F.25Vaillant
Vaillant F.25: the flue gas temperature is too high
Checked against Vaillant's own documentation: Vaillant: boiler fault codes list and 1 more source. Last reviewed October 2026. How we check
Engineer only
The gases leaving through the flue are hotter than they should be. Because this involves the flue, it is a job for an engineer only.
What you can safely do
There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.
Before you book an engineer
- Write down the code exactly as the display shows it: F.25.
- Find your boiler's model name. It is usually on the front panel or in the user manual. The table below shows why it matters: Vaillant codes can mean different things on different models.
- Note when it started and what the boiler was doing (heating, hot water, or starting up).
- If the boiler is still under warranty, check your warranty paperwork before you book anyone.
Call a Gas Safe engineer if
- Book a Gas Safe registered engineer.
- If anyone feels unwell (headache, dizziness, nausea), get fresh air and call 0800 111 999.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to ecoTEC plus. Codes can differ between models, so check your manual.