What the F9 code means
Before a Vitodens lights, its fan must reach a set speed to push the right amount of air through the burner and clear the combustion chamber. Viessmann's installation and service instructions for the Vitodens 050-W and 100-W describe F9 as "fan speed too low during burner start". If the fan does not get up to speed in time, the boiler will not open the gas valve, and it locks out.
This is a safety check. Without enough air, the gas would not burn cleanly, so the boiler refuses to fire. On the Vitodens 100-W the fault symbol usually flashes with R, and the boiler needs a manual reset once the cause is fixed.
F9 is closely related to FA, which Viessmann describes as the fan not reaching its idle state, and FC, an electrical fan control fault. All three point towards the fan and how it is driven, so if you have seen more than one of them, mention that to the engineer.
Common causes of F9
Viessmann's service instructions point to the fan, the fan connecting cables, the fan power supply and the fan control. Here is what that usually means in practice.
Worn or failing fan
Fans have bearings and a motor that wear with use. A fan that is slow to spin up, noisy or intermittent is a common cause of F9 on older boilers.
Loose or damaged cables
A loose plug, a damaged lead or corrosion on the connector can stop the fan getting power or stop the boiler reading its speed correctly.
Fan power supply or control fault
If the control unit is not sending the right signal or supply to the fan, the fan will not reach the speed the boiler needs. In these cases the control unit may need repairing or replacing.
Power problems
A brief power cut or dip at the moment the boiler starts can occasionally trigger F9. If it happens once and never again, this may be all it was.
Age and running hours
Fan faults become more common as a boiler gets older, particularly in homes where the heating runs for long hours through the winter. A fan that has started to wear often gives some warning first, such as a louder start-up, a whine that comes and goes, or the odd fault that clears on reset. Noticing these signs early gives you time to book a repair before the boiler stops completely on a cold day.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Power blip | One-off fault after a power cut | You, by resetting |
| Worn or failing fan | Whining, rattling or slow start-up noise | Gas Safe engineer |
| Loose or damaged fan cable | Fault comes and goes | Gas Safe engineer |
| Fan control or control unit | Fan and cables test fine | Gas Safe engineer |
Safe checks you can do yourself
- Check the fused spur and power supply to the boiler, and note whether there has been a power cut.
- Check your thermostat and timer settings so the boiler is actually calling for heat.
- Reset the boiler as your manual describes. On the Vitodens 100-W, press and hold R for about two seconds.
- Listen as the boiler starts up. A fan that sounds rough, whines or struggles is useful information for the engineer.
- If the code returns after two or three resets, stop and book an engineer.
Do not remove the casing or touch the fan, its cables or the control unit. The fan is part of the combustion system, and faults here can affect how safely the boiler burns gas. Keep a working carbon monoxide alarm near the boiler. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If F9 comes back after two or three resets, call a Gas Safe registered engineer. Every likely cause is inside the boiler, so this is not a homeowner repair.
The engineer will check the fan, its connecting cables and power supply, and the fan control from the control unit, then replace whichever part has failed. After any fan work they should check combustion with a flue gas analyser to make sure the boiler is burning correctly.
When you book, give the code, your exact model name and any other codes you have seen, such as FA or FC. Mention whether the fault started after a power cut or any electrical work in the house.
Typical repair costs in the UK
If a single reset clears F9 and it does not return, there is no cost. If a part is needed, these ranges are typical. Prices vary by region, engineer and model.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Repairing a fan cable or connector | Typically £80 to £150 |
| Fan replacement | Typically £200 to £400 fitted |
| Control unit replacement | Typically £250 to £450 fitted |
How to stop F9 coming back
Fan wear cannot be avoided completely, but a yearly service helps. An engineer can listen for early signs of bearing wear, check connections and test combustion before the fan fails outright. Our boiler service cost guide explains what a service usually includes. Keep the flue terminal outside clear of plants and debris. If the boiler shows a no flame fault instead, see our page on Viessmann F4, and for a control unit fault see Viessmann FD.