The F14 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F14 brand by brand
The F14 code on Ferroli boilers signals a problem with the heating circuit's secondary NTC temperature sensor — either a short circuit or an open circuit has been detected. The boiler's control board monitors the resistance reading from this thermistor continuously; if that reading falls outside the acceptable range, the board raises the F14 fault and shuts the boiler down to protect the system. On some older Ferroli models such as the Optimax, F14 can instead point to a flue stat sensor fault — a safety device that monitors exhaust gas temperatures near the flue outlet. It's worth knowing that Ferroli uses two broad code categories: codes starting with 'A' are hard lockouts requiring a manual reset, whereas codes starting with 'F' (including F14) are blocking faults that may clear on their own once the underlying condition is resolved. Always cross-check with your specific model's manual, as meanings can vary slightly across ranges including the Modena, Optimax, Bluehelix, and i-Series.
Causes, fixes & costs for Ferroli F14 → · All Ferroli codes
The F14 fault code on Glow-Worm boilers has two distinct meanings depending on your model range, so it is important to identify which boiler you have before attempting any diagnosis.
On Flexicom CX and most modern combi and system models, F14 points to a gas valve control fault. The boiler's PCB has detected that the gas valve is not responding correctly to its commands — it may be failing to open, close, or modulate as required. This is a safety-critical issue: a misbehaving gas valve can cause the boiler to run dangerously hot, fail to ignite at all, or behave erratically. The PCB responds by locking the boiler out entirely, leaving you with no heating or hot water until the fault is resolved by a qualified engineer.
On older CXI, HXI, and SXI models, F14 instead signals that the central heating flow temperature has climbed above 95°C. This overheating condition is typically triggered by a circulation problem — most often a scaled-up plate heat exchanger, a failing pump, or a faulty flow thermistor — rather than a gas valve issue. Real-world engineer reports confirm that a blocked plate heat exchanger is a particularly frequent culprit on models such as the 30CXi.
In both cases the boiler will lock out as a protective measure. Do not repeatedly reset the boiler without understanding the underlying cause, as this could lead to unsafe operation or mask a worsening fault.
Causes, fixes & costs for Glow-worm F14 → · All Glow-worm codes
The F.14 code appears on Protherm boilers (including the Gepard, Panther, and Condens ranges) when the control board cannot obtain a valid signal from the collector or external NTC temperature sensor. This sensor is typically fitted to an external solar collector circuit, a buffer vessel, or a secondary heating circuit. When the signal is absent, out of range, or inconsistent, the board flags the fault and — depending on the model — may prevent normal operation until it is resolved. A single reset sometimes clears the code if it was triggered by a momentary glitch, but a persistent F.14 points to a hardware problem that needs an engineer. Note: older Protherm models such as the Leopard and Jaguar have shorter fault-code lists that do not include F.14; if your boiler is one of these, check your specific manual, as the code may mean something different or may not apply.
Causes, fixes & costs for Protherm F.14 → · All Protherm codes
The F.14 code appears on Vaillant boilers when the control board detects an interruption in the signal from the heating flow temperature sensor — commonly called the flow NTC thermistor. This small resistive sensor monitors the temperature of water leaving the heat exchanger; as water heats up, its electrical resistance drops in a predictable way, allowing the PCB to regulate burner output. When the signal is absent, out of range, or erratic, the boiler locks out and displays F.14 to prevent overheating. Worth noting: on certain Vaillant models such as the ecoMAX Pro range, F.14 sits within a family of NTC-related codes that includes F.00, F.04, F.05, F.10, and F.15 — each targeting a different temperature sensor in the system. If your display shows one of those companion codes, the same diagnostic approach applies, but the specific sensor location will differ.
Causes, fixes & costs for Vaillant F.14 → · 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.
