F.81 Boiler Fault Code: What It Means on Every Brand

The F.81 fault code appears on 3 boiler brands — Glow-worm, Protherm and Saunier Duval — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 3 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.81 means	Severity	DIY?
GWGlow-wormshown as F81	The F.83 code — which on some Glow-Worm displays can be misread as F81 due to the small digits — indicates a 'dry fire' condition. Full guide →	Lockout — engineer likely	Some checks
PProtherm	The F.81 code on Protherm boilers signals a fault with the charger pump — the secondary circulation pump responsible for pushing hot water through the circuit that heats stored domestic… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.81 code indicates that the boiler has detected an abnormal overheating event on the domestic hot water (DHW) circuit. Full guide →	Lockout — engineer likely	No

F.81 brand by brand

Glow-worm F81

The F.83 code — which on some Glow-Worm displays can be misread as F81 due to the small digits — indicates a 'dry fire' condition. When the burner ignites, the boiler expects to see a measurable rise in temperature at the flow or return sensor within a set period. If that temperature change is too small or absent entirely, the boiler concludes that either there is insufficient water circulating through the system, or the temperature sensors are not reading correctly. As a safety measure, the boiler locks out to prevent overheating or damage to internal components. If your display is definitely showing F81 and not F.83, check your model-specific manual or call Glow-Worm technical support directly, as F81 does not appear in standard published references for this brand.

Causes, fixes & costs for Glow-worm F81 → · All Glow-worm codes

Protherm F.81

The F.81 code on Protherm boilers signals a fault with the charger pump — the secondary circulation pump responsible for pushing hot water through the circuit that heats stored domestic hot water in models fitted with an integrated storage cylinder. When the boiler's control system detects that this pump is not operating correctly, it raises the F.81 fault and typically locks out to protect the appliance. This code is specific to Protherm models that include a built-in hot water storage tank; if your boiler is a straightforward combi with no cylinder, this code is unlikely to appear. Always check your specific model's manual to confirm.

Causes, fixes & costs for Protherm F.81 → · All Protherm codes

Saunier Duval F.81

The F.81 code indicates that the boiler has detected an abnormal overheating event on the domestic hot water (DHW) circuit. As a safety response, the boiler locks out and stops producing hot water, although central heating may continue to operate normally. You may notice the pump running and making noise, yet the stored water stays cold (around 23–24 °C) while the heat exchanger outlet climbs to 70–80 °C — a classic sign that heat is being generated but not transferred into the water supply. The boiler display will flash two alternating numbers to indicate the fault. Once the underlying cause is resolved, pressing the reset button should clear the lockout.

Causes, fixes & costs for Saunier Duval F.81 → · All Saunier Duval codes

Frequently asked questions

Is the F.81 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Protherm and Saunier Duval the F.81 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.81 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.81 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.