Kiln material for Rocket Mass Heater

I'm a potter and have material for building a kiln. I'm thinking of doing a rocket mass heater in my studio using the kiln materials. I have three concerns before I begin. First: I have enough insulating fire brick to make a riser but the shape of the brick best lends itself to a square shape riser. From the posts I've seen a circular riser is all I've seen. Is this circular shape critical to a good functioning riser? Second: My studio is limited in available floor space. Can the thermal mass be constructed vertically up the wall instead of horizontally along the base of the wall? Finally: I have an existing chimney that is about 14 feet tall on the outside of the studio but with a thimble access on the inside at about 8 feet high. Would this present a condensation problem with the water vapor running down the inside of the chimney? In our bitter cold winters can this present a potential for freezing of the condensate in the chimney?
Dave Lyon

I think the square shape is fine. In Ianto Evans book there is brick risers. He also states that some turbulance in the riser is good.

For the rest, i imagine everything is depending on the implementation. The chimney in particular. To use the chimney safely, you need to give it some heat. That means designing your rmh to be less efficient, with shorter piping, but how much is anybodies guess.

You could add some specifics to the question so somebody more experienced can give an estimate.
like, what is the chimney made of?
What dimensions do you build your system? 6" 8"?
What material you build the thermal mass?

It also depends how often will you burn that rmh. If its daily used, there is less of a chance that condensation will happen in the chimney, since the flue gases are warmer.
For this reason i plan to avoid using a chimney, even if i have to put a small fan to aid draft. (im not planning to, i build it to make its own draft, but one cant be sure enough )

Thanks for the reply. I intend to use concrete bricks, pea gravel, rock, clay, and broken pottery for most of my thermal mass. I suspected that having a chimney outside would present condensation problems. I believe I'll abandon the idea of using it in favor of getting more efficiency. Heating a chimney instead of putting the heat into the room kind of defeats the purpose of the high efficiency. I thought a 6 inch system would be adequate for my 600 sq foot studio. Thanks again for your thoughts.

No problem.
In the meantime something occurred to me. If you use an inner tube in the chimney it could work. Provided you make it from like stainless or aluminum and solve the problem of disposing the condensated water adequately.