Vaillant F.77 fault codewhat it means and how to fix it
The condensate pump or the flue non-return flap has not responded. The boiler cannot clear its condensate or confirm the flue, so it has shut down.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.77
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What sort of installation is this?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
The boiler is below the drain and there is a small pump nearby
The pump is the likely subject of the code. Listen for it — a working condensate pump runs briefly and audibly every few minutes while the boiler is firing.
Check the pump is powered and not visibly overflowing, and mention what you find when you book.
It is a flat and the flue is shared with other properties
The non-return flap is the more likely half of this code. On communal systems this may be the responsibility of the building rather than yours alone.
Tell the engineer it is a shared flue before they attend, and inform your managing agent or freeholder.
There is water pooling under or near the boiler
The condensate has nowhere to go and is escaping instead. Left alone it damages flooring and anything below it.
Turn the boiler off, put something absorbent down, and treat the visit as urgent.
What F.77 means
F.77 covers two components that Vaillant groups together because both concern getting waste out of the appliance safely. The first is the condensate pump, fitted where the boiler sits below the drain it discharges into and gravity cannot do the job. The second is the flue non-return flap, which prevents products of combustion from an adjoining appliance travelling back down a shared flue.
If the boiler commands either and gets no confirmation, it stops. Which one applies to you depends entirely on your installation. A boiler in a basement or a converted loft very likely has a condensate pump. A boiler on a shared or communal flue system, common in flats, has the non-return flap. Knowing which you have narrows this code down considerably before an engineer even arrives.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Condensate pump failed or seized | These pumps handle mildly acidic water continuously and eventually clog or burn out, typically after several years. | Common |
| Condensate pump blocked with debris | Sludge from the combustion process settles in the reservoir until the float or impeller can no longer move freely. | Common |
| No confirmation from the flue non-return flap | The flap sticks, or the switch reporting its position fails, so the boiler cannot verify the flue is clear to use. | Occasional |
| Wiring fault to pump or flap switch | A disconnected or damaged lead means the component may be fine while the boiler still receives no signal from it. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Condensate pump £140 to £260 fitted. Cleaning an existing pump often within a call-out of £80 to £150. Shared flue work varies with access and may involve the building owner.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.32Get it checkedThe fan is not running at the speed the boiler expects, so combustion cannot be verified as safe and the boiler has locked out.Related: fan and flue
- F.35Get it checkedThe air intake or flue is blocked. Something is physically obstructing the path combustion products take out of the building.Related: fan and flue
- S.32Get it checkedA protective wait, not a lockout: the fan speed has strayed outside tolerance and the boiler is holding off before it tries to start again. If the wait keeps recurring, F.32 is next.Related: fan and flue
- F.33Stop — call an engineerAn air pressure switch fault: the boiler cannot prove the fan is moving air through the flue. Vaillant check the flue route first, so treat it as a flue fault until proved otherwise.Related: fan and flue
- F.347Get it checkedA 2022 ecoTEC plus has detected condensate inside its combustion chamber and stopped the burner. Vaillant's cause is a blocked condensate discharge — the siphon or the pipe — backing up into the boiler.Related: fan and flue
- F.334Get it checkedA commissioning code on the 2022 ecoTEC plus: the flue system configuration was never completed in the installer settings. F.336, F.337 and F.338 are its neighbours — a configuration set wrongly.Related: fan and flue
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.77
How do I know whether I have a condensate pump?
Look for a small plastic box near the boiler with a thin pipe running from the appliance into it and another leaving towards a drain. It will be plugged into a socket or wired nearby. If the boiler discharges downhill straight into a drain instead, you almost certainly do not have one.
What is the flue non-return flap for?
On a shared flue it stops exhaust from a neighbouring boiler being drawn back into yours. It is a safety device, which is why the boiler will not run without confirmation that it is doing its job.
Can I clean the condensate pump myself?
The pump reservoir sits outside the sealed boiler casing, so cleaning it is not gas work. Isolate the electrical supply first, and be aware the water is acidic — gloves are sensible. If you are unsure at any point, leave it to the engineer.
What is the cost likely to be?
A condensate pump is £140 to £260 fitted. Cleaning one out may fall within a call-out charge. Work on a non-return flap in a shared flue is harder to price and may need coordination with the building owner.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.