There are two main factors that differentiate a Rocket Mass Heater (RMH) from a regular wood-burning stove. One is that the combustion that takes place is far more complete in an RMH, due to the increased draft and insulated burn chamber. An RMH can consequently be up to eight times more efficient, requiring less wood for the same amount of heat. This also results in there being less ash and smoke.
The second factor that makes an RMH so superior is the thermal mass that encases the components that heat up. As biomass is burned and the RMH is heated, it transfers heat to the thermal mass (adobe, dirt, sand, even water). Although it takes a while to heat up, that heat will last hours after the fire has been extinguished. For example, if we have a fire going for just a couple of hours in the evening, the stove will still be hot in the morning.
By the time we started researching the benefits of an RMH, we had already built our house and had a regular wood-burning stove in place. If we’d known about it sooner, we would have built an RMH encased by an adobe bench or something similar into the design of the house. We made the RMH we’ll be describing in this article to fit an existing space, and it has been working great!
As long as the sun comes out in the day, we rarely need a fire, as we designed the house to make use of solar gain. However, for those cloudy, cold days, this stove has been a huge hit. We use a fraction of the wood we used to, we can burn a far greater variety of biomass, and the heat lasts a lot longer. It’s win-win all around. The only disadvantage is that it takes more work to make one than just going to the store and getting a pre-made stove, but it seems like a relatively small price to pay, especially for the Makers out there.
If you use wood to heat your house, you should definitely consider making a RMH. It will make a huge difference to your home and your woodpile. This design can be modified to suit your space and needs. Or you can use the principles we outline to create your own unique heating device.
All in, it cost us $150, though an adobe one would be considerably cheaper. Still, we’ve made that money back in the time and energy saved in collecting and cutting firewood.
This RMH is based around a 6” diameter flue. If your house has a different flue size, then components will have to be rescaled. The cross sectional area of all parts of the system can be no bigger than the cross sectional area of the flue. The burn chamber (the horizontal area where the flame is) should be the smallest cross sectional area of the entire system. The greater area you have, the taller the heat riser should be, as well. So, if you do build one of these, follow these plans exactly or research what things may change with a different flue size.
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.How to build a Rocket Mass Heater