In This Guide
What Does the Vaillant F27 Error Code Mean?
F27 is Vaillant’s flame detection fault code. It appears when the boiler senses a flame signal at a moment when there should be none, for example after the gas valve has closed and the burner should be out. Because the boiler relies on knowing exactly when a flame is and is not present to control gas safely, a false or lingering flame signal means it can no longer trust that picture, so it locks out and shuts down. It is most common on the earlier ecoTEC Plus and Pro models built between 2005 and 2012, where damp reaching the electronics is a frequent trigger. Like the gas valve fault F62, this is a combustion-side fault that a homeowner cannot put right, and it should be seen as a fault needing a Gas Safe registered engineer rather than one to clear on repeated resets. It is one of the Vaillant faults 0800 Homefix is called out to across London and the South East.
How Is F27 Different From F28 and F29?
These three codes all involve the flame, which is why they are easy to confuse, but they describe opposite problems and it helps to know which you are dealing with.
With F28 the boiler tries to light but no flame is detected at all, so it is an ignition failure. With F29 the boiler does light, then the flame goes out during operation, so it is flame loss. F27 is the reverse of F28: the boiler detects a flame when there should not be one. So F28 is no flame when there should be, F29 is a flame that started then stopped, and F27 is a flame signal present when the boiler expects none. The distinction matters because it points the engineer at different components, and because a false flame signal can, in some cases, mean a gas valve is not shutting fully, which overlaps with the F62 gas valve fault.
What Causes a Vaillant F27 Fault?
F27 comes down to the boiler receiving a flame signal it should not be getting, and that is usually the flame sensing electrode, the electronics reading it, or a gas valve letting a trickle of gas through. The common causes are set out below.
Cause
| What happens | Typical fix |
Dirty or failing flame sensing electrode | The ionisation probe sends a false or distorted flame signal to the boiler | A Gas Safe engineer cleans or replaces the electrode |
Moisture on the electronics or PCB | Water or condensation on the board, common on older ecoTEC models, causes false flame readings | A Gas Safe engineer dries and protects or replaces the affected board |
Gas valve not closing fully | A trickle of gas keeps a small flame present after the valve should have shut | A Gas Safe engineer tests and replaces the gas valve |
Faulty PCB | The board misreads the flame signal and triggers the lockout | A Gas Safe engineer tests and replaces the PCB |
Wiring fault between electrode and PCB | A damaged or loose connection distorts the flame signal | A Gas Safe engineer inspects and repairs the wiring |
The reason moisture features so heavily on the older ecoTEC boilers is that condensation or a small leak reaching the electronics is enough to confuse the flame signal, which is why F27 often clears briefly on a reset and then returns once the damp is back. Every one of these causes sits inside the boiler on the combustion or electronics side, so beyond a single reset there is nothing for a homeowner to do here except book a Gas Safe engineer.
What Should You Do When You See F27?
There is very little safe DIY with an F27, so this is short by design rather than padded out.
You can reset the boiler once, holding the reset button to see whether it was a momentary glitch. If the boiler fires and runs normally, it may have been a one-off. What you should not do is keep resetting it. A boiler that repeatedly detects a flame when there should not be one has a real fault with the electrode, the electronics or the gas valve, and forcing it to restart does not address any of those.
If F27 returns, that is the end of the homeowner steps, and the boiler needs a professional diagnosis, which you can arrange through the 0800 Homefix boiler repair service. As with any gas fault, if you ever smell gas, do not rely on the fault code: turn the gas off at the meter, leave the property and call the National Gas Emergency Service on 0800 111 999.
What Does an 0800 Homefix Engineer Do About an F27 Fault?
An 0800 Homefix engineer works out why the boiler is seeing a false flame rather than swapping parts on assumption. They inspect and clean or replace the flame sensing electrode, check the electronics and connectors for the moisture that so often causes F27 on older ecoTEC boilers, and test the gas valve to confirm it is closing fully rather than letting gas trickle through. Where the board itself is misreading the signal, they test and replace the PCB, and they check the wiring between the electrode and the board. Once the faulty component is dealt with they run a full combustion check with a flue gas analyser to confirm the boiler is detecting and controlling the flame correctly again. Every engineer is Gas Safe registered, and the team covers London and the South East seven days a week, 365 days a year, so an F27 lockout does not have to leave you long without heating and hot water.
How Much Does It Cost to Fix a Vaillant F27 Fault?
0800 Homefix does not charge a call-out fee. The visit is charged as the first hour of labour, paid in advance, and any further repair work, whether that is a flame electrode, a dried or replaced board or a gas valve, is quoted and agreed with you before it begins, so nothing is a surprise once the engineer is on site. Because F27 can range from a simple electrode clean to a PCB replacement, the figure depends on the diagnosis, and the rate is set out in full on the rates page. For peace of mind after the repair, 0800 Homefix guarantees its labour for 12 months, and the manufacturer guarantees any replacement parts for 12 months.
How Do You Prevent the F27 Fault Coming Back?
Because F27 on the older ecoTEC boilers is so often about damp reaching the electronics, the most useful prevention is keeping moisture away from the boiler and having it serviced annually. An annual service lets a Gas Safe engineer clean the flame electrode, check the condition of the electronics and connectors, and spot early corrosion or a weeping seal before it triggers a lockout. You can book this through Vaillant boiler servicing. If your boiler is one of the 2005 to 2012 ecoTEC models where F27 is most common, that yearly check is the single most effective thing you can do to stay ahead of it, along with dealing promptly with any leak or damp near the appliance.
Frequently Asked Questions
Can I reset a Vaillant boiler after an F27 fault?
You can reset it once, holding the reset button until the boiler tries to re-fire, to rule out a momentary glitch. If F27 returns, do not keep resetting. A boiler that repeatedly detects a flame when there should not be one has a genuine fault with the electrode, electronics or gas valve that a reset will not fix.
What does false flame detection mean on a Vaillant boiler?
It means the boiler is receiving a flame signal at a time when the burner should be off and no flame should be present. The boiler cannot tell whether that signal is real, so it locks out on safety. It is usually down to a dirty flame electrode, moisture on the electronics, or a gas valve not closing fully.
Why is F27 common on older Vaillant ecoTEC boilers?
On the 2005 to 2012 ecoTEC Plus and Pro models, condensation or a small leak reaching the electronics is a frequent cause. Damp on the board or connectors distorts the flame signal, which is why F27 on these boilers often clears on a reset and then returns once the moisture is back. It needs an engineer to dry, protect or replace the affected part.
Can I fix an F27 fault myself?
No. Beyond a single reset there is nothing a homeowner can safely do. F27 is a flame electrode, electronics or gas valve fault, and by law that work must be carried out by a Gas Safe registered engineer. 0800 Homefix can diagnose and repair it.
Is a Vaillant F27 fault dangerous?
The lockout is the boiler protecting you by shutting down when it can no longer trust the flame signal, so a boiler sitting on F27 has taken itself safely out of use. The risk is in overriding that with repeated resets. If you ever smell gas, turn the gas off at the meter and call the National Gas Emergency Service on 0800 111 999.
Having an Issue With Your Heating?
If a reset has not cleared an F27, book an 0800 Homefix Gas Safe registered engineer to diagnose it. We cover London and the South East seven days a week, 365 days a year, with no call-out fee and a 12-month labour guarantee. Book online or call the team to get your heating and hot water back on.
Helpful Resources
- Gas Safe Register, to check any engineer’s registration: https://www.gassaferegister.co.uk/
- Vaillant’s own boiler support and fault code help: https://www.vaillant.co.uk/
Spotted Any of These Warning Signs?
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Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
