Vaillant F35 Fault Code: Causes and Step-by-Step Fixing Guide
If your Vaillant boiler is showing the F35 fault code, it means there’s an issue with the air/flue gas duct system. This is a critical part of your boiler that ensures safe combustion and ventilation. When it goes wrong, your boiler may lock out for safety reasons.
Table of contents
What Does the Vaillant F35 Fault Code Mean?
The F35 error points to a problem in the air or flue gas duct. The boiler isn’t able to maintain safe combustion airflow, which could be caused by:
- Blocked or obstructed flue outlet
- Damaged or leaking flue pipes
- Incorrect installation of air/flue routes
- Faulty air pressure sensor or pressure switch
- Incorrect D132 setting in multiple-flue systems
Because this involves both air supply and combustion gases, it’s essential to take the error seriously.
Step-by-Step Troubleshooting & Fixing the Vaillant F35 Fault Code
Time needed: 5 minutes
Here’s how you (or your engineer) can investigate and resolve the issue:
- Check for Flue Obstructions
Go outside and inspect the flue outlet.
Look for blockages such as debris, leaves, bushes, or even birds’ nests.
If blocked, carefully remove the obstruction. - Inspect Flue Pipes for Damage or Leaks
Check both the internal and external flue pipes.
Look for cracks, gaps, or loose fittings.
If damage is found, the section may need replacing. - Verify Air/Flue Gas Route Installation
Ensure that the flue system is correctly installed.
Misalignment, poor sealing, or incorrect routing can trigger the F35 code. - Test the Air Pressure Sensor or Switch
The air pressure sensor/switch monitors airflow.
If faulty, it can send incorrect readings and trigger the F35 code.
A professional can test and, if necessary, replace it. - Check D132 Setting (for Multiple-Flue Systems)
In multiple-flue setups, ensure D132 is configured correctly.
Incorrect settings can cause the boiler to misread air/flue pressure. - Deactivate Air/Flue Pipe Function (D145)
As a last resort, some engineers may adjust the D145 setting to deactivate the air/flue pipe function.
This must only be done by a qualified Gas Safe engineer.
When to Call a Gas Safe Engineer
While you can carry out basic visual checks for blockages and clearance, most F35 fault fixes require professional expertise. Call a Gas Safe engineer if:
- You suspect flue damage or leaks
- The air pressure sensor/switch needs replacing
- The fault persists even after clearing obstructions
Conclusion
The Vaillant F35 fault code signals a serious issue with the air and flue system. While minor obstructions can sometimes be resolved easily, most fixes involve checking and repairing internal flue components something that should only be handled by a Gas Safe registered engineer.
Frequently Asked Questions
This error is received if the central heating inlet temperature sensor is short circuit, open-circuit, or damaged. Domestic water and central heating request is stopped. When this error is deleted, normal operation begins.
What does F72 mean on a Vaillant boiler? The F72 fault code indicates a discrepancy between the flow and return temperature sensors.
F codes are typically the most common and are used to display various issues in your boiler, whether minor or major. The “F” can mean either fault or failure and is the main indication of whether a general issue has occurred in the boiler.
