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Vaillant F35 Fault Code

This page contains the information required to help repair and diagnose the fault codes being shown on the boiler display. The following information, will take you through the fault finding sequence or steps, for rectifying a Vaillant Ecotec boiler with this fault code. Suggested solutions will be detailed further down the page.

Vaillant F35 / F.35 Fault Code

This fault code suggest that there is a problem along the Air/Flue route. According to manufacturers instructions, the exact meaning is that there is ‘air/flue gas duct fault’.

Vaillant F35 / F.35 Fault Code Repair

In order to repair this fault we suggest a few basic checks in order to determine if an engineer visit is required for determining if this fault is being caused by another component.

vaillant flue duct
vaillant flue duct

First check that the flue outlet is not restricted in any way. See diagram above. Check that there are no bushes or tree coverings obstructing the flue. Also from outside, check there are no obstructions in both the inner and outer tube.

vaillant front case
vaillant front case no 2.

If you are unable to inspect the flue, it would be a good idea to remove the front outer case (item 2 shown in diagram above) of the boiler to determine if the flue needs further investigation. This will allow the ventilation air to be taken from the room instead of the outside. The gas flue products will still continue to go directly outside unless that part of the flue is blocked, in which case you would get a different fault code.

There is also a final posibility that there is some form of internal blockage on the inlet of the fan, preventing air from being pulled in. This would require an engineer to investigate further.

Disclaimer: Information on this website is provided for informational purpose therefore we cannot accept liability for subsequent problems. All repair tasks should be carried out by qualified trades people. We may also receive commission payments when you are taken to external websites and complete purchases from third party sellers, once you leave our site, we cannot be held liable for your purchases.