Boiler Fans and Air Pressure Switches
The fan clears combustion gases through the flue, and the air pressure switch confirms that fan is actually moving enough air before the boiler is allowed to ignite.
The fan clears combustion gases through the flue, and the air pressure switch confirms that fan is actually moving enough air before the boiler is allowed to ignite. Together they're one of the most common sources of lockouts that leave the boiler completely dead.
What it does
The fan draws air in and pushes flue gases out; the pressure switch senses the pressure difference the fan creates and only signals 'safe to ignite' once it's confident.
How it fails
Fan bearings wear and get noisy or seize, or the pressure switch's sensing tube blocks with condensate or dust, so it never confirms airflow even with a healthy fan.
Repair or replace
Both are typically replaced as sealed units rather than repaired internally, though a blocked sensing tube can sometimes just be cleared.
What points to a fan or pressure switch fault
- Boiler completely dead, no attempt to fire
- Loud grinding or whining fan noise before shutdown
- Lockout codes referencing air pressure or fan speed
- Fan running but boiler still failing to ignite
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Get a dead boiler diagnosed
Tell us whether the boiler attempts to start at all, and any fault code shown, and we'll check the fan and flue before assuming a bigger fault.
Questions we get asked
Can anyone change a boiler fan, or is it gas work?
Yes, the fan and pressure switch are safety interlocks on the gas appliance and any replacement is carried out under Gas Safe registration.
Can a blocked flue cause the same symptoms?
Yes, and it's checked first — a blocked or badly terminated flue can trip the pressure switch even when the switch itself is fine.
How long do boiler fans usually last?
There's no fixed lifespan we'd quote, but bearing wear tends to announce itself with noise well before complete failure.