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Flue Size for Add-on wood furnace

Posted by s13smith (My Page) on
Sun, Jul 28, 13 at 10:19

Hi Everyone,
I'm new to the forum and was hoping for a little guidance. I just purchased a Harman SF-2500-A wood furnace that I plan to install in the basement and tie into the existing duct (FWIW I do plan on pulling permits and having it professionally installed). My question is in regards to the flue size. We have a flue in the basement that vents to the chimney. The Spec sheet for the furnace lists the Flue size as 7"...when I got home and measured; the internal diameter of our flue is 6.5 inches. I'm assuming the measurements on the spec sheet refer to internal that the case?

I guess what I am really getting at is will my existing flue serve the purpose, or do I need to consider changing my order to the smaller Harman furnace which only has a 6" flue size.

Any input will be appreciated.


NOTE: the Pic needs to be rotated clockwise 90 degrees

Follow-Up Postings:

RE: Flue Size for Add-on wood furnace

I'm not an expert but I have installed two wood stoves and chimneys of my own.

There are several reasons stoves have flue size ratings. If the flue is too big, when cold the unit may not be able to push the cold air slug out of the flue and get a draft going. If too small, obviously it may not be able to handle the volume of exhaust.

I would think way down in the basement that a bit smaller is better than a bit too big, on the 'cold air slug' side. And you're within 10% of the rated. I would think there is at least that much safety margin built into the specs.

You could always ask the manuf.'s technical service people, too.

RE: Flue Size for Add-on wood furnace

You'll be fine with that flue. That's a standard 8/8 tile flue and the ID varries from 6.5 to 7" depending. You'll have no issues with the flue diameter, but be sure you stack is built correctly for optimal operation. It should be 2-4' taller than the HIGHEST part of the roof, if it's not you will have mediocre draft in moderate outdoor temps, poor draft during low fire, smoke pouring out of the loading door, poor performance (heat output) from your furnace and likely draft reversals on "bad air" days. If your stack isn't high enough, fix it now and have a pleasant experience, don't wait until you have an issue or, at the least, mediocre performance before having the stack height remedied. Good luck!

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