Vaillant F29 fault: flame lost while the boiler was running

What F29 means in plain terms

A boiler watches its own flame all the time it is running. A small electrode sits in the flame and carries a tiny electrical signal through it, so the control board always knows the burner is alight. If that signal disappears part way through a heating or hot water cycle, the boiler shuts the gas valve straight away and tries to light again.

F.29 is what you see when that has happened and the relight attempts have also failed. The boiler is doing its job: it will not keep feeding gas to a burner it cannot prove is lit. The code tells you the flame was lost during operation, not why, so the checks below are about narrowing down the reason.

Check 1: is gas reaching the boiler?

Try the hob or another gas appliance if you have one. If that will not light either, the problem is the supply, not the boiler. On a prepayment meter, check the credit, including whether emergency credit has run out overnight. A meter running out while the boiler is firing is a classic F.29, because the flame is lost mid-cycle.

Check the emergency control valve next to the gas meter. When it is on, the handle runs in line with the pipe. If someone has recently had work done on the gas, or the gas company has been out, it is worth confirming the valve was left fully open.

If the hob works but only lights weakly, or the flames lift and flicker, the gas pressure reaching the house may be low. That is not something to adjust yourself. Tell the engineer, because low working pressure at the boiler is one of the things they will measure.

If you smell gas at any point, stop. Open the windows, do not use switches or anything that could spark, turn the gas off at the meter if you can do so safely, leave the property and call the National Gas Emergency Service on 0800 111 999.

Check 2: has the condensate pipe frozen?

Condensing boilers drain water through a plastic pipe, usually white or grey, that often runs outside to a gully or drain. In a hard frost that pipe can freeze. The water backs up inside the boiler, the flame is disturbed and the boiler shuts down. On a Vaillant this can show as F.28 or F.29, sometimes with a gurgling sound from the boiler before it stops.

If the outside section is frozen, pour warm water, not boiling, along the pipe, working from the open end back towards the wall. Then reset once. Our guide to a frozen condensate pipe explains how to find the pipe and what to do if it keeps freezing. A pipe that freezes every cold snap is usually too long, too narrow or too exposed, and rerouting it is a small job compared with repeated winter breakdowns.

Check 3: anything blocking the flue outside?

Look at where the flue leaves the house, without climbing or removing anything. A bird's nest, a bag caught against the terminal, a new fence panel, shrubs grown up around it or building work nearby can all upset the air and flue gases the boiler relies on. If flue gases are drawn back into the air intake, the flame can become unstable and go out.

Do not try to clear anything from inside the terminal. Note what you see and tell the engineer. If the flue runs through a loft or boxing and you notice any damage, staining or a smell of fumes, switch the boiler off and treat it as urgent.

Should you reset it?

Once you have ruled out the gas supply and a frozen pipe, one reset from the boiler's control panel is reasonable. If the cause was an empty meter or a thawed pipe, the boiler should run normally from then on.

If F.29 returns, stop resetting. A boiler that repeatedly loses its flame is telling you something is wrong with the gas it receives, the air it breathes or the way it senses the flame. Our guide on how to reset a boiler explains when a reset is appropriate, and boiler lockout keeps returning explains why repeated resets make diagnosis harder, not easier.

What causes F29 inside the boiler

When the supply is fine, an engineer will usually work through the following. All of them sit behind the sealed casing or involve the gas and flue, so they are Gas Safe work only.

• Low gas pressure at the boiler: a partly closed valve, an undersized gas pipe or a faulty meter regulator can starve the burner once it is at full rate.
• Flue gas recirculation: a damaged, poorly sealed or blocked flue can let exhaust gases back into the air supply and snuff out the flame.
• Blocked condensate trap or siphon inside the boiler, even when the outside pipe is clear.
• Worn or dirty ignition and flame-sensing electrode, or a damaged lead, so the boiler loses the flame signal even though the flame is there.
• A faulty earth on the appliance, which Vaillant's manual lists alongside an interrupted gas supply as a cause of F.29.
• A gas valve or control board fault, usually confirmed only after the simpler causes are ruled out.

What to write down before the engineer arrives

A few details save time on the visit. The exact code, including the F and the full stop. The model name from the front of the boiler. Whether the fault appears at the start of a cycle, after it has been running for a few minutes, or only when heating and hot water are both on. What the weather was doing. Whether the hob works normally. How many times the boiler has been reset and whether any other codes, such as F.28 or F.75, have appeared.

If the boiler is old or has had several faults in a short time, it is fair to ask whether a repair is still good value. Our guide on whether to repair or replace a boiler sets out how to weigh that up, but get a proper diagnosis first. Many F.29 faults are cleared by fixing the gas supply, the flue or the condensate route rather than replacing parts.

Getting a Vaillant F29 fixed

Loyal Plumbers works across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Gas work is carried out by Gas Safe registered engineers. When you call, give us the code, the model and what you have already checked, especially the gas supply and the condensate pipe.

Our call-out charge is £60. The engineer will explain what they have found and what the repair involves before any work starts. If there are vulnerable people in the house and no heating or hot water in cold weather, tell us when you call. More on how we handle breakdowns is on our boiler repair and servicing page.