Boiler
F.24
The F.24 error code on a Worcester Bosch boiler indicates that the flow temperature is rising too quickly — a fault the boiler takes seriously enough to lock out for safety. It’s not an emergency, but you should not ignore it or keep resetting without investigating.
What does this error mean?
Worcester Bosch boilers continuously monitor how fast the water temperature rises inside the heat exchanger. When the flow temperature climbs too rapidly — faster than the boiler’s programmed safe limit — it triggers F.24 and shuts down to prevent overheating. This is a protective lockout, meaning the boiler will not restart on its own; it needs a manual reset after the underlying fault is addressed.
Est. repair cost
£80–£350 for most common causes (sensor, pump, or power flush); new boiler £1,800–£2,800 installed
🔍 Likely causes
🛠️ Step-by-step fix
Check the boiler pressure gauge — it should read between 1 and 1.5 bar when cold. If it’s below 1 bar, repressurise the system using the filling loop (see your boiler manual for instructions specific to your model).
Bleed any radiators that feel cold at the top to release trapped air. Start with upstairs radiators first, then downstairs.
Check that all radiator valves — both TRVs and lockshield valves — are fully open. A partially closed valve anywhere on the circuit can restrict flow enough to trigger F.24.
Once you’ve checked pressure, bled radiators, and confirmed valves are open, press the reset button on the boiler (usually a flame symbol with a line through it). Hold for 3 seconds. If the boiler fires up and runs normally, monitor it over the next 24–48 hours.
If the fault returns after resetting, do not continue resetting. Repeated lockouts risk further damage and the root cause — pump failure, heat exchanger blockage, or a faulty sensor — requires a Gas Safe registered engineer to diagnose.
📞 When to call a professional
If the F.24 error returns after a reset, or if the pressure and radiators appear normal, call a Gas Safe registered engineer — this is a legal requirement for any internal boiler work in the UK. Tell them the exact fault code (F.24), how many times it has locked out, and the age and model of your boiler (e.g. Worcester Bosch Greenstar 30i). Do not attempt to open the boiler casing or touch internal components yourself.
⚖️ Repair or replace?
A pump replacement typically costs £200–£350 including labour; a heat exchanger descale or power flush runs £300–£500 depending on system size; an NTC sensor replacement is usually £80–£150. If your Worcester Bosch boiler is over 10–12 years old and has required repeated repairs, a new A-rated boiler (£1,800–£2,800 installed) is likely more cost-effective than continued repair. For boilers under 8 years old, repair is almost always the right call.