The Vaillant F27 error code means your boiler has detected a flame signal while the burner should be off, so it has shut down and locked out as a safety measure. Unlike a low pressure fault, F27 is not something a homeowner can fix, but there are a few safe checks you can make before calling an engineer.
Quick answer: F27 on a Vaillant boiler means a flame signal has been detected while the burner is off. It is a safety lockout. You can press the reset button once. If F27 comes back, stop and call a Gas Safe registered engineer, because the usual causes, such as moisture on the electronics or a faulty electrode or circuit board, sit inside the boiler.
What Does the Vaillant F27 Error Code Mean?
F27 is a false flame or flame detection fault. Vaillant’s official boiler fault codes list describes it as a flame signal that has been detected while the burner is off. On the boiler display it usually appears as F.27.
A condensing gas boiler constantly checks whether a flame is present. When it lights, it expects to see a flame; when it switches off, it expects that signal to disappear. If the control board still reads a flame signal when the gas should be shut off, it cannot be sure the burner is behaving as it should. Rather than carry on, the boiler locks out and shows F27.
The commonly reported causes, such as moisture or a faulty electrode, produce a false signal rather than a real flame. Even so, only a qualified engineer can confirm what is happening, which is why Vaillant treats F27 as a job for an expert.
What Causes the F27 Error?
Vaillant’s own pages give the meaning of F27 but not a list of causes, so the causes below are those named consistently by UK heating engineers’ guides. Treat them as possibilities that an engineer will test, not a diagnosis.
- Moisture on the electronics: water reaching the control board or connections, for example from a leak above or inside the boiler, can create a false flame signal.
- Damaged ignition electrode or lead: a worn, cracked or poorly connected electrode or lead can send an incorrect signal.
- Ignition or spark generator fault.
- Faulty main printed circuit board (PCB): the board that reads the flame signal may itself be at fault.
An engineer may also check the gas valve to confirm it is closing fully when the burner switches off. Check your model’s manual and your engineer’s findings, because the exact cause varies between boilers.
How Do You Fix the F27 Error Yourself?
You cannot repair an F27 fault yourself, but you can make these safe checks from outside the boiler casing and try one reset, as the Vaillant operating instructions allow for fault messages.
- Write down the code and when it appeared, for example straight after heavy rain, after a bath overflowed upstairs, or after recent building work.
- Look at the outside of the boiler and the area above it for water: drips from pipes, a damp ceiling, or water running down the wall. Do not open the casing.
- If you find water coming from above, switch the boiler off at its mains switch or fused spur and leave it off. Water and electrics do not mix.
- If there is no sign of water, press the reset button once, as described in your manual, and see whether the boiler restarts normally.
- If F27 appears again, do not keep resetting. Leave the boiler locked out and book an engineer.
Safety: One reset is the limit. Do not remove the casing, dry or touch the circuit board, or adjust the electrode, ignition lead or gas valve. Vaillant UK says this is a case for an expert and that you should not try to solve it yourself as it can be dangerous. Any work inside a gas boiler must be done by a Gas Safe registered engineer. If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
When Should You Call an Engineer?
Call a Gas Safe registered engineer as soon as F27 returns after a single reset, or immediately if you have seen water on or around the boiler. F27 points to the flame sensing system, which is part of the boiler’s gas safety controls, so it should always be checked properly rather than cleared and ignored.
When you call, tell the engineer the boiler make and model, the exact code, how often it has appeared, and anything that happened just before it. This helps them bring the right parts. If the boiler is still under a Vaillant guarantee, contact Vaillant or your installer first; the Vaillant UK fault codes page lists customer service on 0344 736 0049.
How Is F27 Different From F28 and F29?
All three are flame related, but they describe opposite problems. F27 means the boiler sees a flame when it should not. F28 and F29 mean the boiler cannot see a flame when it needs one. The table uses Vaillant’s official descriptions.
| Code | Vaillant’s description | Homeowner step |
|---|---|---|
| F27 | A flame signal has been detected while the burner is off | One reset, then Gas Safe engineer |
| F28 | The flame signal was not detected during the ignition phase | Check gas supply and condensate pipe, reset, then engineer |
| F29 | The ignition following a flame loss during operation was unsuccessful | Check gas supply and condensate pipe, reset, then engineer |
| F62 | The flame switch-off was detected after a delay | Gas Safe engineer |
| F20 | Safety cut-out has interrupted the gas valve because a temperature limit was exceeded | Gas Safe engineer |
| F22 | No or insufficient water in the product, or water pressure too low | Top up pressure; engineer if it returns |
If your boiler shows a low pressure code instead, our guide on how to repressurise a combi boiler walks through the safe steps.
How Can You Stop the F27 Error Coming Back?
The most effective prevention is an annual service by a Gas Safe registered engineer, which Vaillant recommends to keep the boiler running correctly and spot problems early. During a service the engineer inspects the burner, electrode and connections.
- Fix leaks above or near the boiler quickly, including bathroom seals and overflow pipes upstairs.
- Keep the boiler cupboard dry and ventilated, and do not store wet items against the boiler.
- If F27 appeared after a repair, ask the engineer who did the work to return and check their connections.
- Keep a simple log of any codes and dates. Patterns, such as faults only in wet weather, are useful clues.
Once your boiler is back up and running, the gas bill calculator turns your meter readings into a cost, and if you are thinking about a replacement, the boiler size calculator gives a starting point for the output you need.
Vaillant F27 Error Code FAQs
Can I reset a Vaillant boiler showing F27?
You can press the reset button once, provided there is no sign of water on or around the boiler. If F27 returns, leave the boiler locked out and call a Gas Safe registered engineer. Repeated resetting will not fix the underlying fault.
Is F27 dangerous?
The boiler locks out precisely so that it does not run while its flame reading is uncertain, so the lockout itself is a safety feature. The fault behind it involves the gas safety controls, which is why only a Gas Safe registered engineer should investigate.
Does F27 mean there is a flame when the boiler is off?
Not necessarily. F27 means the boiler has detected a flame signal while the burner is off. The commonly reported causes, such as moisture or a faulty electrode, lead or circuit board, create a false signal, but an engineer must confirm this.
Can rain or a leak cause the F27 error?
Moisture on the boiler’s electronics is one of the commonly reported causes, so a leak above the boiler can trigger it. Switch the boiler off if water is reaching it and call a Gas Safe registered engineer rather than trying to dry anything inside.
Will I have heating and hot water with an F27 fault?
No. F27 is a lockout, so the boiler stops providing heating and hot water until the fault is cleared. Use electric heaters safely in the meantime and avoid leaving them unattended.
Is F27 covered by my Vaillant guarantee?
It depends on your guarantee terms and whether the boiler has been serviced as required. Contact Vaillant or your installer and read your guarantee terms before booking another engineer. Vaillant UK customer service is on 0344 736 0049.
Checked October 2026. Sources: Vaillant: Official boiler fault codes list, Vaillant UK: How to handle common fault codes, Vaillant: ecoTEC plus operating instructions. Causes cross-checked against two independent UK heating guides; check your model’s manual.
