Flue Size for Add-on wood furnace
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 diameter...is 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