Vaillant F28: What the Fault Code Means and Why Your Boiler Won’t Ignite

F28 means your Vaillant boiler tried to light its burner and failed, so it has locked itself out for safety. You will usually have no heating and no hot water until it is resolved. First check the simple things outside the boiler — that the gas supply is on, the pressure is around 1–1.5 bar, and, in cold weather, that the condensate pipe outside has not frozen — then reset the boiler once. If F28 comes straight back, stop resetting: the fault is most likely inside the boiler (gas valve, ignition electrodes or control board) and belongs to a Gas Safe registered engineer.

What F28 actually means

F28 is one of the most common fault codes on Vaillant ecoTEC boilers (you will also see it on the ecoFIT range). When you ask for heating or hot water, the boiler’s control board opens the gas valve and fires the ignition electrodes to light the burner. With F28, that ignition never took hold. After a set number of failed attempts the boiler stops trying altogether and displays F28 instead of keeping on attempting to light — that lockout is deliberate, not a further fault.

That distinction matters for how you read the code. F28 is a start-up failure: the flame never established in the first place. Its neighbour F29 is different — the flame lit, then went out during operation. And F27 points at a flame-detection fault, where the boiler thinks it sees a flame when it should not. They can look alike on a cold morning (no heating, a code on the display), but they point an engineer at different parts of the boiler, which is why noting exactly which code you saw is worth doing before you reset anything.

One reassurance first: F28 on its own is not a gas leak. The boiler has refused to run precisely because it could not light cleanly, and a boiler that will not light is not pouring unburnt gas into your home. The one exception to calm handling is if you can actually smell gas — see the safety box below, which overrides everything else on this page.

Safety, if you smell gas: do not touch the boiler or any electrical switches. Open windows, turn the gas off at the meter only if you can do so safely, leave the property and call the National Gas Emergency line on 0800 111 999.

Why a boiler fails to ignite

Ignition needs three things at the same moment: gas reaching the burner, a spark to light it, and the boiler’s electronics confirming the flame. If any one of them is missing, you get F28. The causes split neatly into the ones you can check from outside the boiler and the ones you cannot.

Outside the boiler — the common, homeowner-checkable causes:

• Gas supply interrupted. The most ordinary cause of all. A meter that has been turned off, a prepayment meter run out of credit, work on the supply in the street, or a tripped emergency control valve all leave the boiler with nothing to light. The boiler is fine; it simply has no fuel.
• Frozen condensate pipe. In cold weather the white plastic pipe that carries condensed water away from the boiler — usually running outside to a drain — can freeze solid. The boiler senses the blockage and refuses to fire, and F28 is one of the codes that can follow. This is the classic “worked yesterday, dead on the coldest morning” fault.
• Low system pressure. If the pressure has dropped well below 1 bar, the boiler may refuse to start at all. Low pressure is its own fault code on Vaillant boilers (F22), but a borderline reading can contribute to a failed start, so it is always worth a glance at the gauge.
• A power blip or a boiler left in an odd state. After a power cut the boiler may sit in lockout until it is reset, even though nothing is actually broken.

Inside the boiler — engineer territory only:

• Worn or dirty ignition electrodes. The electrodes spark the gas and then sense the flame. When they wear, carbon up or crack, the spark weakens or the flame signal never registers, and the board concludes ignition failed.
• A faulty gas valve. If the valve does not open, or opens only partly, the burner gets too little gas to sustain a flame.
• A control-board (PCB) fault. The board sequences the whole start-up — fan, gas valve, spark, flame check. If it mis-times or misreads any step, it aborts the start and shows F28.
• Fan or flue problems. The boiler checks that combustion air is moving properly before it will light. A blocked flue terminal or a failing fan can stop the sequence before ignition is even attempted.

The practical upshot: roughly half the F28 cases a homeowner meets are supply or weather problems outside the boiler, and the rest need the case opened — which is the line you do not cross yourself.

The checks you can safely do yourself

Everything here happens outside the boiler case. Do them in order, and do not be tempted to go further if they do not clear the code — opening the case, touching gas pipework or adjusting anything inside is engineer work, full stop.

1. Check the gas supply is actually on. The quickest test is another gas appliance: try a hob ring. If the hob will not light either, the problem is the supply, not the boiler. Check the meter is on, a prepayment meter has credit, and nobody has turned the emergency control valve off. If the hob lights normally, gas is reaching the house and the fault is with the boiler.
2. Check the pressure gauge. Most ecoTECs show pressure on the digital display; some have a dial. Around 1–1.5 bar when cold is normal. If it reads well below 1 bar, top the system up through the filling loop — our Vaillant F22 guide walks through that check step by step — then try the boiler again.
3. In cold weather, look at the condensate pipe outside. It is usually a white plastic pipe running from the boiler to an outside drain. If it feels frozen solid or you can see ice at its end, pour warm (never boiling) water over the frozen section, or wrap it in a warm cloth, until water starts to drain freely again. Then reset the boiler once. Lagging the pipe afterwards stops the same fault returning on the next frost.
4. Reset the boiler once — and only once. The reset button is usually marked with a flame-with-a-cross symbol, or a dedicated reset control depending on your model; your manual shows which. Press it, let the boiler run through its start-up sequence, and give it a minute. One reset clears a genuine one-off lockout, including after a power cut or a thawed condensate pipe.
5. Note what happens next. If the boiler fires and stays lit, the fault is cleared — but keep an eye on it for a day. If F28 returns immediately, or returns within hours, do not keep resetting.

Why only one reset matters: repeated resets do not fix a gas valve or an electrode fault, and each failed ignition attempt wears the ignition components a little more. Worse, resetting over and over can mask a pattern an engineer needs to see — how quickly the code returns tells them a lot about which part is failing. One reset is troubleshooting; five resets is hoping.

When to call an engineer

Book a repair rather than trying anything else if any of these apply:

• F28 returns after a single reset.
• The hob lights fine, the pressure is normal and the condensate pipe is clear, yet the boiler still will not light — the fault is inside the boiler.
• F28 keeps coming back every few days, even if a reset clears it each time. An intermittent ignition fault is still a fault, and it tends to fail permanently on the coldest night of the year.
• You hear the boiler clicking repeatedly without firing, or it fires briefly and then drops out — both point at ignition or flame-sensing parts.
• You smell gas at any point — leave the property and call 0800 111 999 (see the safety box above), do not wait for an engineer booking.

If you are a landlord, note that an F28 that leaves tenants without heating or hot water in cold weather is urgent — the same duty-of-care timescales as any no-heating breakdown apply, and our landlord gas safety page explains the annual check that often catches tired electrodes before they fail.

What an engineer will do, and what it typically costs

An engineer investigating F28 works through the start-up sequence in order: confirming gas pressure at the boiler, checking the ignition electrodes and their leads, testing the gas valve, and checking the control board’s flame signal. A condensate or supply problem is usually spotted within minutes; an internal fault is confirmed by testing, not by swapping parts at random.

The reassuring part is that the common F28 fixes are ordinary repairs. Electrode replacement, a gas valve or a sensor fault will typically sit within the £70–£250 range that covers most standard boiler repairs on this site. A control-board failure can push a bill beyond that — all the more reason to agree a fixed price after diagnosis. Our guide to boiler repair costs in Walsall breaks down what drives a repair bill up or down, and our boiler repair page explains how a diagnosis visit works.

A note on prevention: ignition electrodes and the condensate route are checked during an annual service, long before they become a cold house and a fault code. If your boiler has not been serviced in a while, booking a boiler service is the cheapest way to keep F28 off the display.

Vaillant F28: your questions answered

Is F28 dangerous?
Not on its own — the lockout is the safety system working, and a boiler that will not light is not leaking gas into the house. What would be unsafe is opening the boiler to investigate, or ignoring a smell of gas. Anything inside the case, or involving the gas supply or flue, belongs to a Gas Safe registered engineer.

Can I just keep resetting it until it lights?
No. One reset is a reasonable test. If the code returns, the underlying fault is still there, and repeated ignition attempts wear the ignition parts without fixing anything. Book an engineer instead.

My hob works, so why won’t the boiler light?
Because gas reaching the house does not mean gas reaching the burner. The fault is then inside the boiler — electrodes, gas valve, fan or control board — and that is engineer territory, even though the supply is fine.

How is F28 different from F29?
F28 is ignition failure at start-up: the flame never established. F29 is flame loss during operation: the boiler lit, then the flame went out. Both end in lockout, but they point at different parts of the start-up sequence, so tell the engineer exactly which code you saw.

Does cold weather really cause F28?
Yes — through the condensate pipe, not the boiler itself. A frozen condensate pipe blocks the boiler’s drainage, the boiler refuses to fire, and F28 (or a related code) appears. Thawing the pipe and resetting once usually clears it; lagging the pipe stops it happening again.

Will topping up the pressure fix F28?
Only if low pressure is part of the problem. If the gauge reads well below 1 bar, top it up (see our F22 guide) and reset once. If the pressure is already normal, topping up will not help — the fault lies elsewhere.