by Rob Avis
Our new rocket mass heater is now functionally done. After installing a proper chimney, it really hums! Above is a video that we produced showcasing its construction.
I have now fired it four times for about an hour to an hour and a half to drive off any remaining moisture in the cob (I should have done this months ago) and the bench is just starting to show signs that it is retaining heat. Once the greenhouse is completely sealed up with thermal curtains* installed we will be testing the rocket to see how long the thermally charged mass bench will keep the greenhouse above freezing. If we are successful we will be working in the greenhouse as early as March this year without fossil fuels to keep the space warm.
Initially the rocket was not functioning properly because the chimney was not tall enough. When a chimney is placed around a building, all sorts of strange low and high pressure zones form around the structure due to wind blowing the building from various directions. For this reason the rule of thumb is to project the chimney at least 2 ft. above all surfaces (usually roofs) within a 12ft radius. It is also advisable to have a high wind cap which ensures that no matter where the wind is coming from there is always proper draw from the fire. Here is a great explanation of chimneys and high wind caps.
The last remaining step to complete the rocket is a render coat which we will apply once we have warmer conditions. The render coat is simply a protective layer with a finer finish applied to the outside of the cob bench to increase its durability.
*Thermal curtains are used inside the greenhouse along the glazing to reduce heat loss at night. They can be bought commercially, or you can use any insulative material, for example a blanket or construction tarp.