F27 Vaillant
Vaillant F27 Fault: Flame Detected in Error
F27 on a Vaillant is the anomaly of the code family: on most models it means the boiler detected a flame signal when no flame should exist — the sensing reported fire with the gas valve closed. It's usually a benign sensing or moisture fault, but because the impossible-if-true reading touches the safety chain, it's treated with more respect than the everyday lockouts.
What the F27 code means
Flame sensing works by ionisation: a small current flows through a real flame, and its presence tells the board the burner is lit. F27 records that current appearing when the burner should be off — which means either the detection circuit is misreading (moisture, contamination, component fault) or something genuinely anomalous is happening at the burner. The control response is a hard lockout, correctly: a boiler that can't trust its flame sensing has no safe basis for operating at all.
Safe steps you can try first
This code family is different from the usual lockouts: the boiler is reporting something that shouldn't be possible — such as detecting a flame when no flame was called for. Codes like this are safety-significant, and the honest homeowner guidance is correspondingly short: don't repeatedly reset against it, don't leave the boiler attempting restarts unattended, and book a Gas Safe engineer promptly. If you ever smell gas alongside any fault, leave the property and call the National Gas Emergency line free on 0800 111 999 before anything else.
The benign explanations — moisture on an electrode, a sensing fault, condensation where it shouldn't be — outnumber the serious ones, but distinguishing them is precisely the engineer's job, with the case open and instruments attached. One reset to see if the code was transient is reasonable; treating a recurring anomaly code as a nuisance to be reset around is not.
What causes it
The benign majority: moisture or condensation reaching the sensing electrode or its connections (with weather-correlated appearances as the tell), contamination on the electrode, degraded leads and connectors leaking current, and sensing-circuit component faults on the board. The serious minority — genuine combustion anomalies or a gas valve not sealing as commanded — is rare, and distinguishing it from the benign cases is precisely and exclusively the engineer's job with the case open and instruments attached.
When to call a Gas Safe engineer
F27 is a prompt-booking code: one reset to test for a transient, then a Gas Safe engineer regardless of whether the boiler seems to run — the anomaly's rare serious causes are why. Diagnosis is systematic and usually ends benignly: electrode cleaned or replaced, moisture path found and closed, connections remade. Don't run a recurring F27 on resets, and as with anything gas: any smell means leaving the property and calling 0800 111 999 before any other call.
Carbon monoxide safety: symptoms, alarms and what to do — essential reading for every home with a fuel-burning appliance.
Frequently asked questions
What does F27 mean on a Vaillant boiler?
A flame-detection anomaly: the sensing reported a flame when none should exist. It's usually moisture or a sensing-circuit fault, but because the reading is impossible-if-true, it earns a hard lockout and a prompt engineer visit.
Is F27 dangerous?
The common causes are benign — moisture, contamination, sensing faults. The code is treated seriously because the rare causes aren't, and distinguishing them requires an engineer. One reset maximum; then book promptly.
Why does F27 appear in wet weather?
Weather-correlated F27 points strongly at moisture reaching the sensing electrode or its connections — a common benign cause. The engineer's job includes finding and closing the moisture path so it doesn't return with the next front.
Can I keep using the boiler with intermittent F27?
Don't run it on repeated resets. If a single reset clears it and it stays clear, fine — but a recurring F27 means the boiler can't trust its flame sensing, and that's a condition to resolve, not manage around.
Why is this code treated more seriously?
Because it reports a condition that shouldn't exist — like flame detected when none was called. The likely explanations are benign sensing faults, but the unlikely ones matter, so the code earns a prompt engineer visit rather than casual resetting.
Can moisture really cause this code?
Yes — condensation or ingress on sensing electrodes is a common benign cause, particularly in certain weather. It still needs an engineer to confirm, dry, and identify why moisture reached where it shouldn't.
When to bring in a Gas Safe engineer
Every guide on this site draws the same line in the same place: homeowner-safe checks stop at the boiler's casing. Re-pressurising via the filling loop, thawing a frozen condensate pipe, bleeding radiators, checking the thermostat and pressing reset once — all yours. Anything requiring the case to come off involves gas, and gas work is legally restricted to Gas Safe registered engineers for reasons that predate any website's advice. Our network handles exactly that side of the line, with engineers matched to the job and prices agreed before anyone starts.