The Vaillant F27 fault code (shown as F.27 on some displays) is a flame detection error, so you may also see it called a boiler F27 error. The boiler thinks it can see a flame when it shouldn’t, or that the flame signal is wrong, so it shuts down as a safety measure. Vaillant lists it as flame simulation on most models. Heating and hot water stop until it’s cleared. Likely causes are moisture on the electrics or circuit board, a leaking gas valve, worn ignition leads or electrode, a faulty flame sensor or a failed control board. None of these are DIY repairs, so after a quick look for water and one reset, you need a Gas Safe registered engineer.
| Question | Answer |
|---|---|
| What does F27 mean? | Flame simulation: a flame signal is detected when there shouldn’t be one, or the signal is wrong |
| Is it a condensate fault? | No. It relates to flame detection, not condensate drainage |
| Common causes | Moisture on electrics, gas valve, ignition leads, electrode, flame sensor, control board |
| Can I fix it myself? | No. You can look for water, reset once, then call an engineer |
What Does F27 Mean on a Vaillant Boiler?
A boiler has a flame sensor that confirms the burner is lit. With F27 the sensor reports a flame at the wrong moment, for instance after the gas should have shut off, or reports a flame signal that doesn’t match what the boiler expects. Vaillant’s code lists call it flame simulation (also described as a false flame signal) on most models and a fault in flame detection on a few others. In both cases the boiler locks out because it can no longer trust what it’s measuring.
The result is no heating and no hot water until an engineer has found the cause. Vaillant lists F27 on several ecoTEC and ecoFIT models, and the wording differs slightly between them, so check your manual for yours.
How F27 Differs From F28, F29 and F62
These codes are easy to mix up because they all involve the flame. Vaillant F28 fault code means the burner never lit at start-up. F29 means the flame went out while the boiler was running. Vaillant F62 fault code points to the gas valve closing late or a flame being detected when it shouldn’t be. F27 sits closest to F62, since both involve a flame signal at the wrong time, and the engineer’s checks overlap. If the boiler simply won’t start and shows no code, see our guide to a boiler not working.
Common Causes of the Vaillant F27 Fault Code
| Cause | What Might Point to It | Who Fixes It |
|---|---|---|
| Moisture on electrical parts or the circuit board | A leak above or near the boiler, damp patches nearby, F27 appearing after a plumbing leak | Engineer, after the leak is dealt with |
| Faulty or leaking gas valve | Gas isn’t shutting off properly, repeated lockouts | Gas Safe engineer |
| Worn ignition leads or spark electrode | Poor ignition, an old boiler that hasn’t been serviced | Gas Safe engineer |
| Flame sensor fault (less common) | A flame signal that doesn’t match what the burner is doing | Gas Safe engineer |
| Control board (PCB) failure | F27 along with other odd behaviour such as pump or fan faults or a flashing display | Gas Safe engineer |
Water Where It Shouldn’t Be
Moisture on electrical components is one possible cause of a flame simulation fault. It can come from a leak nearby, such as a dripping pipe, a failed seal or water entering from above the boiler. Water on a circuit board can make it read a signal that isn’t real.
Gas Valve and Ignition Parts
If the gas valve doesn’t close fully, a flame can persist after the boiler has told it to stop. Worn ignition leads and a tired spark electrode can create stray signals or poor ignition. These sit inside the sealed combustion area, which is why they are always engineer jobs.
Flame Sensor and Control Board
A faulty sensor or a failing control board can report a flame that isn’t there. A failing board often causes other symptoms too, such as odd temperature control, pump or fan faults and a flashing display.
What You Can Do Safely at Home
There’s very little to try yourself. Anything beyond the steps below involves gas or electrics.
Look for Water Around the Boiler
Without opening the casing, check around and above the boiler for drips, damp patches or stains. If water is coming from a pipe or ceiling, deal with the source first. If it’s dripping onto the boiler or its electrics, switch the boiler off at the isolator only if you can reach it without standing in water or touching anything wet, and shut off the water supply to the leak. Then call an engineer. If the pressure gauge has been dropping as well, that suggests a leak, and our Vaillant F22 fault code guide explains low pressure.
Reset the Boiler Once
Reset once only, and only as your manual describes. Because a gas valve may be involved, don’t treat a reset as a fix: even if the boiler restarts, book an engineer check, since the fault can return. If F27 comes straight back, stop resetting and call an engineer.
Don’t Attempt These
Don’t remove the casing, clean or bend the flame sensor or electrode, test wiring with a multimeter, touch the gas valve or replace the circuit board. Pressing reset again and again won’t help either.
Smell Gas or Carbon Monoxide Alarm Going Off?
With a gas valve possibly involved, take any gas smell seriously. Don’t flip switches or reset the boiler. Open doors and windows if you can do it safely, turn the gas off at the meter, leave the building and ring the National Gas Emergency Service on 0800 111 999.
If a carbon monoxide alarm sounds, turn the boiler off, ventilate and get everyone outside. Don’t re-enter until the property is declared safe, and report it on 0800 111 999. Dial 999 for anyone with a headache, dizziness or nausea or who has collapsed, since the gas line can’t treat symptoms.
When to Book a Gas Safe Engineer
- F27 appears again after one reset
- You’ve found water near or inside the boiler
- The boiler locks out repeatedly or shows other codes too
- The boiler is old and hasn’t had a recent service
- You notice a gas smell, soot or scorch marks
- You’re tempted to keep the boiler running with the code showing. Leave it off until it has been checked
- The display flashes or the boiler behaves oddly in other ways
What an Engineer Does for F27
The engineer will look for moisture or corrosion on the electrical parts, test the gas valve and its closing, inspect the ignition leads and spark electrode, check the flame sensor and its signal, and test the control board. They’ll also trace any leak that let water reach the boiler. The code only says the flame reading is wrong, so most of the time is spent working out which part is responsible.
Vaillant F27 Repair Cost
Because every cause here needs a Gas Safe engineer, the cost depends on engineer time and which part is faulty. BeFix labour is £95 an hour from 7AM to 5PM, £125 from 5PM to 9PM and £160 from 9PM to 7AM, with VAT added and no call-out fee. Parts are quoted separately once the engineer has diagnosed the fault. If your Vaillant is still within its manufacturer’s warranty, check the terms or contact Vaillant before arranging a chargeable repair. Drying out and cleaning electrics is a smaller job than a new gas valve or circuit board.
On an older boiler, an expensive circuit board or gas valve may tip the balance towards replacement. Our boiler installation London page covers what a new boiler involves.
Vaillant Fault Codes People Mix Up With F27
| Code | Meaning | Guide |
|---|---|---|
| F22 | Water pressure too low | Vaillant F22 fault code |
| F23 | Large temperature gap between flow and return | Vaillant F23 fault code |
| F27 | Flame simulation or incorrect flame signal | This guide |
| F28 | No flame at start-up | Vaillant F28 fault code |
| F62 | Gas valve closes late or a flame is seen unexpectedly | Vaillant F62 fault code |
| F83 | No temperature rise after the burner lights | Vaillant F83 fault code |
Reducing the Risk of F27
Book an annual service so worn leads, electrodes and the gas valve are checked before they fail. Fix leaks promptly, especially above or beside the boiler, and keep the boiler area dry and ventilated. If the boiler has shown the code once, mention it at the next service even if a reset cleared it.
Arrange an F27 Repair With BeFix
A flame detection fault needs proper diagnosis, and BeFix’s Gas Safe registered engineers handle Vaillant lockouts as part of our boiler repair in London service.
We can often visit the same day, depending on availability, with arrival typically within 1 to 3 hours. Every repair is backed by a 12-month workmanship warranty. When you contact us, mention the code, your boiler model and whether you’ve seen any water near the boiler.
To book a boiler repair, use the online form, call 020 8058 4241 or email info@befix.co.uk. We answer Monday to Friday 7AM to 10PM and Saturday and Sunday 9AM to 8PM from 227B Nether Street, London N3 1NT.
FAQs
What does the Vaillant F27 fault code mean?
The boiler’s flame detection is reporting a flame when there shouldn’t be one, or an incorrect flame signal, so it has locked out for safety. Vaillant lists it as flame simulation on most models.
Is F27 a condensate fault?
No. F27 relates to flame detection, not to condensate drainage. A blocked or frozen condensate pipe is a different problem and doesn’t explain this code.
How do I fix a Vaillant F27 fault?
Check around the boiler for water, reset it once following your manual, and if F27 returns, call a Gas Safe engineer. The causes involve gas and electrical parts, so there isn’t a safe home repair.
Can moisture cause F27?
Yes, moisture on the electrical parts or circuit board is one possible cause. If it comes from a leak nearby, the leak has to be fixed along with the boiler.
What’s the difference between F27 and F62?
Both involve a flame signal at the wrong time. F62 points to the gas valve closing late or a flame detected unexpectedly, while F27 is flame simulation or an incorrect flame signal. An engineer’s checks overlap for the two.
Can I reset a Vaillant F27 fault?
Once, following your manual. If it comes back, stop. Repeated resets leave the cause in place.
Is F27 dangerous?
The lockout is a safety shutdown. The risk comes from a gas valve that isn’t closing properly, so if you smell gas, leave and call 0800 111 999. Don’t try to repair gas or electrical parts yourself.
How much does it cost to fix a Vaillant F27 fault?
BeFix charges £95 an hour from 7AM to 5PM, £125 from 5PM to 9PM and £160 from 9PM to 7AM, plus VAT, with no call-out fee. Parts are extra, and the total depends on whether it’s moisture, a gas valve, leads or the circuit board.
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