Check the following:
- Is the flue system permissible?
- Are there any blockages?
- Is the appliance correctly set up (i.e. air inlet guides, restrictor, throttle rings)?
- Is the correct roof terminal used (DRU brand)?
- Is the terminal located in line with local regulations (i.e. free from any wall or roof obstructions)?
You can find more information about the above in the instructions.
! PowerVent®? Check whether the pressure setting unit is correctly
Is the pilot flame too weak (dirty)? And is it pulled away from the thermocouple when the main burner is ignited?