Boiler
F11
The F11 error code on a Viessmann boiler indicates a flame failure — the boiler is attempting to ignite but cannot establish or maintain a stable flame. This fault will lock out your boiler, leaving you without heating or hot water until it’s resolved.
What does this error mean?
F11 signals that the boiler’s control board detected either a failed ignition attempt or a flame that lit briefly but then cut out. The boiler performs a set number of ignition retries, and when it can’t confirm a stable flame via the ionisation sensor, it enters lockout mode as a safety measure. This is the boiler protecting itself — and your home — from an uncontrolled gas release.
Est. repair cost
£150–£300 for Gas Safe engineer call-out, diagnosis, and parts (ignition electrode or ionisation sensor). Gas valve replacement: £250–£450. New boiler if replacement needed: £1,800–£2,500 installed.
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances in your home (hob, fire) are working normally. If they’re not, call your gas supplier — this is not a boiler fault.
Inspect the condensate pipe (relevant in cold weather): locate the white plastic pipe that exits your boiler and drains outside. If it’s frozen solid, thaw it carefully by pouring warm (not boiling) water over the external section, then reset the boiler.
Reset the boiler: press and hold the reset button (usually marked with a flame/arrow icon) for 3–5 seconds. Refer to your Viessmann manual for the exact location on your model. Allow 10 minutes for the boiler to attempt a restart.
If the F11 returns after one or two resets, stop resetting — repeated failed attempts can stress components. The fault requires a Gas Safe registered engineer; do not attempt to access the gas valve, ignition electrodes, or ionisation sensor yourself.
Before calling an engineer, note down how many times the boiler has locked out, whether the fault appeared suddenly or after a cold snap, and your boiler model number (found on the data plate inside the front panel) — this helps the engineer diagnose faster.
📞 When to call a professional
Call a Gas Safe registered engineer if the F11 error returns after a single reset attempt, or immediately if you smell gas at any point. Tell the engineer it’s a flame failure lockout (F11) and describe whether it happened suddenly or during cold weather — this helps them bring the right parts. Do not keep resetting a boiler in persistent lockout; it will not fix an underlying component fault and may mask a more serious issue.
⚖️ Repair or replace?
For a boiler under 10 years old, repair is almost always the right call — replacing an ignition electrode or ionisation sensor typically costs £150–£300 including labour, which is far cheaper than a new boiler. If your Viessmann is over 12–15 years old and this is one of several recurring faults, replacement becomes worth considering; a new Viessmann Vitodens A-rated combi starts around £1,800–£2,500 fully installed. For a mid-age boiler with no prior history of faults, repair first.