This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Vaillant F.35 fault codewhat it means and how to fix it
The air intake or flue is blocked. Something is physically obstructing the path combustion products take out of the building.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.35
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 F.35 means
Vaillant describe F.35 plainly as the air or flue pipe being blocked, and it is a code of the combustion-sensing boilers: the ecoTEC exclusive tables print it as "fault: air/flue gas duct" with an instruction to check the whole air and flue pipe for permitted design, restriction or blockage, and damage, and the 2022 ecoTEC plus CS/1-5 shows it as F.035. An ecoTEC plus or pro of 2012 to 2021 does not have the row; a blocked flue on those shows up through F.28, F.29 or the fan code F.32 instead.
Every condensing boiler draws combustion air in and pushes flue gases out, usually through a concentric flue where one pipe sits inside the other, and the appliance monitors that path continuously. When resistance rises beyond what the fan can work against, the appliance stops rather than continuing to burn gas with restricted extraction. That is exactly the right behaviour and it is the reason this code should never be worked around.
The causes are almost always physical and often visible from outside. Bird nests are the classic, and spring is the season for them — a flue terminal is a warm, sheltered cavity and it is attractive to nesting birds for the same reasons a chimney is. Beyond that: leaf litter, wind-driven snow packing the terminal, render or insulation forced in during external wall work, and on rare occasions a flue that has partially collapsed internally.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Bird nest in the flue terminal | The classic cause and heavily seasonal towards spring. A terminal without a guard is an inviting cavity. | Common |
| Debris or leaf litter in the terminal | Especially where the flue discharges low down, under an overhang, or near trees. | Common |
| Snow or ice packing the terminal | Wind-driven snow can pack a terminal solid in hours. Usually clears itself as the weather turns, which is why the fault can appear to come and go. | Occasional |
| Building work debris | Render, insulation, or dust forced into the terminal during external wall insulation or rendering. Suspect this immediately if work has been done recently. | Occasional |
| Internal flue damage or collapse | A displaced joint or a sagging section in a long horizontal run. Uncommon, but it is the one that needs proper investigation rather than clearing. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£450
Clearing an accessible terminal is £90 to £180. A terminal guard fitted is £60 to £120. Work at height or a flue investigation with access equipment runs £200 to £450.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
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.77Get it checkedThe 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.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.35
Can I just poke the nest out with a broom?
Please do not. You risk pushing material deeper into the flue where it cannot be reached without dismantling, and if the terminal is at height you are risking considerably more than that. It also matters legally: some nesting birds are protected during the breeding season, and an engineer will know how to handle that properly.
Is it dangerous to have a blocked flue?
It would be if the appliance carried on running, which is precisely why it does not. The safety system has worked. The danger comes from overriding it or from repeatedly resetting to force short bursts of operation, which you should not do.
How do I stop it happening again?
A terminal guard is inexpensive and solves the nesting problem outright. If the fault was snow, the answer is usually about siting rather than guards, and an engineer can advise whether the terminal position meets current guidance.
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 support portal
- Manufacturer technical and service documentation
- Trade body guidance
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.