Vaillant F27 fault: flame detected when it should not be

What flame simulation means

A boiler has to know for certain whether its burner is alight. It does this by passing a small electrical signal through the flame using an electrode: when there is a flame, the signal flows, and when there is not, it does not. That tells the boiler it is safe to keep the gas on, or that it must shut it off because the flame has gone out.

F.27 appears when the boiler sees that flame signal at the wrong time, when the gas valve has been told to close and there should be no flame. Either there really is a small flame because gas is still getting through, or something is making the electronics think there is one. Both are reasons for the boiler to stop, which is exactly what it does.

The usual causes

An engineer will work through the possibilities in a sensible order rather than replacing the most expensive part first.

• Gas valve not closing fully: if the valve lets a small amount of gas through when it should be shut, a real flame can remain. This needs testing by a Gas Safe engineer and the valve replacing if it is at fault.
• Moisture or condensation: water from a blocked condensate trap, a leak inside the casing or heavy condensation can affect the electrode, its lead or the connections and create a false signal.
• Electrode or lead damage: a worn, cracked or badly connected ignition and sensing electrode can give an unreliable signal.
• Printed circuit board fault: the board that reads the signal can itself fail, sometimes after a power surge or water getting onto it.
• Wiring or earth problems: a poor earth or a damaged cable inside the boiler can upset the flame sensing.

Why this is not a DIY fault

With F.83 or low pressure, there are checks you can make yourself. F.27 is different. The possible causes are the gas valve, the ignition and sensing parts and the control board, which are all inside the sealed boiler casing and all involve gas, electricity or both. Work on them must be done by a Gas Safe registered engineer. Taking the casing off to look, drying parts with a hairdryer or following a video to clean the electrode is not safe and can affect the boiler's combustion.

If you have also noticed a smell of gas near the boiler, treat it as a gas emergency, not a boiler fault. Open windows, do not use switches, turn off the gas at the meter if you can safely do so, leave the property and call 0800 111 999.

F.27 means the boiler cannot be sure whether a flame is present. Leave it switched off after a failed reset rather than repeatedly restarting it.

Should you reset it?

One reset from the control panel is reasonable, as the fault can occasionally be a one-off, for example after a brief power interruption. If the boiler then runs normally for days, note the date and mention it at the next service.

If F.27 comes back, do not keep resetting. Our guide on how to reset a boiler explains when a reset is appropriate and when it only hides a fault, and boiler lockout keeps returning explains what a repeating lockout usually means.

What you can usefully do while you wait

You can make the engineer's visit shorter by writing a few things down. The exact code as displayed, including the F and the full stop. The boiler model from the front panel or the data badge. When it first happened and whether anything changed around that time, such as a power cut, a thunderstorm, a leak in the room above, a frozen condensate pipe or recent work on the boiler. Whether you have seen water inside the boiler cupboard. And whether the boiler has shown other codes, such as F.28 for ignition failure.

Also check whether the condensate pipe outside is frozen or blocked if the weather is cold. A backed-up condensate trap can put water where it should not be. Do not open the boiler to check the trap; just tell the engineer what you have seen outside.

What the repair usually involves

The engineer will normally check for signs of water inside the casing, inspect the electrode and its lead, test the gas valve to confirm it shuts off properly, and check the wiring and board. If there is a water source, such as a blocked condensate trap or a leaking seal, that must be fixed too, or the fault will come back.

The cost depends on which part is responsible. Drying out and resealing after a small leak is a very different job from a new gas valve or control board. On an old boiler that also has other problems, it can be worth comparing a repair with a replacement, and our guide on whether to repair or replace a boiler helps with that decision. Ask the engineer to explain what they found and why, not just which part they fitted.

Getting a Vaillant F.27 looked at

Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road, and our gas work is carried out by Gas Safe registered engineers. When you call, tell us the code, the model, how many times it has been reset and anything you noticed around the time it first appeared.

Our call-out charge is £60. The engineer will explain what they have found and what the repair involves before starting. You can read more on our boiler repair and servicing page.