Phoenix Journal · Ductwork
A roof extract fan that has started to shake is telling you something specific - usually either grease throwing the impeller out of balance, or a mechanical part wearing out. Knowing which one you are dealing with decides whether a clean or a repair is the right call.
Rooftop extraction
A commercial kitchen extract fan that has started to vibrate is rarely doing it at random - it is almost always one of two problems, and telling them apart saves you money.
When a roof fan runs true, you barely notice it. When it starts to shake, the whole rooftop unit can buzz, fixings work loose, and the noise travels down the ductwork into the kitchen below. Vibration is not just a nuisance - it loosens bolts, stresses joints, and can drive a bearing or shaft to overheat, which on a grease-laden system is a genuine fire concern.
Broadly, a vibrating extract fan sits in one of two camps. Either the impeller has gone out of balance - nearly always because grease has built up unevenly on the blades - or a mechanical part has worn, such as the bearings, the drive belt, or the rubber anti-vibration mounts that isolate the fan from its frame. Both produce shaking, but they call for very different responses. One is a cleaning and balancing job; the other is a repair or replacement.
The honest way to settle it is to take a reading. Under ISO 14694, the international standard for industrial fan vibration, a rigidly mounted fan of this size should sit within the FV-3 category - a broadband limit of 4.5 mm/s RMS measured on the bearing housings. An imbalance from grease typically shows up as a strong vibration at once per revolution (1× running speed) on a spectrum reading, whereas bearing damage tends to show at higher frequencies. Below we weigh up the two courses of action so you can decide which your fan actually needs.
The first and most common answer is that the fan is simply dirty. Grease carried up the duct settles unevenly on the impeller, and even a small amount in the wrong place upsets the balance and sets the whole assembly shaking. Bringing the system back to the TR19® Grease standard - a mean grease film no greater than 200 microns between cleans - and rebalancing the impeller often puts things right without a single new part.
Pros
Cons
If the fan is clean and still shakes, the fault is mechanical. Bearings wear, belts stretch and slip, and the rubber anti-vibration isolators that cushion the fan from its framing dry out and break down with age - a very common cause of creeping vibration on rooftop units. Left alone, a failing bearing generates friction heat on the shaft and can, in the worst case, become an ignition source. This route means stripping the fault out and putting sound parts back in.
Pros
Cons
In practice the sensible order is to rule out grease first, because it is cheaper, faster, and the likeliest culprit - then move to mechanical repair only if a clean, rebalanced fan still reads over limit. Trying to diagnose it by ear alone tends to send people straight to expensive parts when a degrease would have done.
Three figures tell you whether your fan is a cleaning job or a repair job · keep them to hand before you call anyone out.
Take a reading first, and check the fan against the grease limit, and you will usually know within minutes which camp your problem sits in. A reading inside 4.5 mm/s after a clean means you are done; one still over limit points at the bearings, belt, or mounts. Either way, the guesswork disappears.
Questions
For a short period it may be, but you should not leave it. Vibration loosens bolts and fixings, stresses ductwork joints, and can drive a bearing or shaft to overheat - which on a grease-laden system is a real fire risk. Get it checked and cleaned or repaired promptly rather than waiting for it to fail.
Often, yes. Uneven grease build-up on the impeller is the single most common cause of extract fan vibration, so a TR19 Grease degrease and impeller rebalance frequently fixes it without new parts. If the fan still reads above the ISO 14694 FV-3 limit of 4.5 mm/s once it is clean, the fault is mechanical - typically worn bearings, a stretched belt, or perished anti-vibration mounts.
A vibration reading tells you. Grease imbalance usually shows as strong vibration at once per revolution, or 1x running speed, whereas bearing defects appear at higher frequencies on a spectrum. The practical approach is to degrease and rebalance first, since that is cheaper and the likeliest cause, then take another reading - if it is now within 4.5 mm/s you are done, and if not, you move on to the mechanical parts.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.