Maybe thats the difference. You only have a small section of 6" pipe going back into 8". I would imagine that 6" would be acting like a damper to some extent..? As opposed to going straight up all the way with 6" pipe..
I'm not really sure. I'm just learning this as I go.
Maybe I should just split the difference and go with a 7" flue all the way
8 inch was standard. The reasons were for open door burning with screen in place for the solid door models which the large fireplace opening becomes almost atmospheric pressure allowing smoke to move both up and in. The other reason was when these stoves were designed they were usually connected to existing chimney flues designed larger for fireplaces. A 6 inch outlet wouldn’t let enough heat out for the larger flue. A customer proving they had a chimney system that would work with a modified stove could have them special built, so you can find 8 inch outlets on the stoves normally fitted with 6 inch outlets. If you’re going straight up into an insulated chimney they work fine. They will not allow maximum BTU output due to reducing chimney capacity, explained below.
It is technically illegal to reduce exhaust all in states that have adopted the International Family of Codes. NFPA 211 allowed 1 inch reduction, but the International Mechanical Code no longer allows any reduction.
The difference venting systems create is quite complex. First, the chimney causes the low pressure area in flue, pipe and stove. Atmospheric air pressure PUSHES into this low pressure area or slight vacuum feeding oxygen to the fire. This is due to 3 factors; 1) Buoyancy of flue gases determined by chimney height, 2) Flue temperature difference between inside flue and outdoor temperature, and 3) velocity of rising gases. This is measured as draft at any given point in the system. It is normally highest ( lowest pressure) at stove outlet. NET draft is the most important. Everything else takes away from the low pressure area inside the stove. Resistance to flow is determined by elbows, pipe, outlet screen, cap, flue pipe damper, resistance through firebox, and intake air damper. Added to all these variables is atmospheric pressure at any given time, altitude, fuel moisture content, fuel species, and building depressurization.
Chimney flue diameter determines relative capacity. The larger the diameter, the more capacity, but the more heat required to cause the same draft, with slower movement, temperature drop due to expanding gases, and longer dwell time in flue.
The flue damper is a variable velocity control slowing rising gases that reduces net draft. It is used to control an over drafting chimney. It is a chimney control that affects the stove.
There are tables and charts for all the above mentioned items. Watch eBay for The Woodburners Encyclopedia by Jay Shelton. It’s the best book I’ve found explaining these things in great detail using simple terms.