Signs & diagnostics
The roof fan sits at the very end of the extract run, so grease reaches it last and settles on it heaviest. When it starts to labour, the cause is often the deposit on its impeller - not simply a worn-out motor.
The short answer
The extract fan usually sits on the roof at the very end of the run, which makes it the last component the grease-laden air reaches and the one it loads most heavily. When that fan starts to struggle - moving less air, running hot, vibrating or drawing more effort than it used to - the cause is frequently the grease built up on its impeller and inside its housing, not simply an old motor reaching the end of its life. The grease connection is the reason a fan that was fine a few months ago can begin to labour: it is carrying a load it was never meant to.
Why the roof fan loads last and heaviest
In a grease extract system, every bit of vapour that is not caught by the filters travels the length of the duct and out through the fan. That puts the fan at the end of a funnel - whatever gets past the canopy and coats the ductwork also reaches the impeller, and because the fan is the final point, it collects the residue that has been carried all the way along. Over time the blades load up, the balance is thrown off, and the housing fills with the same greasy film that lines the duct.
A loaded impeller is heavier and out of balance, so it vibrates and demands more effort to spin. At the same time, grease along the ductwork below narrows the free area, so the fan is pulling against a restriction as well as carrying extra weight. The two effects combine: the fan works harder, runs hotter, and sounds and feels different from the smooth machine it was when clean.
Reading the struggle as a grease signal
Because the fan sits at the end of everything, a struggling fan is a signal about the whole system, not just itself. If grease has reached the impeller in enough quantity to make the fan labour, the ductwork feeding it is carrying a substantial load too - the fan is simply where that load becomes measurable, in effort, heat and vibration. So a fan that has started to struggle is worth reading as evidence that the entire run is overdue attention.
The risk of misreading it is that a labouring fan gets treated as a mechanical fault - a motor or bearing to replace - when the real problem is grease. Replacing a fan that is struggling because it is fouled solves nothing if the duct feeding it is left loaded; the new fan simply loads up the same way. The struggle is a grease signal first, and a mechanical one only once the grease has been ruled out.
What to do about it
A struggling fan is a prompt to clean, not automatically to replace. Clearing the impeller and housing rebalances the fan and lifts the extra load off it, while cleaning the ductwork below restores the free area so the fan is no longer pulling against a restriction. Done together, the fan runs balanced and quiet again on a system that breathes freely - and any genuine mechanical wear becomes clear once the grease is out of the picture.
Questions
You often cannot tell from the kitchen - a fouled fan and a worn motor can feel similar. The reliable approach is to clean the impeller, housing and duct first; if the struggle clears, it was grease. If the fan still labours once it is clean and balanced, that points to genuine mechanical wear.
Because it sits at the end of the run, everything the filters and duct do not catch is carried the full length and passes through the fan. It collects the residue accumulated along the whole system, so the impeller and housing tend to be the heaviest-loaded part.
Indirectly, yes. A fouled fan means grease has reached the end of the run, so the whole system is carrying a combustible load, and a fan running hot on a heavy impeller adds its own risk. It also drives down airflow, so capture and clearance suffer throughout the kitchen.
Not before ruling out grease. A fan that struggles because it is fouled will not be fixed by a replacement if the duct feeding it is left loaded - the new fan loads up the same way. Clean first; replace only if a genuine mechanical fault remains afterwards.
Working at height, the fan is isolated and the impeller, housing and blades are cleared of grease so it moves air freely and runs balanced. It is done with proper access and working-at-height provision as part of cleaning the run, not as an optional extra.
Phoenix Duct Clean · by the numbers
A struggling fan is often a grease problem, not just an ageing one. We reach and clean the roof fan and the run below it so it moves air freely and runs balanced again.