F77 Boiler Fault Code: What It Means on Every Brand

The F77 fault code appears on 5 brands — Glow-worm, Heatline, Ravenheat, 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: Stop & check now 5 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 F77 means	Severity	DIY?
GWGlow-worm	The F77 fault code on a Glow-Worm boiler signals a gas valve problem detected by the boiler's control board. Full guide →	Stop & check now	No
HHeatline	The F77 fault code on a Heatline boiler indicates that the boiler has detected a problem with its condensate removal system. Full guide →	Lockout — engineer likely	Some checks
RRavenheat	The F77 code indicates a malfunction in the gas valve motor assembly. Full guide →	Lockout — engineer likely	No
SDSaunier Duvalshown as F.77	The F.77 fault code on a Saunier Duval boiler signals that the boiler's control system has detected a problem with either the flue non-return flap (flue gas valve) or the condensate pump —… Full guide →	Lockout — engineer likely	Some checks
VVaillantshown as F.77	The F.77 code appears on Vaillant boilers when the condensate removal system has encountered a problem serious enough to trigger a safety lockout. Full guide →	Lockout — engineer likely	Some checks

F77 brand by brand

Glow-worm F77

The F77 fault code on a Glow-Worm boiler signals a gas valve problem detected by the boiler's control board. Specifically, the boiler has sensed gas combustion occurring at the burner when no heat demand has been issued — in other words, gas appears to be flowing and burning without the boiler being told to fire. This is a serious safety condition, so the boiler immediately locks out and refuses to restart until the fault has been properly diagnosed and resolved by a qualified professional. Because gas is involved, this code must always be treated as a priority fault: do not repeatedly attempt to reset the boiler, and if you smell gas at any point, turn off your gas supply at the meter and call the National Gas Emergency line on 0800 111 999 straight away.

Causes, fixes & costs for Glow-worm F77 → · All Glow-worm codes

Heatline F77

The F77 fault code on a Heatline boiler indicates that the boiler has detected a problem with its condensate removal system. Modern condensing boilers produce acidic water vapour as a by-product of combustion, which condenses into liquid and must be drained away. When the boiler cannot confirm that this process is working correctly — whether because a pipe is frozen or blocked, or because a fitted condensate pump has failed — it locks out and displays F77 to prevent further operation. Because Heatline is part of the Vaillant Group and shares the same underlying platform, F77 carries the same meaning across Vaillant-family brands including Glow-worm. The fault is particularly common during cold spells when external condensate pipes are vulnerable to freezing.

Causes, fixes & costs for Heatline F77 → · All Heatline codes

Ravenheat F77

The F77 code indicates a malfunction in the gas valve motor assembly. In practice, the solenoid-operated valve has either stuck open or stuck closed, which creates a risk of unsafe combustion or a complete failure to ignite. You may notice a low humming or buzzing sound coming from the valve area before or during lockout. Because the boiler cannot verify that gas is being controlled safely, it shuts down immediately and holds a hard lockout. On combi models, both heating and hot water are lost instantly. This fault always requires the attention of a Gas Safe registered engineer — do not attempt repeated resets once the boiler has locked out on F77.

Causes, fixes & costs for Ravenheat F77 → · All Ravenheat codes

Saunier Duval F.77

The F.77 fault code on a Saunier Duval boiler signals that the boiler's control system has detected a problem with either the flue non-return flap (flue gas valve) or the condensate pump — or both. In plain terms, the boiler is not receiving the expected feedback from one of these components, so it shuts down as a precaution. Because Saunier Duval is part of the Vaillant Group and shares the same F.7x fault code series and control architecture, this definition applies consistently across the Saunier Duval range. The fault can mean the condensate pump is blocked, failed, or not responding correctly, or that the flue non-return flap is not operating as expected — often because of a wiring issue, a failed flap mechanism, or a blockage in the condensate drainage path. In winter, a frozen external condensate pipe is one of the most frequent triggers.

Causes, fixes & costs for Saunier Duval F.77 → · All Saunier Duval codes

Vaillant F.77

The F.77 code appears on Vaillant boilers when the condensate removal system has encountered a problem serious enough to trigger a safety lockout. Vaillant condensing boilers produce acidic water as a natural by-product of the combustion process, and this condensate must be continuously drained away. When the boiler detects that this drainage is not happening correctly — whether because of a frozen pipe, a blocked outlet, or a failing pump — it shuts itself down to prevent internal damage or flooding. The fault is most commonly seen on boilers fitted with a condensate pump (used where the condensate cannot drain by gravity alone), but it can also appear on gravity-drained installations if the pipe becomes blocked or frozen.

Causes, fixes & costs for Vaillant F.77 → · All Vaillant codes

Frequently asked questions

Is the F77 fault code the same on every boiler?

No. F77 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 F77 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 F77 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.