8 Inch to 6 Inch Reduction for Exhaust Gases?

A question about installation for a family member who is deciding between a Hitzer 983 (larger and hopperless, so easier for wood burning) and a 503 insert:

The 983 has, I believe, an 8 inch diameter flue opening for exhaust, while the 503 outlet is 7 inches. Whichever stove is purchased, it needs to connect to an existing 6 inch diameter chimney liner. One installer suggested he would try to avoid reducing from 8 inches to the 6 inch liner--not clear if this is a common code violation or if this reduction poses some danger in some way. As a result, that installer is recommending the 503 because the reduction required (from 7 inches to 6 inches) would be more acceptable.

Does anyone have any experience or opinions about the safety, and regulatory compliance, of such an 8 in to 6 in reduction?

It may work ok for high draft conditions like when you stove is really hot and it's cold out, my opinion is that there may be more issues with draft when temps are warm/stove idle conditions compared to a full 8" all the way.
I'm sure there are others out there more knowledgable than me on regulations...also, maybe call the manufacturer?

Also be a little concerned about ash build up in the pipe right before the 8 to 6 transition.

Thanks squirrel. Will be talking to the manufacturer soon. Since the flue from the insert goes straight up the chimney rather than having any bends, I doubt fly ash will be a problem at the reducer.

Here is a chart to help you with chimney capacity. It was originally posted here.Switching From Power Vent to SS Insulated Pipe - Question Hope it helps.

PostBy: DOUG On: Sun Aug 09, 2009 12:03 pm
Well, it all depends on the height of the chimney as to whether or not the 6" will handle the needed capacity. A 6" flue area is 28 sq.in. and a 8" flue area is 50 sq.in. Your K-6 says that it has a gross BTU output of 144,000 and a net output of 122,000. Here is a chart for chimney capacity.

HEIGHT IN FEET
-----------------------------------------------5"-----6"-----7"----8"---10"---12"--------------------------------------PIPE SIZE
----------------------------------------------19"---28"---38"---50"---78"---113"-------------------------------------AREA IN SQUARE INCHES
10--------------------------------------------56----89---129---175--300--450---------------------------------------Btu per hour in thousands
15--------------------------------------------66---105---150---210--360--540---------------------------------------Btu per hour in thousands
20--------------------------------------------74---120---170---240--415--640---------------------------------------Btu per hour in thousands
30--------------------------------------------83---135---195---275--490--740---------------------------------------Btu per hour in thousands

Interesting table, Doug. On a BTU basis, the 6 " chimney (probably 25 feet tall) liner is more than adequate for either stove.