Vaillant F28 Fault Code: Boiler Failed to Ignite

What does F28 mean on a Vaillant boiler?

F28 is logged on ecoTEC and similar Vaillant appliances as a startup failure: the boiler asked for a flame, went through its full ignition sequence, and the flame sensing electrode reported nothing back. Vaillant allows a set number of attempts, usually three or four in quick succession, before the control board stops trying and displays the lockout. The lockout is deliberate, and it is why F28 needs a manual reset rather than clearing on its own.

The distinction that matters is between F28 and F29. F28 is failure to light in the first place. F29 is a flame that lit and then went out during the burn. They feel identical from the kitchen, but they point at different parts of the appliance, so it is worth reading the display rather than assuming. Our page on the Vaillant F29 fault covers the flame loss side in the same detail.

Other makes log the same event under their own label, so an Ideal boiler L2 is the same ignition lockout written down under a different name. The number on the display changes with the badge on the case, the sequence behind it does not, and the order an engineer works through the causes in is the same whichever make is on the wall.

What the boiler is doing when it locks out

Stand next to it and call for hot water and you can hear the whole sequence in about fifteen seconds. The fan spins up to purge the heat exchanger and prove the flue is clear. The igniter then starts clicking, a rapid tick tick tick as the spark electrode arcs, and at the same moment the gas valve should open and gas should reach the burner.

On a healthy boiler the clicking stops after two or three seconds, because the flame sensing electrode has picked up a flame current and told the board to stop sparking. On a boiler heading for F28 the clicking runs its full window, stops, the fan winds down, and the whole thing starts again. After the last attempt the display shows F28.

What you hear tells you a lot before anyone opens the case. Clicking with no woof of ignition behind it usually means no gas is arriving. A brief woof that dies means gas arrived but the flame was not held or not detected. Silence with no clicking at all points at the igniter or the board rather than the gas side.

A boiler locked out on F28 gives you neither heating nor hot water, and that makes the radiators a free test. If they are still getting warm and only the hot water has gone, what you actually have is no hot water but the heating works, and the boiler is firing, so F28 is not the fault in front of you. Where the appliance on the wall is a combi, no hot water from a combi boiler while the radiators still heat is the better trail to follow than anything on this page.

The same test works at the outlets. A boiler in lockout is not heating water for anything, so cold at every tap and every shower fits the lockout. Hot water at the basin and no hot water in the shower fits a shower valve, a thermostatic cartridge or a scaled up mixer instead, and no amount of resetting the boiler will change it.

Causes of a Vaillant F28, most likely first

This is roughly the order an engineer works in, from most common to rarest. Most F28 callouts end in the first three.

• No gas at the appliance. A prepayment meter out of credit, a supply interruption, or the emergency control valve at the meter left part closed after other work. This is the single most common cause and costs nothing to rule out.
• A gas isolation valve closed under the boiler. Every Vaillant has one on the gas pipe entering the case. Anyone who has worked in the kitchen may have closed it and not opened it fully again.
• Frozen condensate pipe. Common in London where the condensate has been run out through a wall and down an outside elevation in narrow plastic pipe. A blocked condensate stops the boiler firing and can throw a range of codes including F28.
• Air in the gas supply after work on the meter or a new meter fit. The boiler will often light after two or three resets once the air has purged, and then behave normally.
• A dirty or cracked ignition electrode. Carbon builds up on the tip, the spark gap drifts out of tolerance, or the ceramic insulator cracks and the spark tracks to earth instead of jumping the gap.
• A faulty flame sensing electrode or its lead. Gas is burning perfectly well, but the flame current never reaches the board, so the board concludes there is no flame and shuts down. You sometimes see a brief flame through the sight glass on these.
• Low gas working pressure at the appliance. Undersized or partly blocked gas pipework, or a shared supply where a cooker and a boiler both pull at once, can drop the pressure under load below what the burner needs to establish.
• A failed gas valve. Less common than people assume, and only ever diagnosed after the supply and the ignition side have been proved.
• A control board fault. The last thing on the list, and it should be the last thing anybody fits.

How to reset a Vaillant F28, and what to check first

None of the following involves taking a cover off, and all of it is worth doing first. A fair share of F28 calls are resolved on the phone by one of these, and the no hot water checklist before you call runs through the same ground for a property that has lost hot water rather than a boiler showing a code.

If every one of those checks out and F28 comes straight back, the fault is inside the appliance. Stop resetting at that point. Every reset drives another full ignition cycle through an electrode that is already failing, and boilers that get reset twenty times a day tend to arrive at a bigger repair than they started with.

• Light a gas hob. If the hob will not light either, the problem is the gas supply and not the boiler at all. Check the meter for credit and check the emergency control valve handle at the meter is in line with the pipe.
• Look for the small lever valve on the gas pipe going into the underside of the boiler. Its handle should sit in line with the pipe. Across the pipe means closed.
• In cold weather, follow the white plastic condensate pipe. If any of it runs outside, feel along it for a cold solid section, usually at a bend or where it meets an outside drain. A jug of warm, not boiling, water poured along that section will often thaw it.
• Check the boiler pressure gauge is not sitting on zero. A boiler with no water in it will not fire, and that produces a different code, but it is worth a glance while you are there.
• Press and hold the reset button for the few seconds the manual specifies. Give it two attempts, not ten.

If you can smell gas at any point, do not reset the boiler and do not touch any electrical switch. Turn the gas off at the meter, open windows, and call the national gas emergency service on 0800 111 999 before you call a plumber.

What an engineer does on an F28 call

Anything behind the boiler case is gas work and is legally restricted to an engineer qualified for it. Our engineers are qualified for the work described here.

The visit runs in a fixed order so the fault is proved rather than guessed. Gas working pressure is measured at the appliance test point with a manometer, standing and then under full load, because a supply can read perfectly at rest and collapse the moment the burner asks for gas. The ignition electrode is pulled and inspected for carbon, a cracked insulator and the correct spark gap, and the spark is watched directly. Where a flame does establish, the flame current is measured in microamps to see whether the board is actually being told there is one.

Only when the supply, the spark and the sensing side have all been proved does attention move to the gas valve and then the board. Fitting an ignition electrode because F28 is on the display, without measuring anything, is how people end up paying for three parts and still having the fault. That sequence is what our boiler diagnosis service is, and it is charged as time rather than as a part.

The rate is fixed before we attend and the total depends on how long the job takes. What an F28 costs therefore depends almost entirely on whether it turns out to be a closed valve or a gas valve.

Why a Vaillant F28 keeps coming back after a reset

A reset that holds for a week and then fails again is telling you something different from a reset that fails within a minute, and the pattern is worth writing down before anybody attends. An intermittent F28 is diagnosed from its behaviour as much as from anything measured on the day, because the fault may not be present while an engineer is standing in front of it.

The recurring causes fall into a short list. A condensate pipe that freezes overnight and thaws by lunchtime gives a code that appears at breakfast and has cleared by the time anyone arrives. A gas supply that is adequate at rest but collapses under load lets the boiler light and run until something else in the property draws gas at the same time. An ignition electrode with a hairline crack in its ceramic sparks perfectly when cold and tracks to earth once the case is hot. A flame sensing lead with a degraded boot does the same.

What settles all of those is measurement rather than repetition. Working gas pressure is read under full load with every gas appliance running, not standing, and flame current is read in microamps while the burner is alight. Both readings either hold or they do not, and neither can be guessed from the display.

The one thing to avoid while you wait is resetting on a loop. Every reset runs another full ignition sequence through the part that is already failing, and an appliance that has been reset twenty times a day for a fortnight usually needs more replacing than it did at the start.

• Fails first thing on cold mornings and clears later: a freezing condensate pipe
• Fails only when the hob or another appliance is also running: gas supply under load
• Fails once the boiler has been running a while: an electrode or lead breaking down when hot
• Fails at random with no pattern at all: flame sensing or an earthing fault on the supply
• Write down the date, the time and the weather each time it locks out

When F28 means the boiler is worth replacing instead

One F28 on an otherwise healthy boiler is a repair, and usually a small one. F28 on an appliance well past fifteen years old that has already had a gas valve or a board, and is now failing to light two or three times a week, is a different conversation. Parts availability is the practical limit on older Vaillant models, and there is a point at which the next part costs more than it buys. What an honest engineer will tell you is which of the two you are in before you spend anything, and our boiler replacement page sets out what that decision actually turns on.