HETAS Approved Multifuel Flexible Flue Liner With Installation Kit 6. Increasing Adapter From Vitreous Pipe to Flexible Flue Liner. Common problems including insufficient draw to power the stove, lack of heat output and leaking tar/smoke can all be rectified by re-lining the chimney with a Flexible Chimney Liner; an essential product when installing into an existing fireplace and chimney.