Vaillant F28 Fault Code: Boiler Not Firing Up? Here’s the Fix

F28 is the Vaillant code you're most likely to see on a frosty morning. It's not dangerous in itself: the boiler stops because it couldn't light safely. Glow-worm boilers use the same code. This guide covers what F28 means, what to check, how to reset, how it differs from F29, and what repairs cost. For every other code, see our Vaillant fault codes list.

What does F28 mean on a Vaillant boiler?

F28 (or F.28) means "start-up unsuccessful": the boiler failed to ignite. When there's a demand for heat, the boiler opens the gas valve and sparks the ignition electrode. A flame sensor then checks for a flame. If it doesn't detect one after several attempts, the boiler locks out and shows F28.

Both codes have similar causes, and you check them the same way.

Is F28 dangerous?

No: the lockout is a safety feature that stops unburnt gas building up.

The most common causes of F28

1. No gas reaching the boiler

This is the first thing to rule out, and it's often the answer.

• Does your gas hob or fire light? If not, the problem is your gas supply, not the boiler.
• Prepayment meter? Check you haven't run out of credit or emergency credit.
• Is the gas valve at the meter fully on? The handle should be in line with the pipe.
• Recent work on the gas meter or in the street? Air in the gas pipe can stop the boiler lighting until it's purged.

2. A frozen condensate pipe (very common in winter)

Condensing boilers drain acidic water through a plastic pipe, usually white or grey, that often runs outside to a drain. In freezing weather it can ice up; water backs up into the boiler and it fails to light. You might hear gurgling from the boiler. This is one of the most common causes of F28 between December and February.

3. A worn or dirty ignition electrode

The electrode makes the spark that lights the gas, and it also often senses the flame. Over time it wears, gets dirty or its lead degrades. Engineer job.

4. Low gas pressure or a faulty gas valve

If the gas valve inside the boiler isn't opening properly, or gas pressure is too low, the burner won't light. Engineer job.

5. A blocked flue or air intake

The boiler needs air to burn gas. A blocked flue terminal (leaves, birds' nests, snow drifts against a low-level flue) can stop ignition. Check outside that nothing is obviously blocking the flue, but don't try to dismantle it.

6. A faulty PCB (circuit board)

Less common, but a failing control board can cause F28. Engineer job.

How to fix F28: step by step

1. Check for a gas smell. If you smell gas, stop and call 0800 111 999.
2. Test another gas appliance, such as the hob. If that doesn't work, it's a supply issue: check the meter, prepayment credit, or call your energy supplier.
3. In cold weather, check the condensate pipe outside (see below).
4. Check the flue outside isn't blocked by snow, leaves or debris.
5. Press the reset button on the boiler and wait a couple of minutes for it to try to light.
6. If F28 returns, stop resetting. Repeated resets won't fix a worn electrode or faulty gas valve. Book a Gas Safe engineer.

How to thaw a frozen condensate pipe

1. Find the plastic condensate pipe where it leaves the boiler and runs outside.
2. Look for the frozen section, usually at the lowest point, a bend, or the open end near the drain.
3. Pour warm (not boiling) water over it, or hold a hot water bottle against it. Boiling water can crack the plastic.
4. Once it's clear, reset the boiler.
5. To stop it happening again, ask an engineer about lagging the pipe, shortening the outside run, or upsizing it to 32 mm.

F28 on a Glow-worm boiler

Glow-worm boilers are made by the same group as Vaillant and use F28 for ignition failure as well. The checks and fixes in this guide apply in exactly the same way.

How much does it cost to fix F28?

If your boiler is more than 10–12 years old and needs a gas valve or PCB, it's worth getting a quote for a replacement too, especially if it's had several faults recently.

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