What the Vaillant F28 fault code means
F28 is an ignition lockout. The boiler asked for gas, tried to spark it into a flame, saw no flame, and stopped. Vaillant's own fault code list gives the meaning:
| Code | Vaillant's definition | Vaillant's listed causes |
|---|---|---|
| F28 (written F.28 in Vaillant's documentation) | "The flame signal was not detected during the ignition phase." | Not enough gas reaching the boiler (for example, low gas pressure or insufficient credit on a pre-payment meter), condensate backed up in the combustion chamber from a blocked condensate pipe, a faulty ignition electrode, a faulty PCB, or an issue with the flue system. |
Source: Vaillant UK, boiler fault codes.
Two things follow. The boiler is not broken in the way people fear: it has done what it is designed to do, locking out rather than releasing more gas into a chamber where nothing is burning. And F28 is a symptom, not a part. It tells you ignition failed, not why, and the cause ranges from a meter with no credit to a circuit board.
While F28 is showing you have no central heating and no hot water, because both come from the same burner. An electric shower or immersion heater is on a separate supply and still works.
Is a Vaillant F28 dangerous?
The lockout is a safety feature working correctly, so the code is not a reason to panic. What matters is what you do next.
If you can smell gas, feel unwell, or suspect carbon monoxide, stop reading and act. The Health and Safety Executive's advice is to call the National Gas Emergency Service freephone number, open all doors and windows, and shut off the gas supply at the meter control valve if you can find it. That number is 0800 111 999 (HSE, domestic gas safety FAQs).
Otherwise the rule is simple: do the outside-the-boiler checks below and leave everything inside the casing alone. The HSE is unambiguous about who may open it: "Anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work." Most of Vaillant's listed causes sit inside the casing, where only a Gas Safe registered engineer may work. So do not remove the front panel, do not clean the ignition electrode, and do not touch the gas valve or any wiring.
Safe checks to make before you call anyone
Vaillant publishes two homeowner checks for F28, plus its own cold-weather advice on the condensate pipe. None of them costs anything, and none of them means opening the boiler.
1. Are your other gas appliances working?
Try the hob, or any other gas appliance. If nothing gas-powered will run, the problem is the supply into your home rather than the boiler, and your gas supplier is who you need. If everything else runs normally, the gas is arriving and the problem is closer to the boiler.
2. If you have a prepayment meter, check the credit
Vaillant lists "insufficient credit on a pre-payment meter" among the causes of F28. No credit means no gas, no gas means no flame, and no flame means F28. Top up, then follow the meter's instructions to restore supply. The boiler can still need a reset afterwards, because the lockout does not clear itself.
3. In cold weather, check the condensate pipe outside
A condensing boiler drains waste water through a plastic pipe, usually white or grey, that often runs outside to a drain. When it freezes, the condensate backs up into the combustion chamber and the boiler stops. A frozen condensate pipe is a cold-weather fault, which is why Vaillant tells you to check for ice blockages in severe weather.
Vaillant's own instructions are to pour warm water along the affected section, using something like a watering can, until the ice clears or the pipe drains. Their explicit warning: "Do not to use boiling water from a kettle as this can damage or crack the pipe, causing damage that will be costly to repair and render your boiler non-functioning for the duration." A hot water bottle held against the pipe works too, and insulating it afterwards helps stop a repeat (Vaillant condensate pipe advice).
Only do this if the pipe is reachable from the ground. Never climb to it. If the pipe is blocked rather than frozen, our guide to fixing a blocked condensate pipe covers what else to look for.
How to reset a Vaillant boiler showing F28
Reset after the checks, not before. Vaillant's guidance is that "often resetting your boiler can resolve a fault or temporarily get your boiler working again", and they point you to your model's user manual for the procedure, because the reset control differs between an ecoTEC pro, an ecoTEC plus and an ecoFIT. Two cautions matter more on F28 than on most codes:
- Reset once, then stop. Every reset asks the boiler to release gas and try again. If it locks out a second time, the cause has not gone away.
- Note the word "temporarily" in Vaillant's own wording. A reset that works but fails again the next morning has not solved anything, and that pattern is worth telling the engineer: it points at intermittent gas supply or a condensate pipe that refreezes overnight.
If the code clears after you topped up a meter or thawed a pipe, that is the fault genuinely fixed. If it clears for no reason you can identify, treat it as borrowed time.
What causes F28, and what an engineer checks
Vaillant lists five causes. Here is what each one means in practice.
Not enough gas reaching the boiler
This covers low gas pressure at the appliance as well as no gas at all. An engineer measures the working gas pressure at the boiler and traces it back, checking the supply pipework and the meter or regulator. If the supply itself is at fault rather than the boiler, that is a job for your gas supplier rather than a repair engineer.
Condensate backed up in the combustion chamber
Covered above, with one addition: a condensate pipe can block through debris or a trapped bend, not only ice, so this cause does not disappear in summer.
A faulty ignition electrode
The electrode produces the spark that lights the gas. It wears, and eventually the spark is no longer reliable enough to light the burner. It is a consumable part, and Vaillant lists a faulty ignition electrode as one of the five causes of F28.
A faulty PCB
The printed circuit board decides when to open the gas valve and when to spark. If it is not sequencing those correctly, ignition fails even though the gas and the electrode are fine. It is the most expensive cause on the list, and the one an engineer reaches last, by elimination.
An issue with the flue system
The burner needs air in and combustion gases out. A blocked, restricted or badly fitted flue upsets that balance and can stop a stable flame establishing. A blocked flue is also the scenario where combustion products can enter the room, which is another reason this belongs with a registered engineer.
Vaillant treats some related faults as their own codes, including F29, F54 and F88, which are set out in the table below. If your display has shown more than one code, tell the engineer all of them.
What an F28 repair involves, and what it costs
We will not quote a figure we cannot stand behind, because F28 has no single fix. The cost depends on which of the five causes it turns out to be, and the spread is wide:
- Nothing at all if it is a frozen condensate pipe you thaw yourself, or a meter you top up.
- A parts-and-labour repair for an ignition electrode.
- A significantly larger job for a PCB, where the part alone dictates the bill and availability varies with the age of the model.
Two questions are worth asking. What was tested to reach the diagnosis, so you are not paying for a part swapped on a hunch. And whether the part is likely to be on the van, because ordering a circuit board for an older ecoTEC often means a second visit.
If the boiler is still inside its manufacturer warranty, check that before you pay anyone. Vaillant warranties depend on registration after installation: our guide to checking a Vaillant boiler warranty shows how to confirm where you stand.
Let us fix your boiler.
Smart Plan works with a national network of thousands of Gas Safe registered engineers. We aim to get an engineer out to you within two working days, with cover-plan call-outs booked Monday to Friday, 08:00 to 18:00. With boiler and central heating cover on your plan, a covered breakdown includes parts and labour up to £500 in a 12 month period if the boiler is under seven years old, or £200 if it is over seven, and the plan's exclusions still apply. A £95 call-out fee, paid in advance, applies in defined cases including the first 30 days of a new plan, during which your cover still applies.
F28, F29 and F54: how the ignition codes differ
Vaillant uses several codes around ignition and gas. These meanings are Vaillant's own:
| Code | Vaillant's meaning | What it points at |
|---|---|---|
| F28 | Flame not detected during ignition | The boiler never lit in the first place |
| F29 | Unsuccessful reignition after flame loss | It lit, then lost the flame while running and could not relight |
| F54 | Gas supply fault | The supply itself, often seen alongside F28 or F29 |
| F88 | Electrical connection to gas valve interrupted | Wiring to the gas valve rather than the gas |
Source: Vaillant UK, boiler fault codes.
We cover the Vaillant F29 fault code and the Vaillant F54 gas supply fault separately, and the full Vaillant fault code list covers the rest.
More questions about the Vaillant F28
Why does F28 keep coming back after a reset?
Because a reset clears the lockout, not the cause. Returning within minutes usually means the gas or the spark is still absent. Returning on cold mornings but not mild ones is the signature of a condensate pipe refreezing, and insulating that pipe is the fix. Returning every few weeks points at an electrode nearing the end of its life.
Which Vaillant boilers show F28?
F28 is in Vaillant's current fault code set for its domestic gas boilers, so it turns up across Vaillant's domestic gas boiler ranges. The meaning does not change between models; the reset procedure does, which is why Vaillant sends you to your model's manual. You will also see it written F.28. Same code.
Does F28 mean I need a new boiler?
Not on its own. F28 says ignition failed, and the causes Vaillant lists include a consumable part, a blocked condensate pipe and gas not reaching the boiler. It becomes a replacement question only when the diagnosis is a major electronic component on a boiler well into its second decade, and even then it is the repair bill against the boiler's remaining life. Get the diagnosis before the sales pitch.
Can I use my hob while I wait?
Your gas hob is a separate appliance and is unaffected by a boiler lockout, so yes, you can cook. Never use a hob or oven to heat a room, though: burning gas in an unvented space produces carbon monoxide. A plug-in electric heater is the safe way to keep one room warm.
What to do next
Do the free checks above. If none of them explain it, or the code returns after one reset, you need a Gas Safe registered engineer. Tell us what your boiler is doing and we will arrange a callback. If you smell gas, call 0800 111 999.
