How do I figure out what length of chimney liner I need for an insert?

Top of the flue to the insert collar

Liner length for an insert is the vertical run from the very top of the chimney down to the insert's flue collar, plus enough to make the connection at the bottom and terminate cleanly at the cap on top. A modern insert install runs a continuous liner the full height of the chimney — it isn't a partial piece.

So the length is essentially your chimney's height measured from the crown down to where the insert sits in the firebox, with a little added on each end. Measure a touch long rather than short; liner can be trimmed, but a short run is scrap.

Chimneys are rarely a clean plumb drop

The straight-shot number only holds if the flue is truly vertical. Many masonry chimneys have an offset at the smoke chamber or a bend partway up, and every deviation from plumb adds length the tape doesn't show if you just measure floor-to-crown.

A liner also has to negotiate those bends, which is another reason the real measurement happens at the chimney, not on a calculator. This is where sending someone up top with the right tools pays off — an inch of extra liner is cheap; a job stopped halfway because the liner won't reach is not.

We size the whole system

Every insert we install goes in with a correctly sized, full-length liner — that's non-negotiable for safe, efficient venting and part of why an insert dramatically outperforms an open fireplace. We measure the flue, confirm its inside size fits the liner diameter your insert needs, cut it to length, connect and cap it, pull the permit, and have it inspected. One company, one accountable job. Call (828) 545-0525.