F.25 on a Glow-worm is a safety switch-off because the flue gas temperature is too high.

This one is worth taking seriously, because an abnormally hot flue can indicate the heat exchanger is not transferring heat properly.

What F.25 actually means

On a condensing boiler the flue gas should leave relatively cool, because most of the heat has been transferred into the water.

If the flue gas is hot, heat is going up the flue instead of into the system. That is inefficient and it can indicate a real problem inside the appliance.

What you can safely check yourself

Nothing safe inside the appliance.

If you have noticed the boiler running noisily, or much more steam than usual from the flue outside, mention it when you call.

When to stop, and why

What causes it beyond the simple checks

A heat exchanger fouled with sludge or scale so heat cannot transfer into the water, poor circulation, a flue sensor reading incorrectly, or a flue problem. Because a genuinely hot flue points at the heat exchanger, an engineer will want to establish whether the reading is real. Glow-worm is part of the Vaillant Group and the Energy range shares its fault code structure with Vaillant appliances.

How Vantage can help

We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.

Frequently asked questions

Is F.25 dangerous?

The boiler has stopped, which is the safe outcome. The reason needs establishing properly.

Does it mean a new heat exchanger?

Not necessarily, and it should not be assumed. A faulty sensor gives the same code.

Would a power flush help?

If the heat exchanger is fouled, it can. That is a diagnosis rather than a guess.

Related fault codes

Other Glow-worm codes: F.20 · F.22 · F.23 · F.24 · F.27 · F.28 · F.29 · F.32 · F.61 · F.75 · all boiler advice. See also boiler repair. The same family on other makes: Baxi E28. See also power flushing.

In Glow-worm’s own words

Glow-worm use different code sets across their ranges and generations. The definition above comes from The Glow-worm Energy range fault finding table. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.

When to call an engineer

If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.

Need help today?

Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.