This past week I had the pleasure of traveling to Oak Harbor, WA (on beautiful Whidbey Island) to build a masonry heater with master mason Steve Cohen of Hot Rock Masonry. We built a soapstone Contraflow with added passive bells on both sides of the chimney up channel. Soapstone is the most efficient material for masonry heater building as its density allows for maximum heat storage capacity. This heater contained (I believe) around 9,000 lbs of soapstone so it should store quite a bit of heat. Due to the massive amount of weight (ie thermal mass) every heater needs its own foundation. We built up a foundation out of CMU and then poured a concrete slap up to the height of the subfloor. We did this a few weeks before the actual heater build so no pictures.
The base slabs of the heater were put in first, followed by the bottom course of the down channels which connected to the two heated benches the wrap around the heater. An ash dump doubling as an air input into the fire box was built in the center.
Next the core (interior) bench channels were capped
From there we put the exterior bench in place and began building the firebox and down channels. The holes on the back of the firebox provide an additional air input into the back of the fire to further aid a complete combustion