Boiler
F15
The F15 error code on a Viessmann boiler indicates a fault with the burner or ignition system — specifically a flame supervision or burner control unit issue. It’s a lockout fault, meaning your boiler has shut itself down as a safety measure and won’t restart without intervention.
What does this error mean?
F15 signals that the Viessmann boiler’s burner control board (BCU) has detected a problem during the ignition or burner monitoring sequence — most commonly that the burner failed to ignite or that the flame signal was lost during operation. The boiler locks out to prevent unburned gas from accumulating, which is a built-in safety response. On Viessmann Vitodens and Vitopend models, this fault is directly tied to the gas valve, ignition electrodes, or the ionisation sensor that confirms a flame is present.
Est. repair cost
£150–£300 for electrodes/ionisation rod; £300–£500 for gas valve; £400–£600 for PCB/BCU — all including Gas Safe labour
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Locate the reset button on your Viessmann boiler (usually a flame symbol button, held for 3 seconds). This clears the lockout and attempts a fresh ignition cycle. Watch whether it fires up or locks out again.
Step 2 — Check your gas supply: Confirm other gas appliances in your home (hob, gas fire) are working normally. If they’re not, contact your gas supplier — the fault may be with the supply, not the boiler.
Step 3 — Check the condensate pipe (winter months): A frozen condensate pipe can cause lockouts that display various fault codes. Locate the plastic white pipe running outside and gently thaw it with warm (not boiling) water, then reset the boiler.
Step 4 — Do not attempt further DIY: All remaining causes — electrodes, gas valve, BCU, or wiring — require a Gas Safe registered engineer. Do not open the boiler casing or attempt to inspect internal components yourself. This is a legal requirement for gas appliances in the UK.
Step 5 — Book a Gas Safe engineer: If the fault returns after a reset, call a registered engineer and tell them your boiler model (e.g. Vitodens 100-W) and the exact fault code F15. This helps them arrive with the likely parts.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler locks out again after a single reset, or if you smell gas at any point — in which case, do not reset the boiler; call the National Gas Emergency Service on 0800 111 999 instead. Tell the engineer it’s a Viessmann F15 fault and your boiler model; a competent Viessmann-experienced engineer will check the ignition electrodes, ionisation rod, gas valve, and BCU as the primary suspects.
⚖️ Repair or replace?
Replacing ignition electrodes or an ionisation rod on a Viessmann boiler typically costs £150–£300 including parts and labour; a gas valve replacement runs £300–£500, and a new PCB/BCU can reach £400–£600 fitted. If your boiler is over 10–12 years old and facing a BCU or gas valve failure, a new Viessmann Vitodens 050-W or 100-W (from around £1,800–£2,500 installed) is likely better value long-term than investing in major repairs on an ageing unit.