I have 8" smoke pipe coming out of the back of the DF-520 to the draft damper (7"), then to a 90 degree bend, then vertical straight piece, then to another 90 degree bend connecting to a smoke pipe reducer to 7" then to the flue opening. I'll post pictures later. The draft regulator is adjusted.
The draft at the output of the DF-520 in the smoke pipe is -0.01 running idle, and -0.02 after its been running for a couple of minutes. The boiler is in a basement with walkout slider. The draft goes from -0.02 to -0.03 if I open the slider door to the basement.
The chimney is roughly 26 feet tall going from basement to top of first floor roof of a cathedral ceiling family room right above the boiler room or basement. The family room above the boiler room has a 16x24 foundation off the main house. The chimney is capped for rain, etc. The chimney is lower than the top of the 2nd floor roof by about 12 feet.
I plan to remove the chimney cap, and seal off the whole house fan opening as a starter to improve the draft.