Safety: Flue and combustion air faults can lead to dangerous operation. Only a qualified engineer should work on these systems.

Inspect flue terminal: From outside, check that the flue terminal is not blocked by debris, snow, or vegetation.
Check condensate drain: Ensure the condensate trap and pipe are not blocked or frozen, as this can affect flue gas flow.
Examine pressure switch tubing: With power off, inspect the silicone tubes from the fan to the air pressure switch for splits, kinks, or blockages. Replace any damaged tubing.
Test air pressure switch: Using a manometer or continuity tester, verify that the switch changes state when the fan runs. Replace the switch if it does not operate within Vaillant’s specified pressure range.
Verify flue installation: Check that flue length, bends, and slope comply with Vaillant instructions. Correct any non-compliant sections.
Reset and test: After rectifying issues, reset the heater and confirm stable operation without F.33 reappearing.

F.33 air/flue fault; appliance does not ignite or shuts down shortly after starting

Possible Causes