Nothing ruins a cold British morning faster than a Vaillant boiler flashing F27 on the display. Whether you’ve got an ecoTEC Pro, ecoTEC Plus or one of the older TurboMAX or ecoMAX models, that particular fault code sends a shiver down most homeowners’ spines – and not just because the heating’s off.
This plain-English guide is for every UK homeowner, landlord or tenant who’s googling “Vaillant F27” at 7 am in their dressing gown. We’ll explain exactly what F27 means, why it happens, the other common Vaillant error codes you’re likely to see, safe DIY steps you can take today, and – crucially – when you need to call a Gas Safe engineer before you make things worse.
Why Vaillant Boilers Show Fault Codes
Vaillant is one of the most popular boiler brands in British homes – 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 isn’t right, instead of just dying quietly like an old cast-iron boiler, they lock out and display an F-code (Fault) or S-code (Status).
This is actually helpful – it tells your engineer exactly where to start looking.
Vaillant F27 Fault – What It Really Means
F27 = “No flame detected after ignition” – but the boiler thinks gas IS reaching the burner.
In simple terms: the boiler opened the gas valve, tried to light, but the flame supervision device didn’t see a proper flame… even though the boiler believes gas is flowing.
The Most Common Causes of F27 (in real UK homes)
- Moisture on the PCB or flame probe – Condensation drips from the trap or sump onto the board (very common on boilers over 8 years old).
- Cracked or perished ignition electrode / flame sensing probe – The ceramic insulator cracks and the probe gives false readings.
- Faulty or drifting gas valve – The valve doesn’t fully close, so a tiny amount of gas seeps through when it shouldn’t.
- Water ingress into the gas valve solenoid – Rain coming down the flue on older vertical flues.
- PCB failure – The board misreads the flame signal (expensive, but less common than people think).
F27 is almost never low gas pressure – that usually gives F28 or F29.
Other Common Vaillant Fault Codes You’ll See
| Code | Meaning | Typical Cause in UK Homes |
| F28 / F29 | Ignition lockout / failed to light | Frozen condensate, low gas pressure, air in gas line |
| F22 | Low water pressure | Leak, recent radiator bleed, dripping PRV |
| F75 | Pump / pressure sensor fault | Seized pump or faulty sensor (common on ecoTEC Plus) |
| F62 | Delayed ignition / gas valve issue | Similar causes to F27 |
| F54 | Flame loss during operation | Strong gusts of wind on balanced flue |
| F83 | No temperature rise when burner on | Airlock, sludge, pump failure |
Safe DIY Checks You Can Do Before Calling an Engineer
These steps fix a surprising number of Vaillant call-outs:
- Reset the boiler properly
Press and hold the spanner/reset button for 5 seconds. If F27 comes straight back, there’s a real fault.
- Check for obvious condensate leaks
Look 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 your gas meter
Make sure the meter isn’t in emergency control (lever parallel to pipe = ON).
- Repressurise if pressure is below 0.8 bar
Find the filling loop (blue or grey handles or a silver key). Top up slowly to 1.2–1.5 bar. See our step-by-step Vaillant repressurising guide.
- Thaw a frozen condensate pipe
The external 22 mm white pipe – pour warm water over it if it’s icy.
If the fault clears after any of these, great. If F27 returns within minutes, stop and call an engineer.
When You MUST Get a Gas Safe Engineer (Don’t Even Think About DIY)
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 flame probe
- Dry out or replace the PCB
- Carry out combustion analysis after repair
Trying to “clean the burner” yourself is illegal and extremely dangerous.
Check your engineer is properly registered or ask to see their card (Vaillant faults should only be touched by someone with ACS qualifications and Vaillant-specific training if possible).
How Much Does It Usually Cost to Fix F27?
Realistic 2025 prices from UK engineers:
- New ignition/flame sensing electrode: £140–£220 fixed price
- Gas valve replacement: £320–£480
- PCB replacement (worst case): £450–£680
- Drying out a wet board (if caught early): £120–£180
Many engineers now offer fixed-price F27 repairs because the fault is so common.
Prevent F27 and Other Vaillant Problems
- Have your boiler serviced every year – a proper service includes cleaning the sump and checking for condensate leaks.
- Fit a condensate pro or pump if your boiler is in a loft (stops drips onto the PCB).
- Consider a magnetic filter (Vaillant or MagnaClean) to keep sludge out of the heat exchanger.
- If your boiler is over 10 years old and getting regular F27/F62 faults, start budgeting for replacement – a new ecoTEC Plus 30 kW is around £1,200–£1,500 supplied and fitted.
Frequently Asked Questions
Will resetting the boiler fix F27?
Sometimes once or twice, but if it keeps returning there is an underlying fault.
Is F27 covered by Vaillant warranty?
Only if your boiler is still in its 5–10 year warranty period AND has a full service history.
Why does F27 happen after heavy rain?
Water runs down an old vertical flue and into the gas valve or onto the PCB.
Can low pressure cause F27?
No – low pressure gives F22. F27 is always flame-detection related.
My boiler is 12 years old – repair or replace?
If it’s just electrodes (£180) then repair. If it needs a gas valve + PCB (£800+) then seriously consider a new A-rated Vaillant.
Staring at F27 right now? Drop your exact model (ecoTEC Pro 28, Plus 32 etc.) and postcode in the comments and we’ll let you know roughly what you’re looking at.
Stay warm,
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.