Vaillant Boiler F.29 — Flame Loss During Operation

What Does This Error Mean?

F.29 means the burner lit but the flame was lost during operation. Common causes are low gas pressure, a dirty flame sensor, restricted flue, or condensate trap issues.

Affected Models

Vaillant ecoTEC Pro
Vaillant ecoTEC Plus
Vaillant ecoFIT
Vaillant Turbomax
Vaillant ecoMAX

Common Causes

Low gas pressure (low LPG tank, partially closed valve)
Dirty ionisation/flame sensor electrode
Blocked or partially blocked flue
Frozen or blocked condensate trap
Faulty gas valve cycling on and off

How to Fix It

Reset the boiler.

Press and hold the reset button for 5 seconds. F.29 may clear after a one-time disturbance. If it returns, the cause needs investigation.
Check the condensate pipe.

A blocked or frozen condensate pipe causes flame loss. If in winter, thaw the external pipe with warm water. If blocked by debris year-round, an engineer can clean the trap.
Check the gas supply.

Verify the gas valve at the boiler and meter are open. LPG tanks below 25% can deliver inadequate pressure. Test other gas appliances to rule out a supply problem.
Call a Gas Safe engineer.

F.29 caused by ionisation sensor or flue issues requires professional service. The engineer will measure gas pressure, clean the flame sensor electrode, and inspect the flue for blockages.

When to Call a Professional

Persistent F.29 requires a Gas Safe engineer to test gas pressure, clean the flame sensor, and check the flue. Internal boiler work is not safe for non-registered persons.