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.77 is used on appliances with a flue non-return flap (cascade / some larger or concentric accessories) or an external condensate pump with a feedback contact. The PCB did not see the expected response.
Vaillant meaning: Flue non-return flap / condensate pump fault
Possible causes
- Flue non-return flap not opening / no end-switch signal
- Condensate pump failed, unplugged, or float switch stuck
- Broken signal cable from flap or pump to the boiler
- Accessory fitted but not required, or missing link where the range expects a link
Possible solutions
- Check whether this GC number actually uses a flue flap or condensate-pump interlock. If a link should be fitted, refit the correct link.
- Test the condensate pump: lift the float, confirm it runs, and that the alarm contact is in the boiler circuit.
- Manually check the flue flap moves and that its switch changes state.
- Replace the failed accessory rather than the boiler PCB first.
Possible parts needed
- Flue non-return flap (if that accessory is fitted)
- Condensate pump
- Link plug where the manual specifies one
- Signal cable