Vaillant F25 Fault Code: The Flue Gas Got Too Hot

What does F25 mean on a Vaillant?

A condensing boiler is designed to strip as much heat as possible out of its exhaust before it leaves through the flue. When the exhaust leaves hot, something is wrong with how heat moves from the burner into the water. F.25 is the boiler protecting itself and the flue by shutting down.

Vaillant’s two main sources describe F25 slightly differently, and it is worth knowing both. Vaillant’s owner fault code page lists a circulation fault (pump, valve or blockage), a defective wiring harness, or the flue gas temperature monitor triggering, and says to contact a heating installer. The installer manuals list a break in the wiring harness or its plugs, or the optional flue gas temperature limiter (where fitted) having triggered, and the 2021 edition also tells engineers to check the flue route, the water level in the heating circuit and the pump. In practice that means F25 is usually a genuine overheat from poor water flow or a restricted flue, or a false alarm from a connection that has worked loose.

What you can safely check

There is less to check yourself with F25 than with the ignition codes, because the causes sit inside the boiler or in the heating circuit. Two things are worth a look.

First, the water pressure, shown on the display of current models or on a gauge on older ones. Vaillant’s ecoTEC plus operating instructions recommend at least 1 bar and say to top up if it falls below 0.8 bar. If it has dropped low, poor circulation becomes more likely; our guide to Vaillant boiler pressure shows how to top it up. Second, make sure the heating circuit is open: radiator valves not all turned off, and no zone valve or controls left in an odd state after someone has worked on the system.

Then reset once. Note when F25 comes back. Straight away suggests a wiring or sensor fault. After a period of running suggests the boiler is genuinely overheating because water is not moving.

Do not open the casing to look at the flue sensor or plugs. Behind the front panel are gas, mains electricity and hot flue parts. If you smell burning, switch off at the fused spur. If you smell gas, leave the switches alone, open windows, turn off at the meter if safe, get out and call 0800 111 999.

Why would a Vaillant overheat and show F25?

The circulation faults behind F25 are the same family as those behind the temperature codes F.23 and F.24: a pump that is seized or weak, air trapped in the boiler, a diverter valve stuck part-way, or sludge restricting the heat exchanger or pipework. A loud boiler pump noise or radiators that stay cold at the bottom before the fault appeared both point this way.

On older systems, sludge is often the root cause. A new pump will not last long if the water is still dirty, so an engineer may recommend a cleanse and a magnetic filter as well as the repair.

What the engineer does

The engineer reads the fault history, then checks the wiring and plug connections to the flue gas sensor and harness, because a loose plug is the quickest fix. If the connections are sound, they test pump speed and flow, bleed air from the boiler, check the diverter valve and look at the heat exchanger for blockage.

Gas work is carried out by Gas Safe registered engineers. There is a £60 call-out, and the repair price is agreed with you on site before any work starts. For other codes, see our Vaillant boiler repair service.