Jesse Keller

So, I am planning on building a rocket mass heater in my house basement going upstairs (6' ceiling in the basement) through an old gravity heat floor vent to a breakfast nook bench (the mass) then back through the floor downstairs and out the side of the house.

2 questions:

1. Since the hottest part of the combustion exhaust pipe will be a 6' pipe extending upstairs to the mass, will all the heat be lost before it has a chance to even make it to the mass?
2. Is it fine to change from a 8" pipe to a 6" pipe once I get to the mass (due to the small size of the mass)?

thanks

Jesse Keller