F28 Vaillant
Vaillant F28 Fault: Ignition Failure Explained
F28 on a Vaillant is the ignition failure lockout on most models: the boiler attempted its start sequence and could not establish a flame, so it shut down and now requires attention plus a manual reset. Alongside F22, it's the code ecoTEC owners meet most — and in winter its leading cause is one you can fix with warm water.
What the F28 code means
The ecoTEC start sequence runs fan-proof, gas valve, ignition, then flame confirmation via the sensing electrode within a strict window. F28 records failure to establish the flame at all (its sibling F29 covers a flame that lit and then died mid-operation). The lockout is hard by design — the boiler will not keep attempting ignition against an unknown failure, because unburned gas attempts are precisely what the interlock exists to prevent.
First moves before booking anyone
Three homeowner-safe checks cover most recoverable cases of this code. Frozen condensate: in a cold snap, the external white pipe is suspect number one — warm water along its length, then reset. Gas supply: if the hob won't light either, the boiler is innocent; look at the meter display and call the supplier or, for any smell of gas, the National Gas Emergency line on 0800 111 999. Meter credit: prepayment meters that run dry or hit an error stop the boiler mid-season more often than any component does.
After any of those, one reset gives the boiler a clean ignition attempt. If the code returns, note exactly what the display shows and how far the boiler gets (pump noise? ignition clicking? brief flame then cutout?) — those observations, passed on when you book, meaningfully sharpen the engineer's first visit.
What causes it
Winter first: a frozen condensate pipe is the seasonal F28 factory, blocking the condensate route and stopping ignition on cold mornings. The rest of the list: gas supply problems (meter errors, prepayment credit, supply interruptions — check the hob), a gas valve fault, ageing ignition electrodes or leads, and flue anomalies. Vaillant's diagnostics are good and the fault memory helps: an engineer reading the boiler's stored history can usually see whether F28 is a one-off event or the latest of many attempts, which changes the conversation.
When to call a Gas Safe engineer
Beyond the condensate thaw and supply checks, F28 is engineer work: gas valves, electrodes and flue investigation sit behind the case, legally and practically. Ignition components are stock items for ecoTEC-fluent engineers, and single-visit fixes are the norm. When booking, say whether the boiler clicks without lighting or fires briefly, and whether the hob works — those two facts pre-sort the likely causes before the van moves.
Carbon monoxide safety: symptoms, alarms and what to do — essential reading for every home with a fuel-burning appliance.
Frequently asked questions
What does F28 mean on my Vaillant?
Ignition failure: the boiler tried to light and couldn't establish a flame, so it locked out. In freezing weather check the condensate pipe first; otherwise gas supply and ignition components are the leading causes.
What's the difference between F28 and F29?
F28 is failure to light at all during start-up; F29 is a flame that established and then died during operation. They share suspects — condensate, gas supply, sensing — but the distinction helps the engineer, so report which you're seeing.
Can I fix F28 myself?
Only the outside-the-case causes: thaw a frozen condensate pipe, restore meter credit, confirm gas at the hob, then one reset. Everything further — valves, electrodes, flue — is Gas Safe engineer territory without exception.
Does F28 mean my boiler is finished?
Rarely — it's usually a component or condition, not a verdict. Electrodes, leads and even gas valves are routine repairs. On an elderly ecoTEC stacking F28 onto other faults, the engineer will give you honest repair-versus-replace numbers.
Could it be my gas supply rather than the boiler?
Genuinely often, yes: check the hob. If other gas appliances are dead too, look at the meter for error messages or credit, and contact your supplier — a heating engineer can't fix a network or meter issue, and the visit would be wasted.
How much does fixing an ignition fault cost?
If it's condensate freezing: often nothing beyond the thaw, plus lagging the pipe to prevent repeats. Component-level fixes — electrodes, leads, sensing parts — typically land at the modest end of repair pricing once diagnosed.
Where written advice reaches its limit
Guidance like this can tell you what usually causes a symptom, but only an engineer with instruments on your actual appliance can confirm it. If what you're seeing doesn't quite match any pattern described here — or matches several at once — that ambiguity is itself useful information to hand over. Our engineers do this triage daily, and an honest one will tell you on the phone when your problem sounds like a cheap fix, a bigger job, or something that isn't the boiler's fault at all.