Saunier Duval F.77 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.77 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Frozen or blocked condensate pipe Common
During cold weather, the water-laden condensate that collects in the pipe can freeze solid, particularly in sections of external pipework. This backs up the drainage path and triggers the F.77 lockout. Even in milder weather, a partial blockage from debris or sludge build-up can cause the same result.
- Condensate pump failure or obstruction Common
Where the boiler cannot gravity-drain condensate away — for example, when the boiler is installed below the drain level — a condensate pump is fitted as an accessory. If this pump becomes clogged with sludge, suffers a mechanical failure, or develops a wiring fault, the boiler detects no response from it and raises F.77.
- Faulty flue non-return flap or its wiring Sometimes
The flue gas non-return flap prevents flue gases re-entering the boiler when it is off. If the flap itself jams, corrodes, or its electrical connection becomes loose or damaged, the boiler's control board receives no confirmation signal and triggers F.77. This is more common on older installations or where the flue connection has not been disturbed for many years.
- Wiring or connector fault on the accessory circuit Sometimes
Both the condensate pump and the flue flap communicate with the PCB via a dedicated accessory circuit. Corroded terminals, loose plugs, or damaged wiring between these components and the board can mimic a component failure, causing F.77 even when the pump or flap itself is perfectly serviceable.
- PCB fault Rare
In rare cases where all external components and wiring have been checked and found to be in order, the PCB itself may be misreading the feedback signals from the condensate pump or flue flap circuit. This is uncommon but does occur on older boards.
How to fix it
- Check whether the condensate pipe is frozen or blocked DIY safe
Locate the condensate pipe — usually a white or grey plastic pipe (22 mm or 32 mm) that exits the boiler and runs to an external drain. Feel along its length for any sections that are frozen solid or visibly blocked. If frozen, gently apply warm (not boiling) water using a watering can or cloth along the pipe from the outlet end back towards the boiler. Never use a naked flame. Once thawed, allow condensate to drain freely for a minute or two.
- Check the condensate pump (if fitted) for obvious issues DIY safe
If your boiler has an external condensate pump accessory, check that it is powered on and that there are no visible obstructions at the pump inlet or outlet. Listen for unusual noises. Check that the pump's float is not stuck. Do not open the pump internals — if it appears blocked with sludge or is making a grinding noise, this is a job for an engineer.
- Reset the boiler DIY safe
Once you have thawed the condensate pipe or identified an obvious external issue, press and hold the reset button (usually marked with a flame symbol or the word 'Reset') for around three seconds until the display clears. Allow the boiler to complete its ignition sequence. If the F.77 code returns immediately or after one or two firing cycles, do not continue resetting — further attempts are unlikely to resolve the underlying fault and may mask a safety issue.
- Check boiler system pressure DIY safe
Glance at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up via the filling loop (refer to your boiler manual for the location). Low pressure alone will not cause F.77, but it is worth ruling out as a contributing issue before calling an engineer.
- Do not attempt to work on the flue flap, condensate pump internals, or wiring Gas Safe engineer
Accessing the flue non-return flap, dismantling the condensate pump, or probing the accessory wiring loom requires knowledge of the boiler's internal layout and electrical circuits. These tasks must be carried out by a Gas Safe registered engineer with the appropriate training and tools.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler continues to display F.77 after the homeowner checks above, a Gas Safe engineer should attend. They will use the boiler's diagnostic mode to confirm which component is at fault, test the condensate pump and its wiring, inspect and test the flue non-return flap and its connector, clear any internal sludge blockages, and replace any defective components. Ensure the engineer is on the Gas Safe Register (check at gassaferegister.co.uk) before work begins.
Parts you may need
- Condensate pump (external accessory) · from £65
- Flue non-return flap assembly · from £55
- Condensate pump wiring harness / connector · from £20
- PCB (printed circuit board) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £90–£300, depending on the underlying cause.
Frequently asked questions
Can I fix a Saunier Duval F.77 fault myself?
The one DIY fix that genuinely works for many F.77 faults — especially in winter — is thawing a frozen condensate pipe. This is safe to do yourself using warm water poured along the pipe. You can also try a boiler reset (up to two or three times) after clearing a blockage. However, if the code persists, the cause is most likely a failed condensate pump, a faulty flue non-return flap, or a wiring fault — all of which require a Gas Safe registered engineer.
Why does F.77 keep coming back on my Saunier Duval?
If F.77 returns repeatedly, it usually means the root cause has not been fully resolved. A condensate pipe that was thawed but not insulated will freeze again in cold weather. A condensate pump that is partially blocked may clear temporarily on reset but will fail again as sludge accumulates. A wiring fault on the flue flap circuit can be intermittent. Have a Gas Safe engineer carry out a thorough inspection so the underlying fault is properly fixed rather than just reset each time.
How much does it cost to fix a Saunier Duval F.77 fault in the UK?
Most homeowners pay between £90 and £300 to resolve an F.77 fault. Thawing and clearing a frozen condensate pipe typically costs £90–£150 including a call-out. Replacing a condensate pump costs roughly £100–£250 for parts and labour. Replacing the flue non-return flap is usually in the £150–£300 range. In rare cases where the PCB is at fault, costs can rise to £400 or more including parts and labour — but this is uncommon and a good engineer will exhaust all simpler causes first.
Is the F.77 code the same on Vaillant boilers?
Yes. Saunier Duval is part of the Vaillant Group and the two brands share the same control system and fault code structure. The F.77 code carries the same meaning on Vaillant boilers — a fault with the flue non-return flap or condensate pump accessory — and the diagnostic and repair approach is identical. If you see F.77 on either brand, the same checks and fixes apply.