Your boiler is struggling to clear its exhaust gases properly, which can be dangerous and indicates a potential issue with your chimney or ventilation.

Technical description

Fumes and puffing during starting (conventional flue). Check the condition of the chimney, ensure it is not blocked and is high enough to produce the required draught. Check that there is an adequate air supply near the burner and that a kitchen fan is not drawing products out of the burner.