can you downsize an 8\" prefab s/s chimney?

RTB

Insulated, prefab, 20 years old, needto go to 5" for a Buderus 115 (flue temp low 300's, which is too low.), is it ever possible to line it to accomplish this, or must it be ripped out and replaced?
1- I know there are variables, I'm looking for a general answer, to give me a start
2- this is eastern LI.
Thanks

[Deleted User]

reducing vent size

is commonly done in masonary chimneys either to line them or to match them more closely with the requirements of newer and smaller appliances. I do not see how using your oversized vent material as a chase for the new replacement vent would be any different.

Please do not assume that the vent connector on the appliance that you are considering determines the size of your vent. Always use your metalbestos tables or the tables in your local venting code to determine the ultimate vent size. If that can fit through the old vent pipe, go for it.