Rocket mass Heater Pipe Sizes question

I have a question about a rocket mass heater. I built (or I'm building) a rocket mass heater out of 8 inch pipe. That pipe snakes through the mass (about 12ft).. once I exit the mass, I have 8 ft ceiling and want to know if I can REDUCE that 8 inch pipe to a 6 inch pipe to punch through my roof (with a double walled pipe.... Is that advisable and/or possible?

I think if you had posted this in the Rocket Stoves forum, it would have been noticed and answered earlier.
It sounds like you have a very short horizontal run, and if you are going straight up through the roof you may have enough draft to make this work. In general, you don't want to reduce the duct size throughout the system, but I have heard of soe who have done it successfully. What is your rooftop situation like? Will the chimney exit near a high point, and are there any big trees/tall buildings/hills nearby to complicate winds?


Just noticed that this is a duplicate of a thread in the Rocket Stoves forum.

Glenn Herbert wrote:I think if you had posted this in the Rocket Stoves forum, it would have been noticed and answered earlier.

Ooops, I missed this. I've added it to the rocket stoves forum now.