F.25 Boiler Fault Code: What It Means on Every Brand

The F.25 fault code appears on 3 boiler brands — Glow-worm, Protherm and Vaillant — 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.25 means	Severity	DIY?
GWGlow-wormshown as F25	The F25 code on a Glow-Worm boiler signals that the appliance has detected a problem with water circulation through the heating system. Full guide →	Lockout — engineer likely	Some checks
PProtherm	The F.25 fault code on Protherm boilers is a safety lockout that can have two related meanings depending on your model. Full guide →	Lockout — engineer likely	No
VVaillant	The F.25 fault code on a Vaillant boiler indicates that the safety temperature limiter (overheat thermostat) has tripped. Full guide →	Lockout — engineer likely	No

F.25 brand by brand

Glow-worm F25

The F25 code on a Glow-Worm boiler signals that the appliance has detected a problem with water circulation through the heating system. When water cannot flow freely around the circuit, heat builds up rapidly inside the boiler and the flue gas temperature climbs beyond safe limits. The boiler's safety controls recognise this and trigger a protective lockout before any damage can occur. In practice the two meanings — poor circulation and elevated flue gas temperature — are two sides of the same coin: restricted or inadequate water movement is what causes both. The boiler will remain locked out until the root cause is addressed and the fault is cleared by a reset.

Causes, fixes & costs for Glow-worm F25 → · All Glow-worm codes

Protherm F.25

The F.25 fault code on Protherm boilers is a safety lockout that can have two related meanings depending on your model. On chimney-flue (open-flue or natural-draught) models such as the Gepard MOV and Panther, it signals that the flue gas or draft thermostat has detected combustion products escaping into the room and has cut the boiler off to prevent poisoning. On all Gepard and Panther models it can also indicate that the boiler's flow temperature has risen dangerously quickly — typically five degrees per second or more — often because system pressure is dropping at the same time. In either case the boiler locks out and will not restart until the root cause is resolved. Do not keep resetting the boiler and ignoring this code; both triggers represent genuine safety hazards.

Causes, fixes & costs for Protherm F.25 → · All Protherm codes

Vaillant F.25

The F.25 fault code on a Vaillant boiler indicates that the safety temperature limiter (overheat thermostat) has tripped. This happens when the boiler's flow temperature climbs far too rapidly — typically around 5°C per second — which is a sign that heat is not being carried away from the heat exchanger quickly enough. As a safety measure, the boiler locks itself out completely and will not restart until the root cause is resolved and the limiter is manually reset by a Gas Safe engineer. This is a hard lockout, not a soft reset fault, so simply pressing the reset button on the boiler will not clear it permanently if the underlying problem remains. The code is most frequently seen on Vaillant ecoTEC Pro and ecoTEC Plus models.

Causes, fixes & costs for Vaillant F.25 → · All Vaillant codes

Frequently asked questions

Is the F.25 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Protherm and Vaillant the F.25 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.25 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.25 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.