- Boilers
- Gas boiler
347
Viessmann 347 on a Vitodens boiler: flue gas recirculation
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On a Vitodens 100-W, code 347 means the boiler has detected flue gas recirculation and is in a fault state. Keep air openings clear, do not reset repeatedly and have a gas technician inspect the flue system.
Safety first
Stop and check this first
- Code 347 concerns combustion or flue gas discharge: if a carbon monoxide (CO) alarm sounds or you feel headache, dizziness or nausea, get into fresh air, call emergency services and do not go back in until told it is safe.
- If you smell gas while code 347 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
- With 347, do not keep resetting: the user manual warns that repeated resets of an unresolved fault can have fatal consequences.
Safe checks you can perform
- Record code 347
- Make sure air openings are not covered
- Act on any smell of flue gas
- Professional flue inspection
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flue leak check
- Do not attempt this professional task: Look for flue gas back pressure
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.
Flue leak check
What they doA qualified gas technician checks the Vitodens flue system for leaks and repairs them before the boiler is returned to service.
They must show youthe leak check result for each flue section.
In writingany leaks found and how they were repaired after 347.
Look for flue gas back pressure
What they doThe technician checks flue fall, constrictions and blockages that create back pressure, and corrects the routing where needed.
They must show youthe measured fall and any blockage or constriction found.
In writingthe correction made and the final reset with combustion verified.
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.
Record code 347
- Who:
- you, from the Vitodens display.
- How:
- open the fault list as the user manual explains, note 347 and whether the burner lockout symbol is flashing.
- What matters: making clear to the technician that the Vitodens detected flue gas recirculation rather than another lockout.
- If the code persists:
- if 347 remains, leave the boiler off and call the technician without further start attempts.
- If it has cleared:
- if 347 has cleared, keep your note: the flue still needs inspecting.
- Evidence to keep:
- dated photo of code 347 on the display.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Make sure air openings are not covered
- Who:
- you, looking without removing anything.
- How:
- the Viessmann user manual forbids blocking or closing air inlet openings; check that no furniture, clothes or covers sit in front of them, leaving the Vitodens flue untouched.
- What matters: ruling out something you placed restricting air while you wait for the 347 inspection.
- If the code persists:
- if nothing needs moving, 347 points to the flue and only the technician checks it.
- If it has cleared:
- if you moved something, tell the technician: 347 still requires the flue to be inspected.
- Evidence to keep:
- photo of the clear air openings after code 347.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Act on any smell of flue gas
- Who:
- you, if you notice combustion fumes.
- How:
- the user manual says to switch off the heating system, ventilate the room where the boiler is installed and close the doors to living areas if you smell flue gas.
- What matters: not staying exposed to fumes while the Vitodens shows 347.
- If the code persists:
- if the smell persists after ventilating, leave the home and call emergency services and the service company.
- If it has cleared:
- if there is no smell, keep the Vitodens off with 347 until the technician visits.
- Evidence to keep:
- time 347 appeared and whether you noticed flue gas smell.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Professional flue inspection
- Who:
- qualified gas technician with access to the Vitodens flue.
- How:
- checks the whole flue for leaks and looks for back pressure from insufficient fall, constrictions or blockages, as the table for 347 requires.
- What matters: finding the exact point in the flue causing 347.
- If the code persists:
- the technician must correct the flue before resetting and checking combustion.
- If it has cleared:
- ask for written confirmation of the inspection even if 347 does not recur during testing.
- Evidence to keep:
- job sheet with the flue section inspected and the correction made for 347.
Removing, sealing or altering the Vitodens flue is work for a qualified gas technician only.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
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
- Viessmann
- Product type
- Gas boiler
Known code variants
- Viessmann 347
- Viessmann error 347
- Vitodens 100-W 347
- Vitodens 347
- Vitodens fallo 347
What it means
The Viessmann service table assigns 347 to flue gas recirculation, with the burner in a fault state. The measures focus on the flue system: check it for leaks and for flue gas back pressure, for example caused by an insufficient fall in the flue, constrictions or blockages. Only after that is corrected is the appliance reset. The user manual also treats a blockage in the air supply or flue outlet as a matter for the installer.
Warnings and stop conditions
- Criticaleditorial warning
Code 347 concerns combustion or flue gas discharge: if a carbon monoxide (CO) alarm sounds or you feel headache, dizziness or nausea, get into fresh air, call emergency services and do not go back in until told it is safe.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Criticalsource verified
If you smell gas while code 347 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026)
- Highsource verified
With 347, do not keep resetting: the user manual warns that repeated resets of an unresolved fault can have fatal consequences.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Symptoms and causes
Probable causes · in order
- Leak in the flue systemPossible
Viessmann directs a check of the flue system for leaks, with any leak repaired before code 347 is reset.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Back pressure in the fluePossible
The table gives an insufficient fall in the flue, constrictions or blockages as examples that stop flue gases leaving normally.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Virtual technician
Does 347 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
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