Chimney Flue ?...OD or ID

[SeanS]

Hi Everyone,

Not sure if this is the best place for this topic, but since I have a wood furnace I figured I would start here. I'm hoping the collective knowledge on this forum can help to clear something up for me.

My wood furnace (Harman 2500-A) has a 7" round flue outlet...and thus I have 7" stove pipe running to my chimney flue. I was reading the manual and it states to vent the furnace to a flue "not less than 8" x 8" in size." I was confused on why it would need to be vented to a flue larger than the exhaust pipe...and worried that I spent a bunch of money on a system that won't work with my setup as my chimney flue is 6.5" x 6.5"

Anyway, I was looking online and some sites suggest that the 8" x 8" refers to the Outside diameter...others say the internal diameter. When I measured the outside diameter of me flue, sure enough it is 8 x 8.

My question is, can anyone tell me if the measurements of a flue typically refer to the ID or the OD. In some ways the OD makes sense because an 8 x 8 OD with 6.5 x 6.5 ID measurements gives roughly the same cross-sectional area as a 7" round pipe (but maybe that's just my wishful thinking hoping that I have more than a $3000 paper weight in the basement.)

If more info is required please let me know.

Sean

 

[brenndatomu]

Normally the ID is referred to, but in your case, IMO they have to be talking OD. I say this because 7" round is ~37 sq in, 6.5" x 6.5" is ~42. 8" x 8" ~48 sq in. You normally don't wanna tie into a chimney too much bigger than the stove pipe connection on the appliance. The 6.5" x 6.5" is a better match. Somebody else may be along shortly that can prove me wrong...

 

[STIHLY DAN]

Does it actually say 8X8 or just 8. Should contact the dealer or company to be sure.

 

[joes169]

As a masonry contractor, I can tell you that clay flues have always been classified by there exterior dimensions. I think what you have is just fine.......

 

[SeanS]

Thanks for the feedback, this gives me more confidence going forward.

The manual actually says 8 x 8...I'll follow up with the dealer as well. Haven't had much luck with technical related questions in the past but we'll see.

Thanks again for helping a newbie out!

Sean