F11 Boiler Fault Code: What It Means on Every Brand

The F11 fault code appears on 6 brands — Ferroli, Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 6 brands use this code Meaning differs by brand

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 F11 means	Severity	DIY?
FFerroli	The F11 code on a Ferroli boiler signals a problem with the domestic hot water (DHW) NTC temperature sensor — the thermistor responsible for monitoring hot water supply or return… Full guide →	Engineer may be needed	No
GWGlow-worm	The F11 fault code on a Glow-Worm boiler indicates a communication breakdown between the User Interface PCB (the board behind the controls and display) and the Main PCB (the board that… Full guide →	Lockout — engineer likely	No
HHeatline	The F11 code on a Heatline boiler indicates that the control board has detected a problem with the return heating temperature sensor — the NTC thermistor that monitors the temperature of… Full guide →	Lockout — engineer likely	No
PProthermshown as F.11	The F.11 fault code on Protherm boilers indicates that the control board has detected a short circuit or open circuit in the CH return water temperature sensor, known as NTC5. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F11 fault code on Saunier Duval boilers covers two distinct scenarios depending on your model. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.11	The F.11 code appears on Vaillant boilers when the control board detects a short circuit in the flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for… Full guide →	Lockout — engineer likely	No

F11 brand by brand

Ferroli F11

The F11 code on a Ferroli boiler signals a problem with the domestic hot water (DHW) NTC temperature sensor — the thermistor responsible for monitoring hot water supply or return temperatures. The boiler has detected either a short circuit or an open circuit within that sensor circuit, meaning the sensor is either sending an implausible reading or no reading at all. Because F11 is an 'F'-type blocking fault (as opposed to an 'A'-type lockout), the boiler may recover on its own if the condition clears, but a recurring F11 almost always points to a genuine component fault that needs an engineer's attention. The exact sensor affected can vary slightly between model families — on some ranges such as the Modena and i-Series it flags the hot water flow sensor, while on others such as the Bluehelix and Optimax it may relate to the return sensor — so always cross-reference with your specific model's documentation.

Causes, fixes & costs for Ferroli F11 → · All Ferroli codes

Glow-worm F11

The F11 fault code on a Glow-Worm boiler indicates a communication breakdown between the User Interface PCB (the board behind the controls and display) and the Main PCB (the board that governs the boiler's core operation). When these two boards lose the ability to talk to each other reliably, the boiler locks out to prevent unsafe operation. On CXI, HXI and SXI models this can also cause the display to go completely blank. If you have a Flexicom model, F11 can additionally point to a fault with the flow or return thermistors — the temperature sensors that monitor water moving through the heat exchanger — rather than a pure PCB-to-PCB communication issue.

Causes, fixes & costs for Glow-worm F11 → · All Glow-worm codes

Heatline F11

The F11 code on a Heatline boiler indicates that the control board has detected a problem with the return heating temperature sensor — the NTC thermistor that monitors the temperature of water coming back into the boiler from your heating circuit. When the PCB cannot get a reliable reading from this sensor, it triggers F11 and locks the boiler out as a precaution, cutting off both heating and hot water until the fault is resolved.

Causes, fixes & costs for Heatline F11 → · All Heatline codes

Protherm F.11

The F.11 fault code on Protherm boilers indicates that the control board has detected a short circuit or open circuit in the CH return water temperature sensor, known as NTC5. This sensor sits on the return pipework and feeds the boiler's electronics with real-time data about the temperature of water coming back from the radiators. When the signal from NTC5 disappears entirely or falls outside the expected range — whether due to a short circuit across its terminals or a complete break in the circuit — the boiler interprets this as an unsafe or unreliable condition and locks out. The fault may also appear in connection with the accumulator sensor circuit on certain Protherm models. Either way, the boiler will not resume normal operation until the sensor circuit has been verified and restored to a healthy state.

Causes, fixes & costs for Protherm F.11 → · All Protherm codes

Saunier Duval F11

The F11 fault code on Saunier Duval boilers covers two distinct scenarios depending on your model. On most Condens, Themafast, and Isofast models, F11 signals a communication breakdown between the main PCB and the user interface (display) board — the two boards have lost contact, usually due to a wiring or connector problem. On certain other Saunier Duval models, F11 instead indicates a primary circuit overheat or safety thermostat trip, where the boiler has detected dangerously high temperatures — often because water isn't circulating properly — and locked itself out as a precaution. If you're unsure which interpretation applies to your appliance, check the front of your manual or the model sticker inside the casing.

Causes, fixes & costs for Saunier Duval F11 → · All Saunier Duval codes

Vaillant F.11

The F.11 code appears on Vaillant boilers when the control board detects a short circuit in the flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for measuring the temperature of water leaving the heat exchanger. At room temperature (around 20°C) a healthy flow NTC should register approximately 12KΩ of electrical resistance. When the PCB reads a value far outside that range, it interprets this as a short circuit and locks the boiler out as a safety precaution. Without a reliable flow temperature reading, the boiler cannot regulate heat output safely, so it shuts down rather than risk overheating.

Causes, fixes & costs for Vaillant F.11 → · All Vaillant codes

Frequently asked questions

Is the F11 fault code the same on every boiler?

No. F11 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F11 fault myself?

Not usually. On every brand listed, F11 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F11 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.