Vaillant F77 fault code: condensate pump or flue non-return flap

What does the Vaillant F77 fault code mean?

Vaillant F77 means your boiler has had no response from the flue non-return flap, or the condensate pump is defective. Without that signal it cannot confirm waste water is being pumped away, so it blocks the burner and locks out. Stopping protects the boiler, but a part has usually failed. Reset once, then book a Gas Safe registered engineer.

Vaillant's installation and maintenance instructions for the ecoTEC plus 80, 100 and 120 kW (document 0020134823_10, dated 27 February 2025) set the code out as "F.77 | Fault: Flue non-return flap/condensate pump | No response from flue non-return flap or condensate pump defective". The same wording appears in the domestic ecoTEC plus manual 0020209591_00 and the ecoTEC pro manual 0020209589_01, so the meaning is the same whichever of those boilers you have. Vaillant writes the code with a dot, as F.77, so what you are reading on the display is the same code.

You will not find F.77 on Vaillant's public fault-code page at vaillant.co.uk/service/boiler-fault-codes/. When we checked that page in August 2026, the F.7 range carried only F.70, F.72 and F.75. F.77 appears in the installer and servicing manuals instead. If you have already checked the maker's own website and come away with nothing, you did not make a mistake and you are not reading the display wrong. The code is real.

In plain terms, your boiler will not fire until it has confirmation that the waste condensate water is being pumped away, or that the flue flap has opened. Without that signal it locks out. The lockout itself is protection rather than damage caused by carrying on, which is worth knowing on a cold evening. It does not mean nothing is broken: on most of the causes below, something has failed and needs replacing or clearing.

Key facts

If you only read one part of this page, read this.

• What it means: Vaillant's manual wording is "No response from flue non-return flap or condensate pump defective". The boiler blocks the burner until it gets that signal back.
• Can you fix it yourself? No. One reset is the only safe action. The other steps on this page are look-only checks, so you can tell the engineer more when they arrive.
• It becomes an engineer job the moment F77 returns after that single reset, and straight away if you can see water pooling around or under the boiler. Keep clear of water near the boiler and its electrics, and keep children and pets away from it.
• Which boilers: Vaillant ecoTEC plus (domestic 612, 615, 618, 624, 630, 637, 824, 831 and 837), the ecoTEC pro range, older open-vent ecoTEC plus, and the commercial ecoTEC plus VU GB 806, 1006 and 1206 at 80, 100 and 120 kW.
• It usually appears where a condensate pump is actually fitted, or where a motorised flue non-return flap is driven by the VR 40 "2 in 7" multi-functional module, which is normally a cascade or commercial system. It can also appear with neither of those, if the bridge on the boiler's "burner off contact" terminals is missing after a fit or a controls change.
• Not on the maker's public list: when we checked Vaillant's consumer fault-code page in August 2026, the F.7 range showed only F.70, F.72 and F.75, so F.77 has to be looked up in the installation and servicing manual.
• Cover would not pay for this repair. The boiler and central heating cover plan excludes condense pumps, any repair relating to flues, and any works where the cause of the issue is freezing, which is most of what F77 points at. A fault your boiler is already showing also counts as pre-existing. For the F77 on your display, book a one-off call-out and diagnosis.
• If you are thinking about cover for future faults: boiler and central heating cover pays parts and labour up to £500 in a 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and only for the modules listed on your cover sheet. Anything above that limit you pay yourself. Smart Plan is a service plan, not insurance.
• A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days. An engineer attends outside those hours only for a genuine emergency breakdown, so a Friday-evening or weekend F77 can wait until Monday. One-off appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays.
• Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.
• Smart Plan cover and our one-off appointments are for domestic boilers only. Commercial boilers are not included.

Why does my Vaillant boiler keep showing F77?

One code, two quite different parts. F77 covers both the condensate pump and the motorised flue non-return flap. The flap only applies where a VR 40 "2 in 7" multi-functional module drives it, which is normally a cascade or commercial installation, so on a single domestic boiler the condensate pump is the part the code points at.

That matters because the two send you in opposite directions. A condensate pump that has failed, seized or flooded is a small component with a clear job. A flue flap that gives no feedback is usually a wiring or accessory question on a bigger installation. The engineer needs to know which one your system actually has, so it is worth telling them the boiler model when you book.

Cold weather is the other pattern worth knowing. A condensate pipe that runs outside can freeze, water backs up into the pump, and the pump's float switch trips the boiler off. That is why F77 turns up on frosty mornings on a boiler that was fine the night before.

Here is what usually sits behind the code, what you would actually see or hear, and who is allowed to deal with each one.

F77 causes and who fixes them

Can I fix a Vaillant F77 myself?

Very little, and it is fairer to say so plainly than to pad the list out. The condensate pump, the condensate route, the flue non-return flap and the wiring behind them are all engineer work, whether they sit behind the boiler casing, inside an accessory or outside on the condensate pipe. So there is one safe action, and a handful of look-only checks that let you tell the engineer more when they arrive.

Work through them in this order.

1. Reset the boiler once, using the reset button and the method printed in your Vaillant user guide. If F77 comes straight back, stop there and call a Gas Safe registered engineer. Do not keep resetting it.

2. Read the pressure gauge on the front of the boiler and write the figure down, so you can pass it on. Do not top up or drain anything.

3. Check your room thermostat is calling for heat and that its batteries are working, so that is ruled out before the visit.

4. Look for obvious signs of water on the floor around or under the boiler, and for ice on the plastic condensate pipe where it runs outside. Look only. Do not touch, thaw, move or open anything, and do not take the boiler casing off. If there is water on the floor by the boiler, keep clear of it, keep children and pets away, do not touch the boiler or any nearby electrics, and tell us when you book.

5. Check whether your other appliances have power and gas, so you know the problem is the boiler and not the supply.

Stop at that point. Inspecting, testing, cleaning or replacing the condensate pump, clearing a blocked or frozen condensate route, testing or replacing the flue non-return flap and its feedback switch, checking the cable to the VR 40 module, refitting the bridge on the "burner off contact" terminals, opening the electronics box, reading diagnostic codes or touching the PCB are all engineer work. So is the D.050 minimum fan speed offset.

Thawing a frozen condensate pipe is the one people ask about most, so here is the straight answer. Plenty of guides do describe pouring warm water over an accessible outside condensate pipe. We do not give it as a step on this page, because F77 is not simply a frozen pipe: the code is raised by the pump and the feedback signal behind it, and those need checking whatever the ice is doing. Look for the ice, report it when you book, and let the engineer deal with the pipe and the pump together.

If the heating is off and the house is getting cold while you wait, use a plug-in electric heater in the room you are actually sitting in, and keep it clear of furniture and curtains. Never use a gas hob or an oven to heat a room.

Is F77 the same as F75, F76, F78 or S.44?

No, and F75 is the one people mix it up with most. Vaillant's installation manual describes F75 as no pressure change detected when the pump starts, caused by the water pressure sensor or the pump, air in the heating system, or too little water in the product. Vaillant's public consumer page words the same code as the pressure change being too low at start-up. F75 is one of the few F.7 codes Vaillant does publish on that public page, so it is easy to land on by accident when you are hunting for F77.

F76 is different again: the overheating protection on the primary heat exchanger has responded, caused by the cable or connections for the safety fuse in the primary heat exchanger, or by the heat exchanger itself being defective. F78 is an interruption to the hot water outlet sensor at the external controller, which happens where a UK link box is connected but the hot water NTC has not been bridged.

The closest relative to F77 is not an F code at all. S.44 is a status code for exactly the same condition, described by Vaillant as "Flue non-return flap response blocks burner operation (only in conjunction with accessory VR40) or condensate pump defective; heat requirement is blocked". Your boiler shows S.44 while it is holding the burner off, and F77 once it has locked out. If you noticed S.44 earlier in the day, that was the same problem building.

F72 is worth naming for a different reason. It is a flow or return temperature sensor fault, and it is on Vaillant's public consumer page, so people searching that page for F77 often end up reading F72 instead and wondering why nothing matches what their boiler is doing.

When should I call a Gas Safe registered engineer?

As soon as F77 comes back after a single reset. There is no second reset worth trying, because the boiler is not confusing itself, it is waiting for a signal that is genuinely not arriving. Call sooner if you can see water pooling around or under the boiler, since that points at an overflowing pump. Keep clear of the water and away from the boiler's electrics until someone has looked at it.

It helps to know what the visit involves, so the quote makes sense when you get it. The engineer works out first whether your system has a condensate pump, a motorised flue non-return flap, or both. On a home boiler it is usually the pump, so they can check whether it has power, whether it runs, whether its reservoir is full and whether its outlet or the condensate route is blocked or frozen. On a cascade or commercial system they can test the flap, its feedback switch and the cable to the VR 40 "2 in 7" module, and check the bridge on the "burner off contact" terminals. Only when the pump and flap are both proven good do they look at the boiler's feedback input and the PCB.

That range is why F77 cannot be priced from the display. Clearing a blocked condensate route and replacing a control board are the same code and very different pieces of work, so the fault has to be diagnosed before anyone can tell you what it takes to put right.

For the F77 showing now, book a one-off call-out and diagnosis, or call us on 0333 772 6247. A Gas Safe registered engineer attends, diagnoses the fault and quotes any parts and extra labour before that work starts. One-off appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays, and we confirm the appointment with you by phone before an engineer is sent. Two things to know up front: one-off works carry no workmanship guarantee, and if we cannot get access on the day the appointment is cancelled and not refunded. A fixed price repair does not include the condensate pump, flue components or the boiler PCB, so an F77 fix is quoted separately after the diagnosis.

Cover is a different product, and it will not pay for this one. Boiler and central heating cover excludes condense pumps, any repair relating to flues, and any works where the cause of the issue is freezing, so an F77 would not normally be a covered repair. Cover is also for future breakdowns, not a fault your boiler is already showing, so an F77 on the display today counts as pre-existing. For the faults the module does cover, it pays parts and labour up to £500 in a 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and anything above that limit you pay yourself. Using a service starts a 12-month agreement period, a £95 call-out fee applies in your first 30 days and for faults the plan does not cover, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance.

One safety note, whatever your boiler is doing. F77 is a condensate and flue-flap interlock, not a gas fault. But if you ever smell gas, or you suspect carbon monoxide, do not wait for an engineer. Leave the gas alone, open a window, get everyone out and call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F77 fault code FAQs

Seeing F77? Let's get a call-out and diagnosis booked.

For the F77 showing on your boiler now, book a one-off call-out and diagnosis. A Gas Safe registered engineer attends, works out whether it is the condensate pump, the condensate route or the flue non-return flap, and quotes any parts and extra labour before that work starts. One-off appointments run Monday to Friday, 09:00 to 17:00, excluding bank holidays, and we confirm the appointment with you by phone before an engineer is sent. One-off works carry no workmanship guarantee, and if we cannot get access on the day the appointment is cancelled and not refunded. A fixed price repair does not include the condensate pump, flue components or the boiler PCB, so an F77 fix is quoted separately after the diagnosis. Cover is a different product. It is for future breakdowns, not a fault your boiler is already showing, so an F77 on the display today counts as pre-existing, and the boiler and central heating module separately excludes condense pumps, repairs relating to flues, and any works where the cause of the issue is freezing. For the faults it does cover, it pays parts and labour up to £500 in a 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and anything above that limit you pay yourself. Cover is modular, so you pick only what you want, and Smart Plan is a service plan, not insurance. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits and for early annual-service requests. Under a cover plan we aim to get an engineer out to you within two working days, and cover-plan call-outs run Monday to Friday, 08:00 to 18:00, with an engineer attending outside those hours only for a genuine emergency breakdown. Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. We work with a national network of thousands of Gas Safe registered engineers, and Smart Plan comes from UK Boiler Company Ltd, trading since 2014. Cover and our one-off appointments are for domestic boilers only, so commercial boilers are not included. Prefer to talk it through? Call 0333 772 6247 or email hello@smart-plan.com.

Helpful links

• Vaillant boiler help and guides
• All Vaillant fault codes
• Vaillant F72 fault code
• Vaillant F73 fault code
• Vaillant F71 fault code
• Vaillant fault-code help hub
• Boiler repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register