I built my chimney transition out of firebrick. My opening ended up being 15 inches wide by 4.5 inches deep. A 30 degree angle created a taper over 4 layers to the 13.5 x 13.5 inch top layer of firebrick; cut with a circular opening for the 8 inch pipe anchor plate. Try stacking fire brick and draw a line to mock the taper. Then measure the angle. In my case, the 30 degrees worked in both dimensions as I reduced to 8.5 inches and widened to 8.5 inches for the anchor plate opening. The 8 inch anchor plate is 12.5x12.5. The chimney transition stays flat across the front of the vent arch and tapers back over the dome as it widens. I kept the chimney transition off of the dome. I was concerned about the expansion forces. I'm not sure if others were as well.
I then cut some tapered wedges to minimize the dead corners near the top. And coated the inside with mortar to make a smooth flowing surface.
Your 6x10 inch vent should work as it is larger than the 50 square inches (inner area) of the 8 inch pipe.
The red lines follow the inner taper.
I know this is apples to oranges, but the Duratch info has a table that indicates a 20 inch fireplace opening should have an 8 inch pipe. The fireplace people said they install primarily 6 inch. I have no experience to back up one of the other. I'll have 7 feet of chimney over the transition. We'll see how it vents tomorrow after the chimney is installed. I did 2.5 days of propane curing at 300, 350, and 400 degrees. Time for wood and higher heat. I don't trust the propane hose at the higher temps. An all metal propane weed wand might be the trick though.
Good luck.
09-25-2009, 09:45 PM
Steel Oven Vent or anchor plate Specs for 42 inch 
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