HETAS Approved 316 Flexible Chimney Liner 6" (150mm). Approvals: HETAS and CA. Flexible Chimney Liner for all stove installations into existing fireplaces. 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.