Error F.24

Safety first

- Do not open boiler panels or touch internal electrical components. That work must be done by a qualified engineer. Do not attempt gas or PCB repairs yourself. If you smell gas, leave the property and call the emergency gas number immediately.

- Before doing any external checks, switch the boiler off at the programmer or the boiler on/off button. If you need to operate valves or the filling loop, turn the boiler off and follow the manual. If you are not confident, call a Gas Safe engineer.

Initial homeowner checks you can safely do

1. Note when the fault appeared. If it happened immediately after bleeding radiators or topping up the system, an airlock or trapped air is likely.

2. Check the boiler display and try a single reset following the instructions in your boiler manual. If the boiler immediately returns to F.24, stop and proceed with the checks below — do not keep repeatedly resetting.

3. Check system pressure on the boiler gauge. Normal operating pressure is roughly 1 to 1.5 bar when cold. If pressure is very low (well below 1 bar) the system may not circulate properly. If you know how to and are comfortable doing so, you can re-pressurise using the boiler filling loop following the manual; otherwise call an engineer.

4. Check the isolation valves under the boiler and on the pump. These are usually small slotted or lever valves beneath the boiler. When the slot or lever is parallel to the pipe the valve is open; when across the pipe it is closed. If any are closed, open them. Only do this with the boiler off and if you are confident.

5. Bleed radiators to remove trapped air. Start at the highest radiator and work downwards. Use a radiator key, open the bleed screw slowly until air hisses out, then close when water appears. After bleeding, check and restore system pressure if needed and reset the boiler.

6. Listen for pump noise and look for obvious signs of a problem. A working pump normally hums quietly. If the pump casing is extremely hot, vibrating heavily, or silent when heating should be running, note that but do not attempt to dismantle the pump or remove covers.

More specific diagnostic steps and when to stop

- If opening isolation valves, re-pressurising, and bleeding radiators clears the fault and the boiler runs normally, monitor for recurrence. If the fault returns, stop further DIY checks and call an engineer.

- If the pump seems to be failing (no rotation, hot or unusually noisy) or if you suspect sludge/debris blocking the pump or heat exchanger, these are not DIY repairs. A competent engineer can inspect, clean, replace the pump, or flush the system as needed.

- If you suspect electrical or PCB communication faults (intermittent faults, inconsistent behavior, or error persists after circulation checks), do not attempt electrical troubleshooting. These require a Gas Safe engineer with the correct diagnostic tools.

When to call a professional

- Call a Gas Safe registered heating engineer if: your checks did not clear the F.24 lockout; the pump is hot, noisy, or not running; you suspect a blocked heat exchanger or heavy sludge; you need pump or PCB replacement; you are not confident performing the simple checks above; or the boiler repeatedly locks out.

- Explain to the engineer that F.24 indicates a rapid rise in flow temperature due to poor circulation so they can prioritise checking pump operation, isolation valves, airlocks, system pressure, heat exchanger condition, pump speed setting and electrical communication to the PCB. They may perform a system flush, fit or clean a magnetic filter, replace the pump or replace faulty components.

Final notes

- Treat F.24 as an urgent fault but not typically an immediate life-safety emergency. Avoid repeatedly resetting the boiler as this can cause additional wear. Use the basic checks listed above only if you are comfortable, safe, and have turned the boiler off first. For any pump, heat exchanger, gas or electrical work, always use a Gas Safe registered engineer.