What it means
What F5 is telling you
F5 means the Vitodens fan isn't reporting the speed the control unit asked for, so the boiler won't allow ignition.
Worn bearings are the usual cause and there's normally an audible warning first — a rattle, whine or grinding noise for a few weeks before the lockout.
Blocked flues, damaged fan wiring and, less often, a control unit fault give the same code.
Before you call
Safe checks you can do yourself
- Check the flue terminal outside is clear of nests, leaves and snow
- Note any recent noise from the boiler — useful information for the engineer
- Reset once, then book a repair if it returns
Never remove the boiler casing or work on the sealed combustion chamber yourself — by law that’s Gas Safe registered engineers only. If you smell gas, call 0800 111 999 immediately.
How we fix it
What we do on the visit
- Fan speed and current draw measured before any part is quoted
- Genuine Viessmann fan fitted where replacement is needed
- Flue inspected for blockage, slope and seal integrity
- Combustion re-analysed and recorded after the repair
How urgent is it?
The boiler is locked out. Book a repair rather than repeatedly resetting.
Costs
What a repair costs
Our diagnostic visit is £99 inc VAT and it comes off the bill if we carry out the repair. We give you a fixed price before touching anything — no hourly rates, no surprises on the invoice.
We’re Gas Safe registered, Which? Trusted Trader endorsed and rated 4.9 across 206+ reviews. We carry the common Worcester Bosch and Viessmann spares on the van, so most faults are fixed on the first visit rather than booked back in.
Common questions
How much is a Viessmann fan?
Typically £260–£450 inc VAT fitted depending on the model. We give you the fixed price before starting.
Do you use genuine Viessmann parts?
Yes. Pattern parts can invalidate your manufacturer guarantee, so we fit genuine components on guaranteed appliances.
Will this affect my Viessmann guarantee?
Not if it's repaired by a registered engineer with genuine parts and your annual services are up to date — which is exactly how we work.
Book it in
Get an engineer out
Step 1 / 5
60 second boiler quote
No callbacks unless you want one
What can we help with?
Pick the option that fits — we'll tailor the next questions.
More help
Other fault codes
Worcester Bosch
EA
No flame detected — ignition lockout
Worcester Bosch
E9
Overheat — the boiler has got too hot
Worcester Bosch
A1
Pump fault or poor water circulation
Worcester Bosch
C6
Fan fault — fan speed out of range
Worcester Bosch
D5
Control board or communication fault
Viessmann
F2
Temperature limiter — overheat lockout