Start with what changed, not with the blower
Most calls about a range hood not pulling smoke have a date attached to them. The hood worked well last winter, and then something shifted: the filters went a season without a wash, a new window went in beside the cooking area, a roofer replaced the cap, or a contractor moved the ductwork during a remodel. Asking what changed around that time narrows the search faster than listening to the motor.
It also helps to separate two complaints that feel alike. A hood that sounds normal but moves little air points to something in the airflow path. A hood that sounds weaker than it used to, or slows down after a few minutes, points to the blower. The first is often an owner's fix; the second rarely is.
Grease filters that have closed up
Every hood pulls its air through the filters first, so a coat of grease there costs airflow on every speed. Zephyr tells owners to clean mesh filters at least every two months, or when the filter indicator asks, partly because a loaded filter is a fire hazard and not just a drag on performance.
- Filter shows light through evenly: not the cause, move on.
- Filter dark in patches or sticky between baffle folds: clean it and test again on high.
- Charcoal filter past six months on a recirculating hood: replace it before judging the fan.
Mesh filters
Hold a mesh filter up to a window after you take it down. Light should show through evenly across the whole panel; a filter that looks amber and blocks the light in patches is doing more blocking than filtering. Soak it in hot water with a degreasing detergent, rinse, and let it dry completely before it goes back up.
Baffle filters
Stainless baffles on professional-style hoods hide their grease inside the folds, so they can look clean from below and still be packed. Pull one and look along the channels from the end. A sticky film on the inside of each fold means it is time for a soak, even if the face shines.
Charcoal filters on a recirculating hood
A hood with no duct to the outside returns air to the room through charcoal filters, and those cannot be washed back to life. Once they are saturated they slow the air and stop catching odors at the same time. Broan's guidance for its non-ducted filters is a change every three to six months, sooner with a lot of frying or wok cooking.
Source: Zephyr — ASL use, care and installation guide — Clean the mesh filters at least every 2 months, or when the filter indicator asks, to prevent a fire hazard
A backdraft damper that no longer swings open
Behind the duct collar on top or at the back of the hood sits a light flap that the moving air pushes open and gravity closes again. Grease can glue it to its seat, a bent hinge can make it bind halfway, and a strip of installation tape can hold it down for years without anyone noticing. The fan then runs at full speed against a door that is barely ajar.
On an under-cabinet hood you can sometimes see the damper from below with the filters out and a flashlight; on a chimney hood it is hidden inside the cover. Look, but do not reach into the housing or unscrew anything above the filter opening. If the flap is not visible, or it is visible and clearly stuck, that is a damper and duct call.
The room, the height and the air around the hood
A hood cannot capture a plume that a breeze has already pushed sideways. Zephyr lists open windows and doors near the hood among the reasons one of its models is not venting out properly, along with a hood mounted too high above the cooking surface. Its answer is a gap between 28 and 36 inches; the higher the hood sits, the wider the plume has spread before it reaches the filters.
A tightly built house adds its own resistance. A strong exhaust fan has to pull replacement air in from somewhere, and when every window is shut and the dryer and bath fans are running too, the hood is competing for the same air. Cracking a window on the far side of the room for a minute is a fair test: if capture improves noticeably, the hood is fine and the house is short of make-up air.
- Close windows and doors next to the hood, then test again on high.
- Open a window across the room instead, and compare.
- Measure from the burners to the bottom edge of the hood and note the figure.
Source: Zephyr — Firenze ZFI-M90S use, care and installation guide — A hood not venting out properly: hood too high (keep it 28 to 36 inches above the cooking surface), wind from open windows or doors, blockage in the duct or its opening, a duct opening facing the wind, a duct too small; a thermally protected motor shuts down when too hot
The duct run and the cap outside
Zephyr's list also names blockage in the duct or its opening, an outlet facing into the prevailing wind, and a duct that is too small for the blower. From the ground you can often see the wall or roof cap. A cap flap painted shut, a nest in the louvers, or a heavy grease stain running down the siding below it all tell you where the air is being stopped.
What you should not do is climb to that cap or crawl into the attic to follow the duct. Grease-lined duct in a hot attic is exactly where a technician wants to look with the right light and footing, and a crushed flexible section or a disconnected joint above the ceiling is repair work, not a check.
Source: Zephyr — Firenze ZFI-M90S use, care and installation guide — A hood not venting out properly: hood too high (keep it 28 to 36 inches above the cooking surface), wind from open windows or doors, blockage in the duct or its opening, a duct opening facing the wind, a duct too small; a thermally protected motor shuts down when too hot
When the blower itself has slowed
If the filters are clean, the damper moves, the room is calm and the hood still barely stirs the steam, the motor is the remaining suspect. A starting capacitor that has weakened lets the motor come up to speed slowly or not at all on low, bearings that have dried out add drag, and a thermal protector that keeps tripping will stop the blower after a few minutes of running and let it restart once it has cooled.
Broan's manuals say that a motor that keeps shutting off and restarting should be serviced. That is the point where the checks end: the blower sits inside the housing or the chimney cover, it is wired to the house, and on an in-line or roof-mounted system it is not even in the kitchen.
Source: Zephyr — Firenze ZFI-M90S use, care and installation guide — A hood not venting out properly: hood too high (keep it 28 to 36 inches above the cooking surface), wind from open windows or doors, blockage in the duct or its opening, a duct opening facing the wind, a duct too small; a thermally protected motor shuts down when too hot; Broan-NuTone — BCSD1 range hood installation, use and care manual — Non-ducted recirculation filters are changed every 3 to 6 months, more often with frying and wok cooking; the thermal overload shuts the motor off when overheated, and a motor that keeps shutting off and restarting should be serviced
