What the boiler fan does
Every modern gas boiler has a fan that pushes air into the burner and drives the waste gases out through the flue. On most condensing boilers it also sets the right mix of gas and air, so the boiler controls the fan speed closely and checks it with a speed signal. If the speed is wrong, too slow or too fast, or the signal is missing, the boiler will not light or will shut down. This protects you, because a boiler without a working fan cannot clear its flue gases safely.
On older non-condensing boilers the fan often worked with a separate air pressure switch, which confirmed the fan was moving enough air before the boiler lit. On modern condensing boilers the fan usually reports its own speed back to the control board through a small sensor built into the fan. That is why a fan code can be caused by the fan motor, the speed sensor, the wiring between them or the board reading the signal.
Symptoms of a failing fan
- A whining, grinding or rattling noise from the boiler, especially at start-up.
- The boiler takes longer than usual to light, or tries several times before it fires.
- Lockouts that come and go, often worse from cold.
- A fan fault code on the display.
- The boiler hums or clicks but never lights, with no flame on the display.
Worn bearings are the most common cause of noise, and the fault often gets worse gradually. A fan that fails completely may be silent. Noise alone does not always mean the fan, though. Kettling or banging usually comes from sludge or scale in the heat exchanger, and a gurgle in cold weather can point to a blocked condensate pipe, so describe the sound as clearly as you can when you book.
Fault codes that point to the fan
| Brand and code | What it usually means |
|---|---|
| Ideal F3 | Fan fault, the boiler cannot confirm the fan speed |
| Baxi E160 | Fan or fan wiring fault |
| Worcester Bosch C6 215 | Fan speed too high |
| Vokera A03 | Fan fault, no correct speed signal on most models |
| Viessmann F9 | Fan speed too low at start-up |
Meanings can vary between models, so check the page for your code. In every case the code tells the engineer where to look. It does not prove the fan itself has failed.
Typical fan replacement cost in the UK
| Repair | Typical cost, fitted |
|---|---|
| Replacement fan | Typically £200 to £400 |
| Repairing fan wiring or a loose connector | Typically £80 to £150 |
| Air pressure switch (older non-condensing boilers) | Typically £120 to £220 |
| Fan and PCB together, if the fan has damaged the board | Typically £450 to £850 |
What pushes the price up or down
- The fan part: genuine fans for premium brands such as Worcester Bosch, Vaillant and Viessmann often cost more than those for budget models.
- Access: on most wall-hung combis the fan is near the front and fairly quick to change, but some models need other parts moved first.
- Seals and gaskets: the engineer should fit new seals where the fan joins the burner and flue, which may add a little to the part cost.
- Region and timing: London rates and out-of-hours call-outs cost more.
- Warranty: if the boiler is still under warranty and has been serviced every year, the fan may be replaced free.
What happens during a fan replacement
The engineer isolates the power and gas, removes the casing and the fan housing, and disconnects the fan from the burner and flue connections. The new fan is fitted with new seals and gaskets, the wiring is reconnected and the casing goes back on. On most wall-hung boilers the whole job takes around one to two hours.
After fitting a new fan on a condensing boiler, the engineer should run a combustion check with a flue gas analyser to make sure the gas and air mix is right. They should also check the case seal is intact, because a poorly sealed casing can let flue gases escape into the room.
Faults that look like a failed fan
A good engineer checks these before ordering a fan:
- Wiring and connectors: a loose plug, corroded pin or damaged cable can stop the speed signal reaching the board.
- Water damage: a leak dripping onto the fan or its connector often causes fan codes. See our guide to a boiler leaking water.
- Blocked flue or air intake: a blocked terminal, bird nest or damaged flue can change the fan load and trigger speed faults.
- Blocked condensate: on some boilers a backed-up condensate trap affects the fan and flue. In freezing weather, check for a frozen condensate pipe.
- The PCB: the board powers and reads the fan, so a board fault can look like a fan fault.
What you can safely check
- Check the power: the fused spur, the plug if there is one and your fuse box.
- In freezing weather, check the condensate pipe outside and thaw it with warm, not boiling, water or a hot water bottle if it is frozen.
- Check the thermostat and timer are calling for heat.
- Reset the boiler as the manual describes, no more than two or three times.
Do not remove the casing or try to free, clean or replace the fan. The fan controls how flue gases leave the boiler, and a fault here can let carbon monoxide build up. Leave all work inside the boiler to a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999, and if your carbon monoxide alarm sounds, open windows, leave the property and get help.
Is it worth replacing the fan?
A fan is a mid-priced repair and is usually worth doing on a boiler under about ten years old, particularly if it has been reliable. On an older boiler with other faults, or if the fan has also damaged the board, compare the total with a replacement. Our guides to repairing or replacing a boiler and boiler repair costs set out the typical numbers.
A noisy fan rarely fixes itself. If you hear a new whine or rattle from the boiler, book an engineer before the winter rush rather than waiting for it to fail completely on a cold morning, when call-outs are harder to get and often cost more.