An F28 fault on a Vaillant boiler means it has tried to ignite and failed, so it has locked out for safety.

There are a few simple things worth checking before you call anyone, and they solve it more often than people expect.

Check these three things first

• Is your gas supply on? Check other gas appliances in the house. If your hob will not light either, the problem is the supply, not the boiler.
• Are you on a prepayment meter? If it has run out of credit, the boiler will show F28. It is a more common cause than most people realise.
• Has your gas been off recently? After a supply interruption there can be air in the pipe. The boiler may need two or three ignition attempts to clear it.

If none of those apply, press and hold reset for three seconds. If it locks out again, stop.

Why you must not keep resetting it

What causes it beyond the simple checks

A failed ignition electrode or a dirty flame sensor, a gas valve fault, a blocked or restricted flue, or low incoming gas pressure. All of these are Gas Safe work and none of them are DIY.

If you smell gas

How Vantage can help

Gas Safe registered, covering Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day.

Frequently asked questions

How many times can I reset it?

Two or three at most. If it will not stay lit, it needs an engineer.

Could it be the weather?

Yes. In very cold conditions a frozen condensate pipe can cause ignition problems, often alongside a gurgling noise.

Is F28 dangerous?

The lockout itself is the boiler being safe. Repeatedly overriding it is where the risk comes in.

What an engineer will do

An ignition fault is diagnosed by working along the sequence the boiler follows, rather than by replacing parts in hope.

Gas first. Working gas pressure is measured at the appliance with the boiler running, not just standing pressure with it off. An undersized or partly obstructed supply pipe gives adequate standing pressure and drops away under load, which produces intermittent ignition failure that gets wrongly blamed on the boiler.

Then the spark and the sense. The ignition electrode and the flame sensing electrode are inspected for wear, cracked ceramic, carbon deposits and correct gap. Worn or dirty electrodes are the single most common cause of ignition lockouts on an otherwise healthy appliance, and they are inexpensive.

Then the flue and condensate. A restricted flue or a condensate trap that has silted up or frozen prevents proper combustion. In winter the condensate is checked early, because it is the most common seasonal cause.

The gas valve last. It is the expensive component and the least likely of the four, so it is tested rather than assumed.

Stopping it coming back

• Have it serviced annually. Electrode wear is exactly what a service catches before it becomes a December breakdown. See boiler servicing.
• Insulate the condensate pipe where it runs outside. A frozen condensate is preventable and it is the most common winter lockout we attend.
• Keep the flue terminal clear of vegetation, nests and stored items.
• If you are on a prepayment meter, keep some credit in hand through cold weather, because a boiler working hard uses more than you expect.

Related fault codes

Other Vaillant codes: Vaillant F22 · Vaillant F75 · Vaillant F29 · Vaillant F27 · Vaillant F20 · Vaillant F23 · Vaillant F83 · Vaillant F32 · Vaillant F62 · Vaillant F61 · Vaillant F72

The same problem on other makes (ignition and flame faults): Ideal L2 · Ideal F2 · Baxi E133 · Glow-worm F28 · Viessmann F4

See also our guide to boiler not firing up, the full boiler advice section, and boiler repair.

When to call an engineer

Still stuck? We repair this fault regularly across Hertfordshire, Bedfordshire and Cambridgeshire. Call 01462 506374 to get an engineer booked, or to talk it through first if you would rather. Genuine 24 hour cover, 365 days a year.