Commercial Kitchen Hood Not Pulling Smoke? Start With the Fan

A kitchen hood that has stopped capturing smoke is rarely a mystery once someone gets on the roof, and the order of investigation matters because the cheapest causes are the most common. Working through it in order beats assuming the commercial kitchen ventilation system needs replacing. In a Twin Ports kitchen there is one seasonal twist worth knowing up front: the make-up air unit.

Start at the fan: is the wheel turning at full speed?

The exhaust fan on a commercial kitchen hood is usually a belt-driven upblast unit on the roof, and the first question is not whether it runs but whether it turns at the speed it should. A belt that has stretched, glazed or lost tension slips on the sheave, and the wheel spins at a fraction of design speed. From the kitchen the fan sounds like it is running, and technically it is.

The checks are belt condition and tension, sheave wear and alignment, bearings, and motor amp draw against the nameplate. Belts live outdoors through a Twin Ports winter, and a set that looked fine in September can be glazed by February. A broken belt is the obvious failure; a slipping belt quietly degrades a kitchen for weeks.

The fan that is running backwards

A centrifugal exhaust fan wired for the wrong rotation still moves air in the correct direction, just far less of it, which is why the fault survives undetected. It appears after any electrical work that disturbed the three-phase supply — a service change, a panel upgrade, a motor replacement — and nobody checks rotation because the fan visibly works.

The tell is a kitchen that was fine one week and marginal the next with no change in equipment or menu. Checking rotation against the direction arrow on the fan housing takes a minute.

A grease-loaded fan wheel moves less air every week

Grease accumulating on a fan wheel reduces airflow two ways: it adds weight the motor must turn, and it changes the blade shape so they no longer move air efficiently. Unlike a belt failure it degrades slowly, so the kitchen adapts and nobody notices until the day the hood visibly fails to capture.

Uneven loading also produces vibration, and vibration kills bearings. How often a wheel needs cleaning depends on what the kitchen cooks: a fryer-heavy or char-broiler line loads one far faster than a bakery. A seasonal kitchen in a small community like Port Wing loads differently than a place open year-round, so the interval should follow the cooking, not the calendar.

Working through a hood that will not capture

Symptom	Most likely cause	Where to look
Performance dropped suddenly, no other change	Broken belt, or fan rotation reversed	Roof: belt, and rotation against the housing arrow
Performance faded over months	Grease-loaded wheel, loaded filters	Roof: wheel condition; kitchen: filters
Doors hard to open, drafts at entry	Building is negative: no make-up air	Make-up air unit status and controls
Smoke rolls out one end of the hood only	Capture area, appliance placement, filter gaps	Hood geometry and filter bank
Fan loud or vibrating	Unbalanced wheel, worn bearings	Roof: wheel, bearings, mounting
Worse in winter than summer	Make-up air unit shut off or locked out	Make-up air unit and its schedule

Filters, and the access panels left open after cleaning

Grease filters in the hood are the first restriction in the system, and a loaded bank of baffle filters costs real airflow. Filters also have to be seated in the track, with no gaps and none missing, and oriented the way the manufacturer intends. A gap in the filter bank is a short circuit: the hood pulls air through the easy opening instead of across the cooking surface.

The one that catches people out is duct access panels. Grease duct has cleanout panels along its run, and after a cleaning visit a panel that is not properly reinstalled and sealed lets the fan draw air from inside the chase rather than from the hood. The kitchen loses capture even though the fan is healthy and the filters are clean. Check after every cleaning, not once a year.

The big one: a kitchen with no make-up air

A hood cannot exhaust more air than the building can supply, and that is the most misunderstood fact in commercial kitchen ventilation. Every cubic foot the fan removes has to be replaced from somewhere. Where a properly sized make-up air unit does that, the system works as designed. Where it does not, the building goes negative and actual hood airflow drops to whatever leaks in around doors and windows.

In this climate the failure is usually deliberate rather than mechanical. A make-up air unit delivers a large volume of outdoor air that has to be heated, and at a −17°F to −21°F design temperature it is often the largest heating load in the building. When it is undersized, poorly controlled, or blowing cold air into the kitchen, staff turn it off, and then the hood stops working and nobody connects the two events. Rural kitchens on propane, in places like Dairyland and Cotton, feel that fuel cost and have the strongest temptation to shut it down.

• Entry doors that are hard to pull open, or that whistle, while the hood is running.
• Cold drafts at doorways and around windows whenever the kitchen is busy.
• Pilot lights going out, or gas appliances struggling to stay lit.
• Smoke that rolls out of the hood only during service, when every exhaust fan in the building is running.
• A make-up air unit that staff have learned to leave off because it blows cold.

The physics and the fixes are worked through in our guide to restaurant make-up air and negative pressure, and where a building needs supply air added, make-up air units are the equipment that does it. Sizing and control separate a unit that gets used from one that gets switched off.

Hood geometry and where the appliances actually sit

A hood only captures what falls within its capture area, and kitchens rearrange themselves over time. An appliance moved a foot to the left, equipment added at the end of the line, or a char-broiler installed where a range used to be all change what the hood is asked to do. Overhang past the front edge of the cooking surface, hood height above the appliances, and a high-effluent appliance sitting under a section designed for something gentler are all real causes of a hood that will not capture.

If a capture problem appeared at the same time as new equipment, the answer is more likely in the layout than on the roof.

Coordinating with hood cleaning and fire suppression

A commercial kitchen exhaust system has several vendors touching it. Hood cleaning handles grease removal from hood, duct and fan. The fire suppression contractor owns the suppression system, including nozzle placement over the appliances. Mechanical service covers the fan, drive, make-up air unit and controls.

The overlaps are where problems live. Moving an appliance changes what the suppression nozzles are aimed at. A cleaning visit can leave an access panel unsealed or a filter out of its track. Electrical work can reverse a fan. When a capture complaint follows any of that work, say so when you call. Where the air balance problem reaches beyond the kitchen, the fix belongs with the commercial ventilation system as a whole.

What kitchen staff can check, and where it stops

There is a genuinely useful in-house list, and all of it is inside the kitchen. Nothing on it involves the roof, the fan, or the fire suppression system.

1. Confirm the exhaust fan and the make-up air unit are both switched on and that nobody has turned the make-up air off.
2. Check that every filter is present, seated in its track, and not loaded with grease.
3. Look for gaps in the filter bank and for filters installed the wrong way around.
4. Note whether the door is hard to open while the hood runs, which points at make-up air rather than the fan.
5. Note when the problem started and what changed then: cleaning, electrical work, new equipment, a remodel.

Past that, the work is on the roof and in the electrical system, and it belongs with a technician: belts, bearings, rotation, wheel condition, motor diagnosis, make-up air controls and air balance. If the kitchen fills with smoke during service, treat it as urgent. A commercial kitchen ventilation visit that begins with airflow measurement rather than a parts guess is what gets a kitchen working as designed.