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Vaillant boiler F29 fault

What is the cause of a Vaillant F29 fault?

A Vaillant boiler triggers an F29 fault when the boiler has ignited the gas but the flame has gone out. It is a very common Vaillant fault.

vaillant f29 fault description — F29 fault on a Vaillant.

No gas credit

The most common reason behind a F29 fault is running out of gas. This doesn’t apply to direct debit gas meters.

If you have run out of gas while using the boiler it will display an F29 fault.

The solution is to buy more gas credit and press the reset button.

Many gas meters lock out when credit runs out as a safety feature. Each meter has a different method of unlocking. It normally displays instructions on the screen if you press a button.

Faulty main PCB

Vaillant boilers have a reputation for being reliable. However their PCBs can be a bit fragile. A Gas Safe registered heating engineer must change the PCB, as it is an expensive part and is safety critical.

Faulty display PCB

The F29 fault (or any other fault) is stored on the small display PCB. If this small PCB develops a fault resetting becomes impossible.

This problem can show itself during a normal service. The engineer must check the FSD (flame supervision device) by turning the gas off which causes a F29 fault.

A Gas Safe registered heating engineer must change the PCB, as it is safety critical.

Faulty flue seals.

The flue of a modern condensing boiler has two pipes, one inside the other. The outer pipework carries clean air for combustion. The inner carries the products of combustion outside.

The seals can fail between the two pipes. This causes the clean air to be contaminated with products of combustion.

The boiler requires clean air to burn the gas properly. If the gas isn’t burning properly an F29 will be displayed.

Changing flue seals is for a qualified gas safe engineer. Do not attempt this yourself.

Blocked condensate pipe or trap.

If the condensate pipe is blocked then the boiler can fill with water. This stops the burning of gas causing an F29 faulty.

Blocked condensates are normally at the connection between the smaller overflow pipework and the main waste pipework in the property.

Newer Vaillant boilers have condensate siphons (traps) that have an internal float. This float can get stuck causing a build up of condensate in the boiler.

To repair this clean the trap.

Faulty ignition electrodes.

Very unlikely again as Vaillant electrodes are pretty reliable.

Blocked flue.

Check the end of the flue to make sure nothing is blocking it. A bit unusual but easy to fix. If the flue is blocked then it must be unblocked then reset the boiler.

Faulty gas valve

A faulty gas valve can cause an F29 faulty. This is one of the more unusual faults as Vaillant gas valves tend to be very reliable. A gas safe engineer must diagnose and fit the gas valve.

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