Signs & diagnostics
A fan that starts fine cold but cuts out once the kitchen is at full tilt is not being temperamental - it is overdrawing. Grease-loaded filters and duct force the motor to work harder and hotter, and at peak service it pushes past its overload point and trips to protect itself.
The short answer
A fan that starts fine cold but cuts out once the kitchen is at full tilt is not being temperamental - it is overdrawing. Grease-loaded filters and duct force the motor to work harder and hotter, and at peak service it pushes past its overload point and trips to protect itself.
The detail
A motor has a built-in overload that cuts power when it draws too much current or runs too hot, so it does not burn itself out. On a healthy system the motor sits comfortably below that limit. On a grease-loaded one, the margin shrinks - and it shrinks most exactly when the kitchen is working hardest.
Two things stack up under load. First, resistance: loaded filters and duct raise static pressure, so the motor has to work harder to move air, drawing more current. Second, heat: at peak service the whole space and the exhaust are hot, so the motor is already running warm before its own workload adds to it.
Grease on the impeller adds a mechanical penalty on top. An unbalanced, grease-coated wheel is harder to turn smoothly and stresses the bearings and shaft, nudging the current draw and temperature up further. Together these push the motor over its overload threshold, and it trips.
That is why it trips under load and not first thing - cold and quiet, the motor stays inside its limits; hot and busy, it does not. Repeated tripping is the overload doing its job, not a nuisance to keep resetting. Each reset runs a labouring motor closer to a real failure.
What it means for you
The instinct is to reset the trip and carry on, but a motor that keeps cutting out is telling you the load is too high for it in its current state. The durable fix is to bring the load back down - and on a greasy system that means cleaning, not winding the speed up or ignoring the trips.
Clearing the filters, duct and impeller lowers static pressure and rebalances the wheel, so the motor draws less current and runs cooler. Often that returns it comfortably inside its overload margin and the tripping stops. If it continues after a verified clean, the motor or its wiring has a fault to investigate.
Phoenix Duct Clean cleans the full system to TR19 Grease condition and flags mechanical issues found on the way, turning a fan that trips mid-service into one running inside its limits again - and giving you the record the Fire Safety Order expects.
The service behind the guide
Sibling guides
Reduced suction at the canopy · Grease spots on the ceiling · Wok and tandoor cleaning frequency
Questions
No. The overload is protecting the motor from overheating, so repeated tripping means it is genuinely overloaded. Each reset runs a straining motor hotter and closer to a real failure. The right response is to find and remove the cause, which is usually grease and resistance.
Because load and heat peak together. Cold and quiet, the motor stays inside its limits. At full service the exhaust and space are hot and the airflow demand is highest, so a grease-loaded system pushes the motor over its overload point precisely when you need it most.
It could. Grease loading is the most common and cheapest cause to rule out first. If tripping continues after a verified clean that has lowered the resistance and rebalanced the fan, the motor, capacitor or wiring likely has a fault that needs an electrician or fan engineer.
If grease and resistance are the cause, yes - lowering static pressure and rebalancing the impeller drops the current draw and running temperature back inside the overload margin. A clean also tells you definitively whether the fault was contamination or something mechanical.
It points to a heavily loaded system, and grease load and fire risk go together. A system restricted enough to trip the fan is usually carrying enough grease to matter for fire too, so the inspection and clean address both at once.
Phoenix Duct Clean · by the numbers
Stop resetting and start diagnosing. Book a clean and inspection before the motor fails.