- Boilers
- Gas boiler
F77
Vaillant F.77 on a gas boiler: flue flap or condensate pump
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Vaillant F.77 means there is no feedback from the flue gas flap or the condensate pump is overflowing. Look for water under the boiler or by the condensate pump and notice any smell of fumes. Call an authorised professional.
Safety first
Stop and check this first
- If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
- If you smell fumes indoors: Vaillant says to open doors and windows fully, switch the appliance off and call an authorised professional. A flue fault can fill the home with carbon monoxide, which has no smell: if you feel headache, dizziness or nausea, ventilate, get out and call for help. A CO alarm helps.
Safe checks you can perform
- Smell of gas or fumes
- Water under the boiler or by the pump
- Condensate drain and funnel
- Flap, pump and wiring check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Condensate pump test
- Do not attempt this professional task: Flue gas flap and feedback switch
- Do not attempt this professional task: VR 40 cable, bridges and fault memory
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Condensate pump test
What they doThe technician checks the condensate pump clears the water and its outlet is not blocked.
They must show youthe pump running and the drain clear.
In writingwhat was cleaned or the pump replaced and its reference.
Flue gas flap and feedback switch
What they doThe technician checks the non return flap moves and its switch sends feedback to the boiler, and checks its wiring.
They must show youthe flap opening and the feedback received.
In writingthe condition of the flap and what was repaired.
VR 40 cable, bridges and fault memory
What they doThe technician checks the cable to the VR 40 module, the contact thermostat bridges and the multifunction module, and reads the fault list.
They must show youthe fault list and the connections checked.
In writingthe codes found and the connection corrected.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Smell of gas or fumes
- Who:
- you.
- How:
- without any flame nearby, notice whether there is a smell of gas or fumes near the boiler.
- Result that matters:
- whether there is any smell.
- If it smells of gas:
- follow Vaillant's gas warning and leave the home.
- If it smells of fumes:
- open doors and windows, switch the appliance off and call an authorised professional.
- Evidence to keep:
- a note of the time and what you noticed.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Water under the boiler or by the pump
- Who:
- you.
- How:
- without taking anything apart, look for water under the boiler or around the condensate pump, if the installation has one.
- Result that matters:
- whether there is water or signs of overflow.
- If there is water:
- do not touch the pump; tell the professional, it points to overflow.
- If it is dry:
- it points to the flap or the wiring.
- Evidence to keep:
- photos of the area.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Condensate drain and funnel
- Who:
- you.
- How:
- look at the condensate drain pipe and the funnel, as Vaillant asks you to check regularly, without taking them apart.
- Result that matters:
- whether you can see or feel anything blocking the flow.
- If there is a blockage:
- do not clear it yourself; have a qualified installer fix it.
- If it is clear:
- tell the technician so the drain is ruled out.
- Evidence to keep:
- photos of the pipe and funnel.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Flap, pump and wiring check by the technician
- Who:
- authorised professional.
- How:
- checks the condensate pump, the flue gas flap with its wiring and feedback switch, the cable to the VR 40 and the bridges.
- Result that matters:
- which accessory is blocking the burner.
- If the pump overflowed:
- it is cleaned or replaced and the drain is tested.
- If the flap does not respond:
- the flap or its wiring is repaired.
- Evidence to keep:
- a report of what was checked and replaced.
The flap is part of the flue system and Vaillant forbids the user from modifying the flue or condensate drain: authorised professional only.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01 · Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Vaillant
- Product type
- Gas boiler
Known code variants
- caldera Saunier Duval F77
- ecoTEC F.77
- Error F77 Vaillant
- F.77
- Saunier Duval F.77
- Saunier Duval F77
- ThemaFast Condens F.77
- ThemaFast F77
- Vaillant F.77
What it means
In the ecoTEC plus table, F.77 is fault: flue gas flap or condensate pump. Vaillant gives two situations: no feedback signal from the flue gas flap, or the condensate pump overflowing. It says to check the cable to the VR 40 accessory, the flue flap (wiring and feedback switch), the condensate pump, the contact thermostat bridges and the multifunction module. It only appears on installations with those accessories; the code does not say which one fails.
Warnings and stop conditions
- Criticalsource verified
If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
- Higheditorial warning
If you smell fumes indoors: Vaillant says to open doors and windows fully, switch the appliance off and call an authorised professional. A flue fault can fill the home with carbon monoxide, which has no smell: if you feel headache, dizziness or nausea, ventilate, get out and call for help. A CO alarm helps.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Symptoms and causes
Probable causes · in order
- Condensate pump overflowPossible
If the installation has a condensate pump and there is water around it or the drain is blocked, the pump has overflowed.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Flue gas flap not respondingPossible
If there is no water and the installation has a flue non return flap, the boiler gets no feedback signal; the flap or its switch may fail.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Accessory wiring or bridgesPossible
If pump and flap work, Vaillant says to check the cable to the VR 40, the contact thermostat bridges and the multifunction module.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Virtual technician
Does F77 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Vaillant · 2017 · manual
- 2
Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Vaillant · 2017 · manual
Was this entry useful?