Vaillant F28 Error Code: Causes and Fixes in Milton Keynes

F28 is one of the more annoying codes a Vaillant owner can see, usually on the coldest morning of the year. The boiler will not fire, there is no heating and no hot water, and the screen flashes F28 no matter how many times you press reset. The good news is that a large share of the F28 calls we go to in Milton Keynes between November and March turn out to be a frozen condensate pipe, which you can clear yourself in ten minutes. This guide explains what the code means, the causes we see most locally, the three checks that are safe to do, and what decides the cost when it does need an engineer.

What F28 means

When a Vaillant fires, it runs a fixed sequence: the fan runs up, the gas valve opens, the electrode sparks, and the flame sensor confirms a flame. F28 means the boiler went through that sequence five times and never saw a flame. It locks out as a safety measure and will not try again until it is reset. Vaillant's own fault code list, in the user manual and on its website, describes it as "failure during ignition".

There are two families of cause: gas is not reaching the burner, or gas is reaching the burner but the ignition side is not doing its job.

The causes we see most in Milton Keynes

• Frozen condensate pipe. By far the most common in winter. The narrow plastic pipe that carries condensate from the boiler to an outside drain freezes in a cold snap, the boiler cannot clear its condensate and it locks out. Older grid-square houses where the boiler was moved to a garage or an outside wall, and new-builds where the pipe runs a long way down an exposed wall, are the usual suspects.
• Gas supply off or interrupted. The emergency control valve at the meter has been knocked closed, a prepayment meter has run out, the meter has been exchanged and not turned back on, or there is a problem in the street. We see this after any work near the meter.
• Worn ignition electrode or lead. The part that makes the spark wears and the ceramic can crack. Common on boilers over about seven years old, and a boiler that has been slow to light for weeks is usually telling you this.
• Dirty or failed flame sensor. The boiler lights, but the sensor does not see it, so it shuts the gas off and tries again.
• Gas valve fault. The valve that admits gas to the burner is not opening properly.
• Circuit board fault. The board is not driving the ignition sequence correctly. Often shows as F28 mixed with other codes.

A related code worth knowing: F29 is flame loss during operation rather than at ignition, and in a cold snap it very often has the same cause, the condensate pipe.

Three checks that are safe to do yourself

None of these involve the boiler casing or any gas pipework. Anything beyond them is a Gas Safe registered engineer's job.

1. Is the gas on? Try the hob or another gas appliance. If nothing works, the problem is the supply, not the boiler. Look at the emergency control valve by the meter: the handle should be in line with the pipe, not across it. If you have a prepayment meter, check the credit. If the gas is off and you do not know why, ring your supplier before you touch anything else.

2. Is the condensate pipe frozen? Find the white or grey plastic pipe, usually around 22mm across, that leaves the boiler and runs outside to a drain or soakaway. If any of it is outdoors and the temperature is at or below freezing, assume it has frozen. Pour warm water along it, never boiling water, which can crack the plastic. Concentrate on bends, the bottom of any vertical drop and where it meets the drain. A jug of water from the hot tap is enough. A hot water bottle held against a stubborn section works too. Then reset.

3. One reset. Hold the reset button for about three seconds. If the cause is gone, it will fire. If F28 returns, stop. Do not reset it five or six times. Each failed attempt cycles the electrode and the board, and repeated resets can turn a cheap fault into a dearer one.

If you smell gas at any stage, leave the house, do not touch light switches, and call the National Gas Emergency Service on 0800 111 999 before you call anyone else.

When it needs a Gas Safe engineer

• Other gas appliances work but the boiler still shows F28.
• You have thawed the condensate pipe and it comes back.
• The boiler clicks, trying to light, but nothing catches.
• The boiler lights for a second and then locks out.
• F28 has been appearing on and off for weeks and is getting more frequent.
• The condensate pipe freezes repeatedly. That is a routing or insulation problem to fix properly, not just thaw.

Our boiler repair service covers all of this, and for no heat and no hot water in cold weather our emergency line puts you at the front of the day.

What decides the cost of an F28 repair

We do not print a price list for F28 because the same code covers a ten-minute fix and a circuit board. What moves the number:

• The part. Cleaning a flame sensor or swapping an ignition electrode is a small part and under an hour. A gas valve is a bigger part and a longer job. The circuit board is the top end.
• The boiler's age and model. Parts for a current ecoTEC plus are on the van or a phone call away. Parts for a fifteen-year-old model can take longer to source.
• Warranty. A Vaillant still under warranty may have the part covered, and the repair may need booking through Vaillant. Have the registration paperwork ready before you ring anyone.
• Whether it is worth it. The honest point: on a Vaillant past twelve or thirteen years old, if F28 turns out to be the circuit board or the gas valve, we will normally give you a replacement figure alongside the repair figure and tell you which we would choose. Often that is the repair. Sometimes it is not, and we would rather say so than fit an expensive part to a boiler with two winters left in it.

Stopping F28 coming back

• Insulate the condensate pipe. Foam pipe lagging from any DIY store on the external run, taped at the joints, stops most winter freezes. Better still is having the pipe re-routed inside or upsized where it runs outside. We do both as part of a winter check.
• Annual service. A service cleans the flame sensor, checks the electrode gap and lead, and picks up a gas valve that is starting to stick. It also keeps the Vaillant warranty valid, which needs a yearly service by a Gas Safe registered engineer.
• Keep the heating ticking over in a cold snap. A low setting overnight keeps warm condensate moving through the pipe so it does not get the chance to freeze.
• Act on slow ignition. If the boiler has started taking two or three goes to light, that is the electrode telling you it is on the way out. Cheaper to replace at a service than on a breakdown visit.

If you are seeing F22 instead, that is low water pressure and a different fix; see our guide to the Vaillant F22 code.

Vaillant F28 repair across Milton Keynes

Plumbline MK is Gas Safe registered (957816) and a Vaillant Advance installer. We carry the common Vaillant ignition parts and we cover all of Milton Keynes including Bletchley, Newport Pagnell, Wolverton, Stony Stratford, Woburn Sands and Olney, plus Buckingham, Leighton Buzzard, Towcester and Bedford. Ring 07805 844 016 and tell us the code, the model and whether the condensate pipe runs outside, and we will tell you on the phone whether it is worth trying the thaw first. Or get in touch here.

Frequently asked questions

Can I reset a Vaillant showing F28?
Yes, once, after checking the gas is on and the condensate pipe is clear. Hold reset for about three seconds. If it comes back, stop resetting and call. Repeated resets do not fix anything and can damage ignition parts.

Is F28 dangerous?
The lockout itself is the boiler doing its job: it will not run gas without a confirmed flame. Never try to bypass it. If you smell gas at any point, leave and ring 0800 111 999.

Why does F28 only happen in winter?
Almost always the condensate pipe freezing where it runs outside. Thaw it with warm water and get the outside run insulated or re-routed before the next cold spell.

Why does F28 keep coming back?
An intermittent gas supply problem, an electrode that works some of the time, or a flame sensor getting dirtier. All three get worse, not better. Once-a-week F28 becomes several-a-day F28, so get it looked at while it is still the cheaper repair.