We are evaluating what size flexible chimney liner to use for a new fireplace (masonry heater). The liner will pass through an existing concrete flue.

The flue is constructed out of 16" square concrete blocks which have an interior of 8 1/4" - 8 1/2". The measurement varies a little depending where we measure.

The flue is not perfectly straight but was constructed with some gradual bends in a few places, I assume to avoid house framing and the roof ridge.

So the question is whether an 8" stainless flexible liner would be able to fit into a flue like this, or would it be too close a fit? The chimney is about 30' long (2 story house).

The next size down seems to be 6" which seems like it would be acceptable but might not draft quite as well, although its substantially cheaper.


  • Are you using a fireplace or an insert? Also what is the stack height?
    – Ed Beal
    Commented Mar 14, 2020 at 13:56
  • What does the wood burning stove instructions say about venting requirements?
    – JACK
    Commented Mar 14, 2020 at 13:59
  • Its essentially a fireplace - technically a masonry heater which is masonry construction. The manufacturer has said that either 6 or 8 would be acceptable, but 8 might perform better for draft. Commented Mar 14, 2020 at 14:03
  • Edited text to clarify the type of wood burning appliance, it was misleading the way I wrote it originally. Commented Mar 14, 2020 at 14:05
  • I my personal experience, too much / too big flue is a more common problem as flue-size problems go.
    – Ecnerwal
    Commented Mar 14, 2020 at 14:40

1 Answer 1


I think you'd have one heck of a time trying to get an 8" into that hole.

Unless incredible care was taken with the inside joints, there's very likely some point where a bit of mortar is poking out that will block the liner, or an offset between blocks at a "slight bend" will, given only 1/4 inch of clearance.

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