Quick answer: Vaillant F27 means the boiler has picked up a flame signal while the burner should be off. Vaillant calls this “flame simulation”, and the boiler locks out for safety. A gas valve that isn’t sealing properly, a fault in the flame-sensing electronics or moisture on the circuit board are the usual causes. You can try one reset, but F27 almost always needs a Gas Safe engineer.
Most boiler faults are about something missing: no gas, no water, no flame. F27 is the odd one out, because the boiler thinks it can see a flame that shouldn’t exist. That makes it one of the codes to take seriously. If your Vaillant is past 8 years old and out of warranty, the parts involved in an F27 repair can make a new boiler fitted by Edinburgh engineers the better value option.
What does the Vaillant F27 error code mean?
F27 means the flame sensor is reporting a flame when the gas valve should be closed. Vaillant’s manuals list it as “Safety switch-off: Flame simulation”, and it has the same meaning across ecoTEC plus, ecoTEC pro and ecoFIT pure models. Newer boilers may show it as F.027.
Your boiler constantly checks for a flame. When it’s heating, it expects one. When it’s idle, it expects none. If it sees a flame signal at the wrong time, it can’t trust its own safety checks, so it shuts down and stays shut down until someone resets it.
What causes a Vaillant F27 fault?
F27 is nearly always caused by the gas valve or the electronics that read the flame signal.
| Cause | What’s happening | Who fixes it |
|---|---|---|
| Gas valve not sealing | A small amount of gas passes through when the valve should be shut | Gas Safe engineer |
| Faulty PCB or flame monitor | The control board misreads the flame signal | Gas Safe engineer |
| Moisture on the electronics | Water from a leak or condensation affects the circuit board | Gas Safe engineer |
| Monitoring electrode or lead | A worn or damaged sensor sends a false signal | Gas Safe engineer |
Is Vaillant F27 dangerous?
The lockout keeps you safe, but the cause needs checking quickly. If the gas valve is letting gas through, that’s a genuine safety concern. That’s why the boiler refuses to run.
If you smell gas at any point, turn the boiler off, open your windows, don’t use light switches and call the National Gas Emergency number on 0800 111 999.
How do you fix a Vaillant F27 error?
There’s very little you can safely do yourself with F27. Try these steps, then call an engineer if the code stays.
- Write down the code and when it appeared. Engineers find it useful to know whether it happens on start-up, at random or constantly.
- Look around the outside of the boiler for water, drips or damp patches. Don’t take the casing off.
- Press and hold the reset button for about 3 seconds, once.
If F27 returns, leave the boiler off and book a boiler repair. The gas valve, electrodes and circuit board all sit behind the casing, and only a Gas Safe registered engineer should work on them.
Can damp or a leak cause F27?
Yes, moisture reaching the control board is a known cause. Vaillant lists it in its own fault tables. Plenty of Scottish boilers live in cold spots, such as lofts, garages, utility rooms and external cupboards, where condensation can form in winter. A slow drip from a pipe above the boiler can do the same.
If you notice water near the boiler as well as F27, mention it when you book. It helps the engineer go straight to the likely cause.
How much does it cost to fix Vaillant F27?
F27 repairs tend to cost more than simple pressure faults, because they usually involve a main component.
| Repair | Typical UK cost |
|---|---|
| Electrode replacement | £40 to £160 |
| Gas valve replacement | £160 to £350 |
| PCB (circuit board) replacement | £250 to £730 |
| Drying and repairing moisture damage | £80 to £200 |
Indicative prices from published UK trade guides, 2024 to 2026. Your quote depends on the model and parts needed.
Should you repair or replace a Vaillant with F27?
If the boiler is under 8 years old or still under warranty, repair it. If it’s older and needs a new gas valve or circuit board, get a replacement quote first.
A gas valve and a PCB together can run to several hundred pounds, and on an ageing boiler other parts are often not far behind. A modern A-rated boiler brings a new manufacturer warranty, better efficiency and fewer surprise lockouts. You can get a free new boiler quote and compare it against your repair cost before you decide.
Does regular servicing prevent F27 faults?
Servicing can’t stop every electronic fault, but it catches the warning signs early. During a boiler service our engineers check the electrodes, test the gas valve and look for leaks or damp around the boiler. Many manufacturers also need a yearly service to keep the warranty valid.
When should you call an engineer for Vaillant F27?
Call a Gas Safe engineer as soon as F27 comes back after one reset, or straight away if you smell gas or see water near the boiler.
Vaillant Boiler Repairs Across Edinburgh and Scotland
The Edinburgh Boiler Company repairs, services and replaces Vaillant boilers right across the Lothians and beyond. We handle boiler repairs in Musselburgh and Broxburn every week, and install new boilers as far out as Stirling and Perth. If your Vaillant is showing F27, call our Gas Safe team and we’ll get it checked.
Frequently Asked Questions About the Vaillant F27 Error
What’s the difference between Vaillant F27, F28 and F29?
All three involve the flame, but at different moments. F27 means a flame is detected when there shouldn’t be one. The Vaillant F28 error means no flame lit at start-up. The Vaillant F29 error means the flame went out while the boiler was running.
Is F27 linked to F62?
They can be. Both point towards the gas valve. The Vaillant F62 error means the flame took too long to go out after the valve closed.
Can I keep using my boiler with F27?
No. The boiler locks out until the fault is cleared, and repeated resets won’t fix the cause. Leave it off and call an engineer.
Does F27 mean I need a new boiler?
Not necessarily. A younger boiler is usually worth repairing. An older Vaillant that’s out of warranty and needs a gas valve or PCB is often better replaced for reliability and lower running costs.
