Boiler
F.23
The F.23 error code on a Worcester Bosch boiler indicates a fault with the temperature differential between the flow and return pipes — specifically that the return temperature is rising too quickly or is higher than expected. This points to a circulation problem and should be investigated promptly to prevent boiler lockout or component damage.
What does this error mean?
Your boiler constantly monitors the temperature of water flowing out to your heating system (flow) and the cooler water returning back (return). F.23 triggers when the difference between these two readings is abnormally small, or the return temperature climbs too fast — suggesting water isn’t circulating properly around your system. In effect, the boiler is protecting itself from overheating by shutting down.
Est. repair cost
£150–£350 for pump replacement; £300–£600 for a full power flush; £80–£150 for sensor or minor repair — all including Gas Safe engineer labour
🔍 Likely causes
🛠️ Step-by-step fix
Press the reset button on your Worcester Bosch boiler (usually a flame symbol with a line through it) and wait 5–10 minutes. If the boiler fires up and runs normally, monitor it closely for recurrence.
Check all lockshield and isolation valves on your radiators and pipework are fully open — a partially closed valve is a common and easily missed cause of poor circulation.
Bleed your radiators using a radiator key to release any trapped air, starting from the radiator furthest from the boiler. This can restore proper flow if an air lock is the issue.
Check your system pressure gauge — it should read between 1 and 1.5 bar when cold. If it’s below 1 bar, repressurise via the filling loop following your boiler manual.
If the error returns after a reset and the steps above, do not attempt further DIY. The likely cause is a pump fault, blocked heat exchanger, or sensor failure — all of which require a Gas Safe registered engineer.
📞 When to call a professional
If the F.23 error returns after a reset, or your radiators are cold, unevenly heated, or the boiler keeps locking out, call a Gas Safe registered engineer immediately. Tell them you have a Worcester Bosch boiler showing error F.23 and that you suspect a circulation fault — this helps them arrive prepared with the right parts. Do not attempt to open the boiler casing, touch the pump, or interfere with the gas or sealed components yourself.
⚖️ Repair or replace?
A circulation pump replacement typically costs £150–£350 supplied and fitted by a Gas Safe engineer; a power flush to clear sludge runs £300–£600 depending on system size. If your Worcester Bosch boiler is under 10 years old, repair is almost always worthwhile. Over 10–12 years old with repeated faults, compare repair costs against a new A-rated boiler (typically £1,800–£3,000 installed) — replacing an ageing boiler often makes better long-term financial sense.