Working area of square chimney liner

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Minister of Fire
Jan 6, 2020
It’s been said that not all of a square chimney liner is used. In fact, I’ve seen information showing the “working area” of a square chimney liner to be different than the “actual area” of the same square liner. That being the case the implication is that the “working area” is a smaller measurement than the “actual area” of the same given liner. That makes sense.

Somewhere a while back I seen some charts on rectangle and square chimney liners. Particularly I’m looking for information on square liners and I can’t seem to find the two charts I had previously seen. Regardless…

For a square liner with inside dimensions of 6.75”x6.75” one chart said this square liner was equivalent to a 5.5” round liner (I forget the website/link) and a different chart showed equivalent to a 7.5” round liner.

One of those numbers has to be right and I was wondering if someone could help me confirm either the smaller or larger number (I’m leaning to the smaller) and provide a link with accurate charts/info. I’m still doing some searching as well.

Also, is it common to see a 6”square I.D. liner to a 8” square I.D. liner (or any I.D. in-between) with an 8” I.D. round thimble? Particularly asking about the smaller square liners to the 8” round clay thimble?
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Here is a chart giving the O.D. and I.D. dimensions as well as the actual area of the I.D., but it mentions nothing of the “working area”.

The “working area” would exclude the corners of a square liner. This is the information I’m looking for, this number to compare with area of round liner.
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I’m surprised no one has replied to this…

I know this type of thing has to have been discussed here before.

Obviously, the corners on rectangular and particularly square liners has to mess with the flow of air exiting the chimney. Surely there’s something scientific on it out there.