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Boiler Combustion fan: What It Does and Signs It's Failing
The fan sits on top of or beside the burner inside the sealed combustion box. It draws in fresh air, pulls in the gas and pushes the burned gases out through the flue. A modern condensing boiler won't fire until the board can see the fan spinning at the speed it asked for.
Who should replace it
Technician job
Related codes
2 error codes on this site point to this part.
How it works
A speed-controlled motor spins an impeller, and a speed signal goes back to the PCB. The faster it runs, the more air and gas reach the burner, which is how the boiler turns its output up and down. If the measured speed doesn't match the target, the board stops the boiler.
Signs it is failing
- Humming, grinding or rattling from inside the boiler when it starts
- Fan runs up but the boiler never tries to light
- Fan speed or fan fault code on the display
- Silence when the boiler should be starting, then a lockout
Fan-speed and fan fault codes point here, along with the wiring harness and the PCB. A blocked flue or air intake can upset the fan's readings too.
Can you replace it yourself?
The fan is part of the sealed combustion system and must be replaced by a Gas Safe engineer (an RGI in Ireland), who will also check the seals and combustion afterward. Ideal suggests a restart for its fan fault, which is fine to try once, but leave the casing on.
Buying the right part
Fans are matched to the boiler's output and burner, so the engineer will use the part listed for your model. A new gasket goes on with it.
Error codes linked to the combustion fan
Last reviewed 2026-10-21.