D Lyon

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.

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

As a potter I would think that the high temps would vitrify your red clay but shrinkage might be a problem making the clay fall out of the joints. The addition of sand or grog would lessen the chances of shrinkage. Having very close fitting joints would also be highly desirable. I would use potter's clay with heavy grog such as clean sand. Some clays might melt at temps higher than 2100 F. Use a high fire clay.