What size chimney pipe

I have recently installed a coal boiler in an out building by my house. My installer is suggesting running a 12 inch chimney pipe out of the boiler. I am having trouble finding 12inch chimney pipe and was wondering if it could be reduced to 8inch. Any thoughts?

What brand boiler...
What is the collar size on the boiler outlet...

I believe it is a Red Square (Montgomery Ward) model. The collar is 12inch. I also have an Iron Fireman stoker setup which reduces the need for draft. My worry is that by reducing the pipe it may not exhaust the smoke quick enough leaving me with a building full of smoke.

Sizing of a chimney depends on btu firing capacity of boiler, fuel type, boiler design, and chimney height.

I doubt a 8" chimeny will work on a boiler with a 12" pipe connection.

Heatfab, Metalfab, Olympia all make metal chimeys 12" in dia. Most suppliers only stock 6 and 8 inch, so you will have to order 12" chimney.

Olympia will make chimney's out of 316 ss which is prefered for coal.

Look them up on the internet.

http://www.olympiachimney.com/