Overview
The Vaillant Turbomax F.24 fault means the boiler has detected poor or no water circulation. Essentially the boiler is not getting enough flow through its primary circuit, so it shuts down as a safety measure. Common causes are low system pressure, closed isolation/gate valves under the boiler, air trapped in the system (airlock), a seized or under-performing pump, or a blockage (sludge, limescale) in the pump, pipes or heat exchanger. Less commonly it can be an electrical fault between the pump and the PCB or a defective PCB reporting a circulation fault. Severity ranges from minor (closed valve or low pressure you can fix yourself) to serious (seized pump, blocked heat exchanger, or PCB failure). Because the boiler locks out to protect itself, you will lose central heating/hot water until the fault is cleared. Some basic checks and simple actions are suitable for a competent homeowner, but faults involving the pump internals, gas parts, electrical wiring or the heat exchanger should be diagnosed and repaired by a Gas Safe registered engineer.
Possible Cause: Low water / Pump Failure / Blockage
Troubleshooting Steps
Safety first: if you are not comfortable working around boilers, stop and call a Gas Safe engineer. Never tamper with gas connections, burner components or wiring unless you are qualified. Turn the boiler off at the control panel when doing visual checks; isolate electrical supply at the consumer unit if you must access internal components. Be careful touching pipes – they can be hot.
Initial quick checks homeowners can do:
1) Note the display and any additional messages, then try a single reset (use the boiler reset button or follow the user manual). If F.24 returns immediately or within a few minutes move to the checks below.
2) Check the system pressure on the boiler gauge. Target pressure for most systems is about 1.0–1.5 bar when cold. If pressure is well below 1 bar, the boiler may show F.24.
3) Look under the boiler at the isolation/gate valves (inlet and outlet pipes). Make sure both are open (valve handles or flats are parallel to the pipe). These can be accidentally closed after maintenance.
4) Check radiators and towel rails. If the top section of radiators is cold while bottoms are warm, there may be air trapped. If many radiators are cold you may have circulation failure.
Safe DIY fixes you can try:
1) Repressurise the system (only if you know where the filling loop is and are comfortable doing it): locate the filling loop (flexible hose or built-in valve), open the valve(s) slowly while watching the pressure gauge until it reaches about 1.0–1.5 bar, then close the valve(s) fully. Never leave the filling loop open. If you are unsure which valves to operate, stop and call an engineer.
2) Bleed radiators to remove airlocks: start with the highest radiators in the property. Use a radiator key, open the bleed valve slowly until you hear hissing, then when water drips tighten it back. Re-check boiler pressure after bleeding and top up if necessary.
3) Ensure radiator valves and TRVs are open and any manual radiator isolation valves are open. Also open any boiler-side gate valves fully.
4) Listen and feel (from a safe distance) for pump operation: with the boiler on you may hear a faint hum or feel mild vibration on pipework. Do not touch hot pipes. If the pump is completely silent and you have power to the boiler, the pump may be seized or electrical supply to the pump may have failed.
Further diagnostic steps and what to expect from a professional:
1) If the above steps do not clear F.24, do not attempt internal repairs. The likely causes at this stage are a seized/failed pump, a blockage/sludge in the pump or heat exchanger, incorrect pump speed setting, or an electrical/PCB fault. These require a qualified engineer to diagnose safely.
2) An engineer will check pump electrical supply and continuity, test the pump rotor and bearings, and may remove and inspect the pump. If the pump is blocked with debris or seized, the engineer may free or replace it. They will also inspect wiring between pump and PCB.
3) If circulation is poor despite a healthy pump, the engineer will check for blockages in the heat exchanger and piping. Powerflushing or chemical cleaning and fitting a magnetic filter to capture debris are common professional remedies. A heavily blocked heat exchanger may require specialist cleaning or replacement.
4) If electrical communication faults are suspected the engineer will test and possibly replace the PCB or sensors. Only a Gas Safe engineer should handle PCB replacement and electrical wiring on gas appliances.
When to call a professional immediately:
- F.24 persists after you have checked pressure, opened isolation valves, bled radiators and tried a reset.
- You suspect the pump is seized, you hear abnormal noises from the pump, or the pump shows no sign of running.
- You cannot locate or safely use the filling loop, or topping up causes leakage.
- You suspect a blocked heat exchanger, repeated sludge problems, or there is evidence of internal leaks.
Final note: simple checks (pressure, isolation valves, bleeding and a reset) are often sufficient for F.24, but pump, blockage and PCB faults require a Gas Safe registered engineer. Do not attempt gas, burner or wiring repairs yourself. If in doubt, switch the boiler off and get a qualified engineer to inspect it.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant Turbomax Plus 824/828E.