Hi Guys. I am replacing my small Franco Belge (25,000BTU)that I installed last year that had a 5" exhaust with a larger stove that has a 6" exhaust. Fireplace installation that has a baro damper off the back of the FB to a 5" black pipe elbow, up through a block off plate, with about 3 feet of black pipe through the smoke chamber. Had to use a grinder to free up an inch or so in the damper area of the chimney to get the 5 inch pipe to fit. Good draw even though I have an outside 8 inch clay-lined chimney. The inspector approved it with out any problems.
Recently picked-up a Chubby (60,000BTU) that has a 6" exhaust because I need more heat. The easiest solution is to use a six to five inch reducer on the back of the stove and basically leave the current piping in place. This would result in an almost 30% reduction of exhaust pipe area relative to a six inch exhaust. Larger FB stoves that have 60,000 BTU outputs also use 5 inch exhausts so I am less concerned about safety but I wonder how it might effect the performance of the Chubby. Not sure the inspector will bless it since the manual says not to use less than a six-inch chimney.
Another alternative is to re-pipe to six inches. Problem is I can not grind anymore to free up space, so a six-inch elbow, regardless of the angle, is not going to fit. Might be able to squeeze some six inch flex pipe through the chimney but it means re-cutting the block off plate, etc. I am reluctant to shell out for an expensive adapter kit.
What do you think? Any problems with using the reducer?
Switching Stoves From 5" to 6" Exhaust: Reduce or Re-Pipe?
Here is some reference material which will help. The first two pictures are from "The Wood Burners Encyclopedia, by Jay Shelton, 1976." The third picture is from "Wood Heat Safety, by Jay W. Shelton, 1979."
According to this information, you should be okay, the set up you are proposing will not be the ideal, but usable.