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The short answer
The Vaillant F28 fault code means ignition failure: the boiler attempted to light and could not, so it has locked out for safety. Common causes are gas supply problems, a frozen condensate pipe, worn ignition electrodes or a faulty gas valve. One reset is safe to try.
What the law requires
- What does F28 mean on a Vaillant boiler?
- F28 or F29? The sibling codes explained
- What causes a Vaillant F28, in order of likelihood
- What you can safely check before calling anyone
- What the engineer will do
- Is F28 dangerous?
- What does an F28 repair cost in London?
- Preventing the next F28
- The first fire of autumn: F28 after the summer off
- A landlord’s note on F28
- How long can I leave an F28 locked out?
- Vaillant F28, quick answers
What does F28 mean on a Vaillant boiler?
F28 appears on ecoTEC, ecoFIT and most other Vaillant models when the boiler runs its ignition sequence and fails to establish a flame. The sequence itself is worth understanding, because every cause of F28 is a failure at one of its links. The fan runs up first and proves airflow. The gas valve opens and meters gas to the burner. The ignition electrodes spark. And the flame-sensing electrode must then detect a stable flame within seconds. If any link fails (no gas, no spark, no detection), the boiler makes its allowed number of controlled attempts and locks out with F28.
Unlike the pressure code F22, which you can usually resolve yourself, F28 is a lockout with safety at its core: a boiler must never keep releasing gas at a burner that is not lighting. That is why the golden rule with F28 is one reset, not ten. A single reset after checking the obvious external causes is legitimate; a reset habit is not, and everything past the checks below belongs to a Gas Safe registered engineer, both practically and legally.
F28 or F29? The sibling codes explained
Vaillant separates its flame faults by timing. F28 means the flame never established at startup. F29 means a flame was burning and was lost mid-operation. They share most causes (gas supply, condensate, electrodes, flue), but the timing is a diagnostic clue: F28 leans toward ignition-side components and supply, while F29 leans toward supply interruptions, flue disturbance and flame-sensing faults. A boiler that alternates between the two is describing one marginal ignition chain, not two faults. Everything in this guide applies to both, with the engineer’s diagnosis weighting the suspects by which code dominates.
What causes a Vaillant F28, in order of likelihood
No gas reaching the boiler
The most common cause in London, and free to identify. An empty or faulted prepayment meter, a gas emergency control valve nudged partially closed, a street-level supply interruption, or in hard frost a frozen regulator at the meter. None of these are boiler faults, which is why they are checked first.
A frozen condensate pipe
The great winter cause. When the small white waste pipe outside freezes, the boiler cannot drain and refuses to sustain ignition; on Vaillants this frequently surfaces as F28 or F29, often with a gurgling soundtrack. If your F28 arrived with sub-zero weather, this is the first suspect, and the thaw is a ten-minute job covered in our frozen condensate pipe guide.
Worn ignition electrodes
The spark components age with every startup, and their decline produces the classic intermittent F28: fine for days, locked out on a damp morning, fine again after a reset. Damp is the tell, because a weakening spark fails first in humid air. Electrode replacement is a routine, cheaper repair, and on the ecoTEC platform the electrodes are a known service item.
A flame-sensing fault
Occasionally the flame lights perfectly and the boiler simply cannot see it: a degraded sensing electrode or its lead. The result is identical from the outside (attempted ignition, then lockout), separated from a spark fault only by watching a live ignition attempt with instruments.
Gas valve faults
The valve can stick or drift out of adjustment, delivering too little gas to establish a flame. Diagnosed with working-pressure tests, never guessed.
Flue and combustion problems
A blocked, obstructed or badly terminating flue disturbs the air-gas balance enough to defeat ignition, and wind-exposed terminals can produce weather-correlated F28s. Less common, but part of every proper diagnosis.
Control faults
Rarely, the PCB or its wiring is the failed link. It sits last on the list because everything cheaper must be eliminated first; an over-the-phone PCB quote is a guess wearing a price tag.
What you can safely check before calling anyone
Check your other gas appliances. If the hob will not light either, the problem is supply, not the boiler. Prepayment customers: check the meter credit and that the meter screen shows supply on; it is the most common F28 resolution in London and costs nothing.
Check the gas emergency control valve near the meter is fully open, lever in line with the pipe.
In freezing weather, inspect the external condensate pipe and thaw any iced section with warm, never boiling, water.
Press reset once and allow a complete, uninterrupted ignition attempt. Listen: fan run-up, clicking, then either the soft roar of ignition or silence. What you hear is diagnostic information worth passing on.
If F28 returns, stop. Do not keep resetting, and do not open the casing. Book a Gas Safe engineer and describe the pattern: constant or intermittent, damp mornings or all conditions, gurgling or silent.
If you smell gas at any point: stop, avoid switches and naked flames, ventilate, and call the National Gas Emergency line free on 0800 111 999 before anyone else.
F28 back after one reset? Our Vaillant specialists carry ecoTEC electrodes and common ignition parts in the van, across every London borough.
What the engineer will do
A disciplined F28 diagnosis follows the ignition sequence in order. Supply first: standing and working gas pressure at the appliance, which clears or convicts everything upstream in minutes. The condensate route second, in any season, because partial blockages mimic electrical faults. Then the ignition chain under a live attempt: the fan proved for run-up and airflow, the spark observed and tested, the flame-sensing circuit read as the burner lights or fails to, electrodes inspected and gapped, the gas valve tested for correct operation, and the flue examined last. Watching one real attempt tells the engineer which link failed in a way no code alone can. The visit ends with a named fault and a fixed quote, and on boilers inside Vaillant’s warranty window, with a check of whether Vaillant should be doing this repair free before you spend anything.
Is F28 dangerous?
The lockout itself is the safety system succeeding: no verified flame, no gas. The risk arrives when people fight it, resetting repeatedly in the hope the boiler relents. It will not, the habit proves nothing, and it wastes the intermittent phase in which this fault is cheapest to fix. Leave the boiler locked out between attempts, run the checks, and hand the rest to the engineer.
What does an F28 repair cost in London?
If the cause is meter credit or a frozen pipe, the checks above cost nothing. Engineer repairs: ignition electrode replacement £120 to £240, flame-sensing electrode or lead £110 to £200, condensate remedial work £95 to £180, gas valve investigation and replacement £220 to £400, flue work quoted on findings, and PCB replacement £300 to £500, all fixed-quoted after diagnosis. On an older boiler where the valve and board are both implicated, ask for the repair and replacement numbers side by side; our repair or replace guide sets out the honest maths.
Preventing the next F28
Two habits remove most future F28s. Insulate the external condensate run before the first frost; the materials cost a few pounds and end the winter lockout ritual. And book the annual boiler service in early autumn, because electrode wear, the leading component cause, is precisely what a service inspects and renews before the damp morning that turns wear into lockout. On newer Vaillants the service also keeps the long warranty alive, which is the cheapest F28 insurance that exists.
The first fire of autumn: F28 after the summer off
There is a version of F28 that engineers can predict to the week. A boiler that spent the summer heating only tap water asks its full ignition chain to perform again on the first cold evening of autumn, and components that drifted over the idle months fail their first real exam. Electrodes that corroded fractionally in summer damp misfire; a gas valve that sat in one position sticks; spiders and debris colonise an undisturbed flue terminal. The result is the seasonal F28 spike every London firm staffs for in late September. The practical lesson runs in both directions. If your F28 arrived with the first cold snap, say so, because the timing itself shortens the diagnosis. And if you want to avoid joining next year’s spike, run the heating for ten minutes once or twice across the summer, and book the annual service for early autumn so the chain is inspected just before it goes back to full-time work.
The same logic covers F28 after a power cut. An interruption mid-sequence can leave the boiler in a lockout it clears with a single reset once power stabilises; that reset is legitimate. An F28 that persists after the power is back, though, is a real fault wearing coincidental timing, and it goes through the same checks as any other.
A landlord’s note on F28
F28 in a tenanted property carries the same trap as every ignition code: the tenant reports a dead boiler, and the actual cause is an empty prepayment meter, a tripped switch or a frozen pipe that thirty seconds of phone triage would have found. Our process runs the meter and condensate questions with the tenant before any visit is booked, free, and a meaningful share of F28 reports end there. When the fault is real, we arrange access with the tenant directly and, where the property’s gas safety record is due, fold the CP12 inspection into the same visit. The prevention side matters doubly for landlords: an autumn service and an insulated condensate pipe across a portfolio converts the winter F28 spike from an emergency-rate certainty into a non-event, which is the cheapest heating strategy a portfolio ever adopts.
How long can I leave an F28 locked out?
The boiler is safe while locked out; the gas valve is closed and nothing is burning, so waiting a day for an engineer carries no appliance risk. Two practical caveats, both about winter. First, a locked-out boiler cannot run its own frost protection, so in a hard freeze an unheated house edges toward frozen pipes territory; keep the property above freezing by other means if the wait runs long. Second, plan the hot water gap: combi households have no cylinder to fall back on, which is why we treat winter F28 calls as same-day work, while cylinder households can bridge with the immersion heater. If your wait stretches, tell the firm your household situation; young children, elderly residents or medical needs move any decent company’s queue.
Vaillant F28, quick answers
Can I fix F28 myself?
Only the external causes: meter credit, the control valve position, a frozen condensate pipe. Everything inside the case is Gas Safe work by law.
How many resets are safe?
Treat it as one, or one more after fixing an obvious external cause.
Why does mine only fail on damp mornings?
That is the weakening-electrode signature, and it is the cheap window: replaced now, it is a £150 morning; ignored until the spark fails completely, it is a cold house in January.
Is F28 the same as F.28?
Yes; the display shows the point, people type both.
Does F28 mean my boiler is finished?
Rarely; most F28s end as an electrode or sensor swap, among the cheaper repairs on the menu.