F15 Boiler Fault Code: What It Means on Every Brand

The F15 fault code appears on 4 brands — Ferroli, Glow-worm, Protherm 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 4 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 F15 means	Severity	DIY?
FFerroli	The F15 code on a Ferroli boiler most commonly points to a fan-related problem — typically an interrupted tachometer signal, a wiring fault at the fan connection, a damaged fan motor, or… Full guide →	Lockout — engineer likely	No
GWGlow-worm	The F15 fault code on a Glow-Worm boiler points to a problem with the return thermistor — the sensor responsible for measuring the temperature of water as it flows back into the boiler… Full guide →	Engineer may be needed	No
PProthermshown as F.15	The F.15 fault code on a Protherm boiler points to a problem with the boiler's fan or its flue exhaust gas sensing system. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.15	The F.15 code appears on Vaillant boilers when the appliance cannot confirm that water is circulating properly around the system. Full guide →	Lockout — engineer likely	No

F15 brand by brand

Ferroli F15

The F15 code on a Ferroli boiler most commonly points to a fan-related problem — typically an interrupted tachometer signal, a wiring fault at the fan connection, a damaged fan motor, or debris obstructing the fan impeller. Because the fan is critical for safely venting combustion gases through the flue, the boiler locks out rather than risk unsafe operation. On some Ferroli model ranges (including certain i-Series and Optimax variants), F15 is instead associated with a flow sensor or temperature sensor fault, so it is worth checking your specific model's manual to confirm which definition applies. If you are unsure, treat the fault as requiring a Gas Safe engineer until confirmed otherwise.

Causes, fixes & costs for Ferroli F15 → · All Ferroli codes

Glow-worm F15

The F15 fault code on a Glow-Worm boiler points to a problem with the return thermistor — the sensor responsible for measuring the temperature of water as it flows back into the boiler after circulating through your heating system. When this thermistor fails or sends readings outside the expected range, the boiler's control board can no longer accurately monitor return water temperature. This disrupts the boiler's ability to regulate its heating cycles correctly, so it shuts down and displays F15 to protect itself. The fault is typically caused by a failed thermistor, a wiring fault in the sensor circuit, or in less common cases, a faulty PCB. While F15 does not usually represent an immediate safety emergency, it will prevent your boiler from running normally until the underlying cause is resolved.

Causes, fixes & costs for Glow-worm F15 → · All Glow-worm codes

Protherm F.15

The F.15 fault code on a Protherm boiler points to a problem with the boiler's fan or its flue exhaust gas sensing system. When the electronic control unit detects that the fan's speed signal is missing or implausible, or that the exhaust gas sensor is returning an out-of-range or short-circuit reading, it flags F.15 and shuts the appliance down safely. On the display, the letter F flashes alongside the number 15. Depending on your specific Protherm model, the fault leans toward one of two interpretations: (1) a fan fault — where the fan motor is not spinning correctly, the tachometric (speed-feedback) signal is interrupted, or the 230 V supply to the fan is absent; or (2) an exhaust gas sensor short circuit — where the NTC sensor monitoring flue temperatures has developed a short circuit in its wiring or element. Both scenarios prevent safe combustion from being confirmed, so the boiler locks out until the root cause is identified and resolved. Because this code involves either the fan, the flue or gas-related sensing components, all diagnostic and repair work must be carried out by a Gas Safe registered engineer.

Causes, fixes & costs for Protherm F.15 → · All Protherm codes

Vaillant F.15

The F.15 code appears on Vaillant boilers when the appliance cannot confirm that water is circulating properly around the system. This typically points to one of three root issues: the system has insufficient water pressure, the circulation pump is not functioning correctly, or the electrical connection to the pump is faulty. When the boiler detects this condition it locks out as a safety measure to prevent overheating or internal damage. F.15 sits within the F.10–F.15 group of pump and sensor-related lockout codes on Vaillant appliances, all of which indicate the boiler cannot safely continue operating without engineer investigation.

Causes, fixes & costs for Vaillant F.15 → · All Vaillant codes

Frequently asked questions

Is the F15 fault code the same on every boiler?

No. F15 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 F15 fault myself?

Not usually. On every brand listed, F15 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 F15 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.