Boiler
F.28
The F.28 error code on a Worcester Bosch boiler means the appliance has failed to ignite and locked itself out as a safety precaution. It’s one of the most common boiler faults in the UK and should be treated seriously — without heat or hot water, and with a potential gas supply issue at play, prompt action is essential.
What does this error mean?
Worcester Bosch uses F.28 to indicate an ignition lockout — the boiler attempted to light but failed to detect a flame within the expected timeframe. The boiler’s control board then shuts everything down to prevent unburned gas from accumulating. This is a built-in safety feature, not a fault with the safety system itself, but it does mean your boiler will not restart until the underlying cause is resolved.
Est. repair cost
£80–£200 for a Gas Safe engineer diagnosis and repair; ignition electrodes typically £100–£150 all-in, gas valve replacement £150–£300+
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: try lighting a gas hob or other gas appliance. If nothing lights, contact your gas network operator (call 0800 111 999 for National Gas in Great Britain) — do not attempt to reset the boiler until supply is confirmed.
If you have a prepayment gas meter, check your credit balance and top up if needed, then wait a few minutes for the supply to restore.
Press the reset button on your Worcester Bosch boiler — typically a flame symbol button held for 3 seconds. Refer to your model’s manual for the exact location. The boiler will attempt to re-ignite.
If the boiler fires up, monitor it for 10–15 minutes. If F.28 returns within a short time, do not keep resetting — repeated failed ignitions can allow unburned gas to build up.
Check that the boiler’s condensate pipe (the white plastic pipe that exits externally) is not frozen or blocked, particularly in cold weather — a blocked condensate can sometimes trigger lockout codes alongside F.28.
If the error persists after one reset and confirmed gas supply, stop here. This is a gas appliance fault that requires a Gas Safe registered engineer — do not attempt to inspect burners, electrodes, gas valves, or any internal components yourself.
📞 When to call a professional
If F.28 returns after a single reset, or if you have confirmed gas supply but the boiler still won’t ignite, call a Gas Safe registered engineer immediately. Tell them the exact error code (F.28), your boiler model (e.g. Greenstar 8000, Highflow 400), and how many reset attempts you’ve made. Do not continue resetting the boiler repeatedly, as this is unsafe and may make diagnosis harder.
⚖️ Repair or replace?
A Gas Safe engineer call-out to diagnose and fix an F.28 fault typically costs £80–£200 depending on the cause — ignition electrode replacement sits at the cheaper end, while a gas valve replacement can push costs higher. If your Worcester Bosch boiler is under 10 years old and otherwise reliable, repair is almost always worth it; Worcester Bosch boilers are well-regarded and parts are widely available. If the boiler is over 12–15 years old and this is not its first major fault, get a replacement quote — a new Worcester Bosch Greenstar starts at around £1,500 fully installed.