Flue size ? for Wood furnace

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New Member
Jul 28, 2013
Rehoboth, MA
Hi Everyone,

I was hoping to get some guidance in regards to wood furnace installation. I just ordered the Harman SF-2500-A wood furnace which I plan to put in the basement and tie into the existing duct. We have a separate clay lined, square flue opening in the basement that vents out though the chimney. The Spec for the furnace lists the flue opening as 7"...turns out the internal width of our flue is only is 6.5" (total width being 8").

Will this be a problem...or is this one of those scenarios where the spec may list 7" but the actual flue used is a little smaller.

I guess what I am really getting at is can I go with the furnace ordered, or should I change it to the next size down (spec only lists a 6" flue size for that one....SF-1500-A)

Any insight will be appreciated.


Pic of basement with Flue opening is below...FWIW I do plan on pulling a permit and having it professionally installed.


  • Flue size ? for Wood furnace
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What you have there is the result of a lazy mason. You should have a thimble. Is there a clean out door in the flue?
Except you can't reduce the flue size and be code compliant.
So I assume then you are saying that a 6.5" square internal width will not be code compliant for a spec requirement of 7". Does the spec refer to internal diameter (circular). Should I consider downgrading a size where the flue requirement is 6"?
Technically speaking a 7" diameter pipe has an area of roughly 38 sq inches while a 6.5 x 6.5 square is about 42. An argument could be made that you are not reducing actual flue size.
Have a sheet metal guy make you a nice heavy gauge adapter from 7" round to 6.5 square.
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