Nothing ruins a cold British morning faster than a Vaillant boiler flashing F27 on the display. Whether it’s an ecoTEC Pro, ecoTEC Plus, or one of the older TurboMAX or ecoMAX models, this fault code sends a shiver down most homeowners’ spines, and not just because the heating is off.
This plain-English guide is for any UK homeowner, landlord or tenant googling “Vaillant F27” at 7am in their dressing gown. It explains exactly what F27 means, why it happens, the other common Vaillant error codes worth knowing, safe DIY steps to try first, and crucially, when it’s time to call a Gas Safe engineer before making things worse.
Why Vaillant Boilers Show Fault Codes
Vaillant is one of the most popular boiler brands in British homes, found everywhere from new-build flats in Manchester to Victorian terraces in London and 1930s semis in the Midlands.
Modern Vaillant boilers, pretty much anything made after 2005, are packed with sensors that constantly monitor flame, pressure, temperature and airflow. When something is not right, rather than just dying quietly like an old cast-iron boiler, they lock out and display an F-code for a fault, or an S-code for a status update. This is actually helpful, since it tells an engineer exactly where to start looking.
What Does the Vaillant F27 Fault Code Mean?
F27 means flame simulation: the boiler has detected a flame signal even though the gas valve is closed and no flame should be present. In simple terms, the flame supervision system is “seeing” a flame that shouldn’t be there, so the boiler locks out for safety.
What Causes the Vaillant F27 Fault Code?
- Moisture on the PCB or flame probe. Condensation drips from the trap or sump onto the board, which is very common on boilers over 8 years old.
- Cracked or perished ignition electrode or flame sensing probe. The ceramic insulator cracks and the probe gives false readings.
- Faulty or drifting gas valve. The valve does not fully close, so a tiny amount of gas seeps through when it should not.
- Water ingress into the gas valve solenoid. This is usually rain coming down the flue on older vertical flue installations.
- PCB failure. The board misreads the flame signal, which is expensive to fix but less common than people tend to think.
F27 is almost never caused by low gas pressure, since that usually produces F28 or F29 instead.
Other Common Vaillant Fault Codes
The table below sets out the other codes a Vaillant boiler is most likely to show, and what typically causes each one in a UK home. For less common codes not listed here, our full guide to Vaillant boiler error codes explains each one.
| Code | Meaning | Typical Cause in UK Homes |
| F28 / F29 | Ignition lockout, failed to light | Frozen condensate, low gas pressure, air in the gas line |
| F22 | Low water pressure | Leak, recent radiator bleed, dripping PRV |
| F75 | Pump or pressure sensor fault | Seized pump or faulty sensor, common on ecoTEC Plus |
| F62 | Delayed ignition or gas valve issue | Similar causes to F27 |
| F54 | Flame loss during operation | Strong gusts of wind on a balanced flue |
| F83 | No temperature rise when the burner is on | Airlock, sludge, or pump failure |
Related Fault Codes Worth Checking
If F27 keeps returning after the checks below, see our guide to the Vaillant F26 fault code, a closely related flame and ignition issue that can present with similar symptoms.
Safe DIY Checks for the Vaillant F27 Fault
These steps fix a surprising number of Vaillant call-outs on their own.
- Reset the boiler properly by pressing and holding the spanner or reset button for 5 seconds. If F27 comes straight back, there is a genuine fault.
- Check for obvious condensate leaks by looking underneath the boiler for drips coming from the white plastic condensate trap. A tiny leak here often lands on the PCB and triggers F27.
- Check the gas meter is not in emergency control, with the lever running parallel to the pipe for on.
- Repressurise if pressure is below 0.8 bar. Find the filling loop, usually blue or grey handles or a silver key, and top up slowly to 1.2 to 1.5 bar.
- Thaw a frozen condensate pipe by pouring warm water over the external 22mm white pipe if it looks icy.
If the fault clears after any of these steps, that is a good sign. If F27 returns within minutes, stop and call an engineer rather than repeating the reset.
When a Gas Safe Engineer Must Be Called for F27
F27 almost always involves gas-side components. By law, only a Gas Safe registered engineer can remove and test the gas valve, replace ignition electrodes or the flame probe, dry out or replace the PCB, and carry out a combustion analysis after any repair.
Attempting to clean the burner without training is illegal and extremely dangerous.
Always check an engineer is properly registered on the Gas Safe Register or ask to see their card. Vaillant faults should ideally only be handled by someone with ACS qualifications and Vaillant-specific training where possible.
How Much Does It Cost to Fix a Vaillant F27 Fault?
Realistic 2026 prices from UK engineers for an F27 repair:
- New ignition or flame sensing electrode: £140 to £220 fixed price
- Gas valve replacement: £320 to £480
- PCB replacement, the worst-case scenario: £450 to £680
- Drying out a wet board, if caught early: £120 to £180
Many engineers now offer fixed-price F27 repairs, since the fault is common enough that most know the likely cause before they arrive.
How to Prevent the Vaillant F27 Fault
- Book an annual service. A proper service includes cleaning the sump and checking for condensate leaks before they cause a fault.
- Fit a condensate pump if the boiler sits in a loft, which stops drips landing on the PCB.
- Consider a magnetic filter, such as a Vaillant unit or a MagnaClean, to keep sludge out of the heat exchanger.
- If the boiler is over 10 years old and getting regular F27 or F62 faults, start budgeting for replacement. A new ecoTEC Plus 30kW typically costs around £1,200 to £1,500 supplied and fitted.
If a replacement involves relocating the boiler, check our guide to boiler moving costs, since repositioning pipework and the flue can add noticeably to the bill compared with a like-for-like swap.
Vaillant F27 Fault Code: FAQs
Will Resetting a Vaillant Boiler Fix F27?
Sometimes, once or twice. If it keeps returning after a reset, there is an underlying fault that needs a proper diagnosis rather than repeated resetting.
Is the F27 Fault Covered by the Vaillant Warranty?
Only if the boiler is still within its 5 to 10 year warranty period and has a full service history. Manufacturers typically expect proof of annual servicing before agreeing to cover a repair under warranty.
Why Does Vaillant F27 Happen After Heavy Rain?
Water can run down an older vertical flue and into the gas valve or onto the PCB, which is why F27 often spikes after a period of heavy rain, particularly on installations more than 10 years old.
Can Low Pressure Cause a Vaillant F27 Fault?
No. Low pressure produces F22 instead. F27 is always related to flame detection rather than system pressure, so topping up the pressure will not resolve it if that is the actual code showing.
Should You Repair or Replace a 12-Year-Old Vaillant Boiler?
If the fix is limited to electrodes, typically around £180, a repair makes sense. If it needs a gas valve and a PCB, likely £800 or more combined, it is worth seriously considering a new A-rated Vaillant instead, since that level of repair cost starts to approach a meaningful share of a full replacement.
What’s the Difference Between Vaillant F27 and F28?
F28 and F29 usually point to the boiler failing to ignite at all, often due to frozen condensate or low gas pressure. F27 is different: it means flame simulation, where the boiler detects a flame signal while the gas valve is closed, which points to a sensor, electrical or gas valve fault rather than a supply issue. If your boiler lights but then loses its flame, our guide to the Vaillant F29 error code walks through the gas supply, flue and condensate checks.
Does an Annual Service Reduce the Chance of a Vaillant F27 Fault?
Yes. A proper annual service checks the sump and condensate trap for leaks, inspects the flame probe, and catches early signs of PCB moisture damage, all of which are the leading causes of F27. Boilers serviced every year are noticeably less likely to develop this fault.
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.