F.24 Boiler Fault Code: What It Means on Every Brand

F24 is a circulation fault: on Vaillant, Glow-worm, Heatline and Protherm it fires when water temperature climbs too fast (a sign of poor flow), and Saunier Duval reads it as little or no flow directly. Same root causes as F23 — pressure, air, sludge, pump — and the same first checks.

Lockout — engineer likely 5 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F.24 means	Severity	DIY?
GWGlow-wormshown as F24	The F24 code appears when your Glow-Worm boiler detects that the water temperature in the heating circuit is climbing far more rapidly than it should. Full guide →	Lockout — engineer likely	Some checks
HHeatlineshown as F24	The F24 code appears when the boiler's PCB detects that water temperature is climbing too rapidly — a clear sign that water isn't flowing properly around the heating system. Full guide →	Lockout — engineer likely	Some checks
PProtherm	The F.24 fault code on Protherm boilers indicates that the boiler's PCB has detected an abnormally rapid rise in water temperature — specifically faster than 10°C per second — within the… Full guide →	Lockout — engineer likely	Some checks
SDSaunier Duval	The F.24 code indicates that your Saunier Duval boiler has detected little or no water flowing through the central heating circuit. Full guide →	Lockout — engineer likely	Some checks
VVaillant	The F.24 code appears on Vaillant boilers when the PCB detects that water temperature is climbing too fast, almost always because water isn't moving around the system properly. Full guide →	Lockout — engineer likely	Some checks

F.24 brand by brand

Glow-worm F24

The F24 code appears when your Glow-Worm boiler detects that the water temperature in the heating circuit is climbing far more rapidly than it should. Under normal operation, the water temperature increases gradually as the boiler fires; when it surges upward too quickly, the flow pipe NTC thermistor picks this up and sends a signal to the PCB, which locks the boiler out to prevent overheating. On certain Glow-Worm models the fault is triggered specifically when the central heating return temperature exceeds 90°C, but the root cause is the same either way: heat generated inside the boiler is not being dispersed into the heating system quickly enough, usually because water is not circulating properly. This is a safety lockout and should not be ignored or repeatedly reset without investigating the underlying cause.

Causes, fixes & costs for Glow-worm F24 → · All Glow-worm codes

Heatline F24

The F24 code appears when the boiler's PCB detects that water temperature is climbing too rapidly — a clear sign that water isn't flowing properly around the heating system. The boiler monitors the difference in temperature between the flow pipe (water leaving the boiler) and the return pipe (water coming back). If circulation slows or stops, heat builds up fast in the heat exchanger and the boiler locks out to protect itself from damage. You'll lose both heating and hot water until the root cause is resolved. Common symptoms alongside the F24 include cold radiators, unusual gurgling or hissing noises, a boiler pressure drop on the gauge, and a pump that feels unusually hot to the touch. Heatline is part of the Vaillant Group and shares fault-code logic with Vaillant boilers, so engineers familiar with either brand will recognise this fault immediately. Important: repeatedly pressing reset without fixing the underlying circulation problem forces the heat exchanger to overheat again and again, which can crack it — a repair that costs far more than addressing the original fault.

Causes, fixes & costs for Heatline F24 → · All Heatline codes

Protherm F.24

The F.24 fault code on Protherm boilers indicates that the boiler's PCB has detected an abnormally rapid rise in water temperature — specifically faster than 10°C per second — within the heating circuit. This tells the boiler's safety logic that heat is not being carried away quickly enough, almost always because water isn't flowing through the system at the rate it should be. Rather than allow the heat exchanger to overheat, the boiler locks out and displays F.24. You'll lose both heating and hot water until the underlying circulation problem is resolved. Protherm boilers share the Vaillant Group control platform, so this code carries the same meaning across the related boiler range.

Causes, fixes & costs for Protherm F.24 → · All Protherm codes

Saunier Duval F.24

The F.24 code indicates that your Saunier Duval boiler has detected little or no water flowing through the central heating circuit. To protect its internal components from overheating or running dry, the boiler locks out and shuts down, leaving you with no heating or hot water until the underlying problem is resolved. Because Saunier Duval is part of the Vaillant Group and shares the same control architecture, this code carries the same meaning across both brands. The boiler will need to be reset once the root cause has been addressed, though repeated resets without fixing the fault will not clear it permanently.

Causes, fixes & costs for Saunier Duval F.24 → · All Saunier Duval codes

Vaillant F.24

The F.24 code appears on Vaillant boilers when the PCB detects that water temperature is climbing too fast, almost always because water isn't moving around the system properly. Without adequate circulation the heat exchanger can't shed heat at a safe rate, so the boiler locks out to protect itself. Until the underlying circulation problem is fixed you'll have no heating and no hot water. The F.24 fault is common across a wide range of Vaillant models — ecoTEC Plus, ecoTEC Pro, TurboMAX, and ecoMAX — as well as some Glow-worm boilers built on the same platform.

Causes, fixes & costs for Vaillant F.24 → · All Vaillant codes

Frequently asked questions

Is the F.24 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F.24 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F.24 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F.24 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.