We are building a traditional rocket heater style build, but with an single bell underneath the burn tunnel. This should provide a lot of heat capture without increasing the footprint of the stove. It is a bell heat capture and not a flue heat capture approach. (you can read more here about the differences between the two approaches)
The exhaust is identical to a standard J-tube heater shown here, except that instead of exiting the back of the heater, it continues to drop down to a “bell” (chamber) below the burner. Because gravity and pressures naturally sort out the hot from the cold gases only the colder gases leave.
The cold gases are exhausted to the chimney from a few inches above the floor in the middle of the bell. We used a piece of stove pipe with an elbow facing the floor. The entire bell chamber, except the floor, is lined with cast refractory or firebrick. Half size bricks are mortared to the CMU walls and 3″ full bricks cover the ceiling, where most of the heat is collected.
Between the fire brick and cement filled CMU bricks there is a lot of thermal mass for heat collection. We considered using 2 chambers underneath, but there is not that much room by the time the bell is lined with firebrick, so we opted for a single chamber.
I have included 3 2″x4″ steel tubes that run through the top of the bell area to both support the fire brick ceiling and provide an avenue for experimentation by block and opening the channels. The steel tubing is wrapped with 1/8″ ceramic fiber gasket material. A piece of expanded metal painted with high temperature paint is laid on top of the steel tubing.
Materials and Construction
Construction techniques are pretty similar to the YouTube video on the 4″ build. (Yes we will have a video on this build.) The combustion system is based on the 6″ Dragon Burner. We used the 6″ Barrel Support with Bell Chamber, and the 6″ Steel and Gasket Kit. Other materials included generic 4″ thick CMU blocks for the main construction, fireclay bricks for the bell lining, mortar, fire clay, and a 55 gallon drum.