Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.25 is the optional flue-gas safety thermostat (STB) circuit. Many UK ecoTECs do not have a flue STB fitted; a break in that plug or loom still throws F.25. High actual flue temperature is less common and points to a dirty or failing heat exchanger / blocked flue.
Vaillant meaning: Safety switch-off: flue gas temperature too high
Possible causes
- Flue-gas STB plug not fitted or pulled off
- Break in the STB loom
- STB itself open (if fitted)
- Genuinely high flue temperature from a blocked or scaled primary heat exchanger or restricted flue
Possible solutions
- Find whether an STB is actually fitted on this GC number. If the manual shows a link plug, refit the correct link.
- If an STB is fitted, test continuity and refit the connector.
- Inspect flue and condensate for blockage. A blocked flue is also F.35 territory.
- If flue temperature is genuinely high, inspect / descale / replace the primary heat exchanger — do not bypass the STB.
Possible parts needed
- Flue-gas safety thermostat (STB) if the range uses one
- Correct link plug where the range is not fitted with an STB
- Wiring harness
- Primary heat exchanger (if flue temperature is actually high)