Vaillant F28 fault code: what it means and what to check

Vaillant's own definition of F28 is precise and worth reading carefully: the flame signal was not detected during the ignition phase. The boiler asked for gas, sparked, and then looked for proof that a flame had actually established. It did not find one, so it stopped.

That is the safety system doing exactly what it is built to do. A boiler that kept opening the gas valve at an unlit burner would be filling a sealed combustion chamber with unburnt gas, so after a set number of failed attempts it locks out and puts a code on the display instead. F28 is not the boiler being broken. It is the boiler refusing to guess.

It is written F.28 on the display and is most often seen on the ecoTEC range, which is one of the most widely fitted boilers in the country and very common across housing here.

Source: Vaillant, boiler fault codes list

What causes it, in the order it usually turns out to be

Vaillant list the possible causes as not enough gas reaching the boiler, condensate backed up in the combustion chamber because of a blocked condensate pipe, a faulty ignition electrode, a faulty circuit board, or a problem with the flue. In practice the likelihood stacks up roughly in this order, and the first two are the reason this article tells you to check things before you call anyone.

• No gas is arriving. This is the most common cause and the one most often missed, because people assume a gas problem would be obvious. It covers a closed isolation valve, an empty pre-payment meter, a supply issue in the street, and gas pressure that is too low at the appliance under load.
• A frozen or blocked condensate pipe. Every condensing boiler produces acidic water that has to drain away. If that pipe runs outside and freezes, the water backs up, and Vaillant's own list is explicit that condensate backing into the combustion chamber will produce this code. It is why F28 arrives in waves during the first hard frost.
• The ignition electrode or its lead. The electrode makes the spark and, on most boilers, also senses the flame. Sooted, cracked, misaligned or corroded, it can spark without ever sensing, which is a perfect F28.
• The flue. A restriction or a recirculation problem upsets the air side of combustion enough to stop a flame establishing cleanly.
• The gas valve or the circuit board. Real, and they do fail, but they are last on the list rather than first, and they should be reached by elimination rather than assumption.

Source: Vaillant, boiler fault codes list

The code alongside it that tells you the answer

Here is something Vaillant publish and almost nobody repeats. Their fault list contains F.54, defined as a fault in the gas supply, and it says in the definition itself that it appears in combination with F.28 or F.29. The causes they give for it are a closed gas isolation cock, gas flow pressure that is too low, or a defective gas valve.

So if your display has shown F.54 as well as F.28, Vaillant have already told you where to look, and it is not the ignition side of the boiler. That single detail changes what a first visit should be spent on. Photograph the screen every time it locks out, including any code that flashes up before or after the one you remember.

Source: Vaillant, boiler fault codes list

What you can safely check yourself

Vaillant give three homeowner checks for F28, and they are genuinely worth doing before you book anything, because two of them are the most common causes on the list.

• Is anything else gas working? Try the hob. If nothing gas in the house works, the problem is the supply, not the boiler, and an engineer visit to the appliance is the wrong call.
• If you are on a pre-payment meter, check the credit and top up if needed. It sounds too simple to be the answer. It regularly is.
• In cold weather, look at the condensate pipe. It is usually a white plastic pipe leaving the wall nearest the boiler and running down to a drain. A gurgling or bubbling noise from the boiler is a strong tell that it is blocked.

Thawing a condensate pipe, the way the manufacturer says to

Vaillant's own method: pour warm water along the frozen section using something like a watering can, and keep going until the ice clears or the pipe drains. They are specific that you must not use boiling water from a kettle, because it can crack the pipe and turn a free fix into a repair. Once it is clear, reset the boiler using the procedure in your own manual, then insulate the pipe so it does not happen again.

Only do this at ground level, from a stable position, on a pipe you can reach standing up. Nothing about a locked out boiler is worth going up a ladder in freezing weather for. If the pipe runs at height or you cannot reach it safely, that is an engineer's job.

Vaillant also make a useful prevention point: pipes are more likely to freeze when the heating has not been running consistently through a cold spell, which is why this catches people who have been away. Leaving the heating on a low timed schedule while the house is empty is cheaper than the alternative.

Source: Vaillant, understanding an F28 fault code

Where it stops being your job

The checks above are the whole list. Vaillant strongly advise that you do not gain access inside the boiler, and they are right: the electrode, the gas valve and the combustion chamber all sit behind a case that only a Gas Safe registered engineer may remove. Nothing in there is a homeowner job, whatever a video suggests.

Reset once, after you have dealt with a cause you actually found. If F28 comes straight back, stop resetting. Repeated resets on a boiler that cannot light are not harmless.

And the hard stop that overrides everything else: if you can smell gas, do not reset anything, do not touch electrical switches, leave the property and call the National Gas Emergency Service on 0800 111 999.

What an engineer should actually do

So you can judge the visit you are paying for. A proper F28 diagnosis works from the gas side inwards rather than starting at the most expensive part.

That means checking the gas working pressure at the appliance with the boiler running at full rate, not just standing pressure with everything off, because an undersized or partly obstructed gas run only shows itself under load. It means gas rating the boiler against its data plate. Then the condensate route inside and outside the appliance, then the ignition electrode and lead for condition and gap, then the flue, and only then the gas valve or the board.

If the visit ends with a new electrode and no pressure reading taken, the fault may well come back, because the electrode was doing its job and reporting no flame.

Who pays for the visit

Worth knowing before you call. Vaillant state on their own fault code pages that they will attend any fault code but reserve the right to charge a call out fee for non-product faults.

That matters more on F28 than on most codes, because look again at the causes: an empty pre-payment meter, a frozen condensate pipe, a supply problem in the street and undersized gas pipework are all things that are not a fault with the appliance. Doing the three homeowner checks first is not just about fixing it yourself. It is about not paying for someone to tell you the meter was empty.

Source: Vaillant, boiler fault codes list

Repair or replace?

For most F28s the honest answer is repair, and often it is not even a repair. A one-off lockout in a cold snap that clears when the condensate pipe thaws is a plumbing problem and an insulation problem, not a boiler nearing the end of its life. An electrode is a routine part.

Four findings change the question, though, and they are worth recognising:

• It locks out on cold evenings and runs fine the rest of the time. That pattern points at gas arriving at too low a pressure when the network and the house are both at peak demand, which is a pipework issue rather than an appliance issue. Fitting a new boiler onto the same undersized run reproduces the fault on the new one.
• The condensate keeps blocking even after the pipe has been thawed and lagged. On an older boiler that is often the condensate route or its fall inside the appliance, which is a more serious conversation than the pipe outside.
• It is the third or fourth part in two winters, out of guarantee. At that point you are not buying a repair, you are subsidising a slow failure, and the money is better aimed at the replacement.
• The boiler is old enough that parts availability is becoming the deciding factor. A cheap part you have to wait a fortnight for in January is not a cheap part.

If this keeps happening

The pattern we see every autumn around Northampton and Milton Keynes is streets of estate stock fitted at the same time and therefore ageing at the same time, with condensate pipes run outside on the coldest wall of the house. The first real frost brings a wave of ignition lockouts across the same postcodes in the same week, and the boilers that recover are the ones whose condensate pipe was lagged properly at some point.

If yours has been locking out into a second or third winter, get the replacement priced while you still have heating rather than in the week it finally stops. Tell us about the job and we will put you in touch with three vetted local installers, with no obligation.