F20 is an overheat / safety-temperature-limiter fault on every brand that shares Vaillant Group electronics (Vaillant, Glow-worm, Saunier Duval, Heatline, Protherm), and on Ferroli's Domiproject range. The cause is nearly always water not circulating — sludge, airlock, tired pump — so the heat has nowhere to go. Cool down, reset once, then an engineer if it returns.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F20 brand by brand
The F20 fault code appears on certain Ferroli boiler models — most notably the Domiproject D/F range and the DOMIcondens HE 26C — and indicates that the boiler has detected a problem with combustion quality or combustion control. In plain terms, the boiler has sensed that the burner is not burning gas in the way it should, so it has shut down as a precaution. Because F20 begins with 'F', Ferroli classifies it as a blocking fault rather than a full lockout: it may clear on its own if the underlying condition resolves, but if it keeps returning you need a Gas Safe engineer to investigate. Do not confuse this with A-series codes (such as A01 or A02), which are hard lockouts always requiring a manual reset. Important note: Ferroli fault code meanings can differ between model ranges (Modena, Bluehelix, Optimax, i-Series, Domiproject). Always cross-reference your boiler's own user manual to confirm the exact meaning for your unit.
Causes, fixes & costs for Ferroli F20 → · All Ferroli codes
The F20 fault code means your Glow-worm boiler has detected an abnormal rise in primary circuit temperature — typically above 95°C — and has shut itself down as a precaution. Some Glow-worm models display the message 'Safety switch off temperature limiter' on the LCD alongside F20, which confirms this interpretation. The boiler will not restart on its own; it requires a manual reset. Because the root cause is excessive heat build-up rather than a gas or combustion issue, the fault is not immediately dangerous, but it should not be ignored: repeated overheating can damage internal components, particularly the heat exchanger. Once the boiler has cooled, a single reset attempt is reasonable, but if the fault returns you need a Gas Safe registered engineer to investigate.
Causes, fixes & costs for Glow-worm F20 → · All Glow-worm codes
The F20 code on a Heatline boiler means the unit has overheated and the internal safety thermal cut-out has triggered. Flow or return temperature sensors (NTC thermistors) detected dangerously high temperatures, so the boiler locked itself out to protect the heat exchanger and internal components. You will lose both central heating and hot water until the underlying cause is found and fixed. Attempting repeated resets without resolving the root issue risks permanent damage and may invalidate your warranty.
Causes, fixes & costs for Heatline F20 → · All Heatline codes
The F.20 code appears when Protherm's built-in safe temperature limiter detects that the primary circuit flow temperature has climbed above 90°C and has cut the boiler out as a precaution. In plain terms, the boiler has overheated and shut itself down to prevent damage or a dangerous situation. Once the water temperature drops back to a safe level the boiler may attempt to restart automatically, but unless the underlying cause is identified and fixed, the fault will keep returning. F.20 sits in a range of codes that Protherm considers serious enough to affect both performance and safe operation, so it should not be ignored or repeatedly reset without investigation.
Causes, fixes & costs for Protherm F.20 → · All Protherm codes
The F.20 code appears on most Saunier Duval boilers — including the Themaplus Condens, Thema Condens, Isomax, and ThemaClassic — when the water temperature inside the heat exchanger climbs past the manufacturer's safety limit of approximately 97°C. At that point, the boiler shuts itself down in a protective lockout to prevent component damage, dangerous pressure build-up, and any risk of scalding or explosion. The display will flash and alternate between two numbers (the second carrying a decimal point) to signal the fault type. Once the underlying cause has been addressed, pressing the Reset button restarts the boiler. Note: on the Condens range specifically, F.20 can occasionally point to a software incompatibility between components — often after an incorrect replacement part has been fitted — rather than a straightforward overheating event. Always cross-reference with your model's manual if the code reappears after a seemingly successful fix.
Causes, fixes & costs for Saunier Duval F.20 → · All Saunier Duval codes
The F.20 code appears when your Vaillant boiler detects that the water temperature at the flow or return NTC thermistor has climbed beyond the safe upper limit — typically around 95–97°C. At that point the boiler's safety logic triggers a hard lockout, cutting the burner to protect internal components from heat damage. The fault can be caused by a genuine overheating event (for example, poor water circulation or a scaled-up heat exchanger) or by a faulty temperature sensor that is reporting an inflated reading to the control board. Either way, the boiler will not restart automatically — you must address the underlying cause before resetting. Until it is fixed you will have no central heating or hot water.
Causes, fixes & costs for Vaillant F.20 → · All Vaillant codes
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.
