Your boiler is struggling to push out fumes correctly when it tries to start, which can lead to dangerous exhaust gases leaking into the air supply.

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.