Boiler
F.25
The F.25 error code on a Worcester Bosch boiler indicates a flue gas temperature fault — specifically that the boiler is detecting dangerously high temperatures in the flue. This is a safety lockout and should not be ignored.
What does this error mean?
Your Worcester Bosch boiler has a flue gas thermistor (temperature sensor) that monitors the heat of gases leaving through the flue. When it detects temperatures exceeding the safe threshold — typically caused by a combustion or heat exchange problem — the boiler shuts down and displays F.25. This is the boiler protecting itself and your home from overheating or a potential flue fire.
Est. repair cost
£150–£300 for thermistor replacement; £200–£400 for heat exchanger service; up to £800 for full heat exchanger replacement
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame and cross symbol) for 3–5 seconds. If the boiler fires up and runs normally, monitor it over the next hour. A one-off lockout after a cold snap or windy day can be benign.
Step 2 — Check the external flue terminal: Go outside and visually inspect the flue outlet (usually a white pipe on an external wall or roof). Look for obvious obstructions such as a bird nest, ice, leaves, or debris. Do not attempt to dismantle the flue — just check what you can see safely from ground level.
Step 3 — Check the boiler’s surrounding ventilation: If the boiler is in a cupboard or utility room, ensure vents are unblocked and the door isn’t sealed shut. Overheating of the boiler casing can contribute to flue temperature faults.
Step 4 — Do not attempt further DIY: F.25 involves the combustion system, heat exchanger, gas valve, and flue — all of which must only be inspected and repaired by a Gas Safe registered engineer. Do not open the boiler casing or tamper with the flue connections.
Step 5 — Call a Gas Safe engineer: If the boiler locks out again after a reset, or if it won’t reset at all, stop using it and call a qualified engineer. Continuing to reset a boiler repeatedly without diagnosing the root cause is unsafe.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler re-locks after a single reset attempt, or if you notice unusual smells, soot marks around the boiler, or the flue terminal appears blocked. Tell the engineer: ‘My Worcester Bosch is showing fault code F.25 — a flue gas high-temperature lockout.’ This helps them arrive prepared with the right diagnostic tools and likely replacement parts such as a flue thermistor or heat exchanger flush kit.
⚖️ Repair or replace?
A flue thermistor replacement typically costs £150–£300 including labour in the UK. A heat exchanger clean or descale runs £200–£400; full heat exchanger replacement can reach £500–£800. If your boiler is over 10 years old and facing a repair above £400, replacement with a new A-rated Worcester Bosch or equivalent (£1,800–£2,800 installed) is worth serious consideration — newer models are significantly more efficient and come with manufacturer warranties of up to 12 years.