660. The number for that is (480) 712-8791, or book from the link above.

Follow-ups

Why would anyone tape a makeup-air damper shut?

Because it opens to outdoor air, and in a Phoenix summer that is unwelcome in a conditioned room. Sky Harbor logged 31 straight days at or above 110 degrees in July 2023. The comfort motive is easy to understand; the consequences are a hood with nothing to draw on and, where a vented combustion appliance shares the space, a backdraft risk.

Will a more powerful blower fix my hood in Laveen?

Usually not, and it frequently makes matters worse. If the room cannot admit replacement air, a larger fan produces more noise and a bigger pressure difference for roughly the same delivered airflow. The sequence is to restore the makeup-air path, correct duct diameter and length, clear the intake screen of field dust, and only then revisit fan capacity.

How do I know it is the ductwork and not the motor?

Run the fan flat out with an exterior door open. If capture improves noticeably, the system was starved and the motor is doing its job. If nothing changes, examine the run itself for crushed flex, a transition narrower than the hood collar, or too many elbows on a long attic route to a roof jack before condemning the blower.

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