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F77

Saunier Duval F77 boiler code: flue flap or condensate pump fault

Verified with sources2 sourcesReviewed Oct 4, 2026
Technician coming? See the 3 steps they must show you
Wall-mounted combi gas boiler installed in a home
Kai KemmannCC BY-SA 4.0Image source

Risk level

High

Professional inspection is recommended.

Urgency

High

Quick answer

On the Saunier Duval ThemaFast Condens, F.77 reports a fault in the flue gas non-return flap or in the condensate pump. Press reset and wait five seconds, once; if F.77 stays, switch the boiler off and call the official service.

Safety first

Stop and check this first

  • If you smell gas while F.77 is on the display, avoid flames and do not use switches, plugs, doorbells or phones. Close the meter valve and, if you can, the appliance gas cock, warn neighbours without ringing, leave the building and call the gas supplier from outside.
  • F.77 involves the flue side: carbon monoxide (CO) has no smell. If anyone has a headache, dizziness or nausea, or a CO alarm sounds, switch the boiler off, ventilate, leave the home and call the official service.

Safe checks you can perform

  • Look at the condensate drain and its funnel
  • Reset once with the fault reset key
  • Watch for a smell of fumes indoors
  • Flue flap and condensate pump inspection

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Test the flue non-return flap
  • Do not attempt this professional task: Check the condensate pump and its drain
  • Do not attempt this professional task: Leave the condensate trap full

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.

  1. Test the flue non-return flap

    What they doThe engineer checks that the combustion gas non-return flap moves and closes as it should.

    They must show youthe flap working after the check.

    In writingthe flap condition and whether it was replaced.

  2. Check the condensate pump and its drain

    What they doThey confirm the condensate pump works and that condensation water flows freely to the drain.

    They must show youthe pump clearing water during a test.

    In writingwhether the pump was repaired or replaced.

  3. Leave the condensate trap full

    What they doBefore closing the boiler, they make sure the condensate trap is filled with water.

    They must show youthe full trap before the casing goes back on.

    In writingthat the trap was checked.

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.

  1. Look at the condensate drain and its funnel

    No special tools

    Who:
    you, by eye, as the care chapter of the user manual asks.
    How:
    look at the condensate drain pipe and drain funnel and check whether you can see or feel anything blocking the water flow; do not dismantle the trap or the pump.
    Result that matters:
    spotting a visibly blocked drain, which the engineer should know about before checking the condensate pump.
    If confirmed:
    if you see a blockage, mention it to Customer Service when booking the F.77 visit.
    If not confirmed:
    if the drain looks clear, the check moves on to the flap and the pump.
    Evidence to keep:
    photo of the drain pipe and funnel as you found them.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

  2. Reset once with the fault reset key

    No special tools

    Who:
    you, with the fault reset key on the panel.
    How:
    press the reset key and wait five seconds, as the user manual fault table says; do it a single time.
    Result that matters:
    seeing whether the ThemaFast runs again or F.77 comes straight back.
    If confirmed:
    if F.77 persists, inform the official service and do not repeat the reset.
    If not confirmed:
    if the boiler starts, note the message so it can be raised at the next service.
    Evidence to keep:
    time of the reset and whether F.77 returned.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

  3. Watch for a smell of fumes indoors

    No special tools

    Who:
    you, paying attention in the boiler room.
    How:
    notice whether there is a smell of fumes or combustion gases near the appliance or in other rooms while F.77 is showing.
    Result that matters:
    ruling out flue gases escaping indoors through a flap or trap problem.
    If confirmed:
    if you smell fumes, open doors and windows, switch the appliance off and call an authorised professional, as the manual says.
    If not confirmed:
    with no smell, keep the boiler off if F.77 stays and wait for the engineer.
    Evidence to keep:
    note of when and where you noticed the smell, if there was one.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

  4. Flue flap and condensate pump inspection

    Tools required

    Who:
    authorised gas engineer or Saunier Duval official service.
    How:
    check that the combustion gas non-return flap responds, inspect the condensate pump and its discharge and make sure the trap is left full.
    Result that matters:
    pinning down which of the two F.77 causes is present.
    If confirmed:
    repair or replace the failing part and test operation with the boiler closed.
    If not confirmed:
    if both parts work, check the connections and run a fresh start-up.
    Evidence to keep:
    job sheet stating whether the fault was in the flap or the pump.

    The flue system and the inside of the boiler may only be worked on by qualified installers; the manual forbids tampering with safety devices.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

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
Saunier Duval
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 ThemaFast Condens error code annex, F.77 is listed as a flue gas flap or condensate pump fault. There are two possible causes: the non-return flap for combustion gases is not responding, or the condensate pump is faulty. Both parts belong to the exhaust side: one stops flue gases flowing back down the duct and the other clears condensation water. The user manual reminds you that the condensate trap must always be full so flue gases cannot escape into the room.

Warnings and stop conditions

  • Criticaleditorial warning

    If you smell gas while F.77 is on the display, avoid flames and do not use switches, plugs, doorbells or phones. Close the meter valve and, if you can, the appliance gas cock, warn neighbours without ringing, leave the building and call the gas supplier from outside.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

  • Criticaleditorial warning

    F.77 involves the flue side: carbon monoxide (CO) has no smell. If anyone has a headache, dizziness or nausea, or a CO alarm sounds, switch the boiler off, ventilate, leave the home and call the official service.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

Symptoms and causes

Probable causes · in order

  • Flue gas non-return flap not respondingPossible

    The ThemaFast Condens table lists as first cause that the non-return flap for combustion gases does not respond. That flap stops flue gases flowing back through the exhaust outlet.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00

  • Faulty condensate pumpPossible

    The second cause of F.77 is a faulty condensate pump. If condensation water is not cleared, the boiler stops running to protect the installation.

    Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00

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.

Start diagnosis

Sources and technical references

  1. 1

    Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00

    Saunier Duval · 2016 · manual

  2. 2

    Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00

    Saunier Duval · 2016 · manual

Sources: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00reviewed 2026-10-04 · verified

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