^ inch pipe thru 8 inch pipe?

I bought a new high efficiency Jotul - 6 inch pipe to replace an old stove with 8 inch pipe (single wall interior and double wall exterior).I'm hoping to avoid the expense of replacing the present double wall exterior 8 inch pipe with double wall exterior 6 inch pipe and avoid buying an adapter for the thimble. I've heard that it is okay to thread the new 6 inch pipe thru the old 8 inch - it's a straight shot. Does anyone know about this? For example, is there a way to seal the gap between the 6 inch and 8 inch pipe at top well enough that I could get away with 6 inch black all the way to the top? Or should I use stainless single wall? Suggestions re how to weather seal the aforementioned gap (between 8 inch and 6 inch pipe at top and bottom ) would also be appreciated. .Tim

Well you can either use 6" for the connector pipe and then upsize it to 8" at the thimble, or you can re-line the 8" with a 6" SS liner down to the thimble. You do not want to put black pipe inside the 8", the black pipe does not have a very long life expectancy and should be inspected yearly for damage.

exterior chimney cross sectional codes allow up to 2X the cross section of the flue collar on the appliance 8" round falls within that range for a 6" collared appliance , you could use the 8" class A as long as it is UL103HT rated without the 6" liner and meet this code requirement. (provided of course that the pipe is in servicable condition).

Hello Tim that is about what I did with mine. Only difference my 8" was triple pipe. I used 6" class A chimney pipe and it was just alittle to big to fit inside the 8" so pulled the 8" out and have the 6" inside a 10 pipe filled the space with vermiculite. It works fine and just enlarged the 8" top cap so the 6" would just fit thru it. Then sealed it with silicone. Here is a picture of it but I have since added 3 more feet and support braces. Also went with a new cap made for the chimney pipe.It has worked just fine.
Don

Thanks for all of the helpful input. One last question - stoveguy, if I understood you correctly you were suggesting that per code the 8" would be fine. I read somewhere that the modern stoves - double-burn high efficiency - require the smaller (6") stovepipe in order to maintain an effective draft. Does anyone know about this? Again many thanks, Tim

a 6" flued device was designed to use a 6" flue , yes but the code on the issue allows cross sectional varience up to 2X the size of the collar opening , this should be acceptable unless the stove manufacturer specifically states that you MUST use 6" flue (i am not aware of a brand out there which makes that demand , however i would check as i dont know the specs for every stove out there.)

Thanks again for the help. I think that I'm getting quite clear about how to install. However, a point of clarification: I consulted the manual that came with my Jotul 500 and it did specify that I must (in bold type, no less) use 6" pipe for the "chimney connector" but made no mention of that dimension in various references to "the chimney". Looking at the diagrams in the manual I get the impression that the "chimney connector" is, in my case, the pipe that runs from my stove to the thimble and that, from there on out, it becomes the chimney?? Is that correct?
Tim