F.27
Vaillant Boiler
Severity: ModerateWhat Does This Error Mean?
Vaillant F.27 is 'sham flame' — the flame sensor signals it sees a flame when the gas valve is closed.
The PCB will not allow operation while it suspects fuel might be entering with no controlled ignition.
Cause is usually moisture on the flame rod, a cracked porcelain insulator, or a gas valve leaking past its main seat — the third is the dangerous case.
Affected Models
- Vaillant ecoTEC Plus
- Vaillant ecoTEC Pro
- Vaillant ecoFIT Pure
- Vaillant ecoFIT Sustain
- Vaillant Turbomax
- Vaillant ecoMAX
Common Causes
- Moisture on the flame rod from a damp combustion chamber
- Porcelain insulator at the base of the flame rod cracked
- Flame rod wire shorted to chassis
- Gas valve leaking past the main seat (rare but serious)
- Damaged ground wire causing false flame signal
- PCB flame-sense input failed
How to Fix It
-
Stop using the boiler.
Switch the boiler off at the spur or the breaker.
Until F.27 is cleared, do not press reset repeatedly — if a leaking gas valve is the cause, you may be raising real risk. -
Call a Gas Safe engineer if uncertain.
F.27 is one of the codes where DIY is not appropriate beyond a single reset attempt.
UK domestic boiler rules require a Gas Safe engineer for any work beyond external resets.
Diagnostic visits run 80-150 GBP. -
Engineer step: dry the chamber.
An engineer opens the combustion chamber.
Visible moisture from a recent flue leak or condensate spill is the most common cause.
Drying the chamber and the flame rod often clears F.27 immediately. -
Engineer step: inspect the flame rod.
The flame rod is a thin metal probe with a white ceramic insulator.
Hairline cracks in the porcelain leak signal to ground.
Replacement rods are 30-60 GBP and clip in.
The boiler often runs perfectly after this swap alone. -
Engineer step: gas valve test.
If chamber and rod are good, the gas valve is tested for leakage past the seat.
This requires a manometer at the test point and pressure observation with the valve commanded closed.
A leaking valve must be replaced — 200-400 GBP for the part. -
Engineer step: check ground bonding.
Flame sensing depends on a clean electrical ground.
Loose ground at the burner or the chassis bonding point causes intermittent F.27.
A 5-minute fix once spotted. -
Reset only after diagnosis.
Once the engineer confirms the cause and corrects it, the reset button on the front of the boiler clears F.27.
Press once — repeated presses do not improve outcomes.
Frequently Asked Questions
Is the boiler dangerous to leave running with F.27?
The boiler will not run with F.27 active — it locks out.
The risk is only present if you bypass or repeatedly reset without finding the cause.
Leave it locked out and call an engineer.