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.
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.
26 gauge sheet metal: 48″ x 18″, 16″ x 109″, 20″ x 89″, 41′ X 36″
16 gauge sheet metal: 31″ x 14″, 32″ x 40″, 33 ½” x 43″, 34″ x 34″
20 ft. of 1″ square tubing
6″ well casing pipe, ¼” thick
Stovepipe and elbow
½” sheet metal screws
Fire bricks (high heat cement if you wish to use mortar)
Some CEB or regular bricks
High temp silicon
High temp paint
Tape measure and marker
Saw (metal and masonry blades)
We made this unit as a self-contained box, one that could be disassembled if necessary (mainly due the fact that is was a prototype). For that reason, we used sand as our thermal mass, and thus the box that held it in needed a bottom.
You need a frame, to which you will attach the sheet metal walls of the unit. We used 1″ square tubing. Because this RMH is effectively comprised of three levels (fire box, sand that surrounds most of the heat riser and stove pipe), this frame is fairly complicated to describe, so make sure you refer to the photos for clarification. As you’re welding everything together, make sure it’s all square and plumb.
As a recap, you will need to cut the following lengths of square tubing: 15″, 15″, 16″, 36 ½”, 19″, 34″, 34″, 28″, 19″, 8 ½”, 7 ½” and 7 ½”, making a total of 20 ft, or one piece of tubing.
This first level of sheet metal comes to the top of the fire box area. It is 16″ tall (1″ above the top of the left-hand square tubing frame).
For this design, we made the ash box to the following dimensions: 6″ wide, 14″ deep and 7″ tall.
Here’s where it gets really tricky to explain, so look closely at the photos.
This next part, which basically involves stacking bricks, has two main functions. The first is to create a path for the smoke, which will travel from the firebox, along and up into the riser, down again to the stove pipe elbow you just put in, and up through the stove pipe to the chimney or exit. The second purpose of the bricks is to retain the sand.
We wanted to make this whole unit capable of being disassembled, so we did it all mortar free. If you don’t plan on ever having to move it, you can mortar in the bricks. Of course, be sure to use bricks and mortar /concrete that are made for high heat. Any places that will not be in touch with high heat (like around the ash box or stove pipe, but not the firebox or riser), can have regular bricks (we used CEBs for those places).
Start off by stacking bricks on either side of the stovepipe’s elbow, and on top of it. The end of the elbow wants to be exposed, but the bricks need to be able to retain the sand that will surround the stovepipe as it rises. Position bricks in a way that does not allow them to shift with weight behind them.
This is one of the most crucial parts of the whole RMH. It is this length of pipe, well insulated, that increases the speed at which the heat rises, thus creating a powerful draft and making the sound that gives a rocket stove its name. This increased draft ensures complete combustion.
We used 6″ well casing pipe for the interior pipe. It’s about ¼” thick and 32″ long.
The final step is to do the last layer of bricks around the top of the firebox. You’ll want to fit and cut bricks, so that it leaves a finished look.
Now it’s time to use this strange looking contraption, but that’s the easy part. Stuff a little paper and some twigs into the box, so they’re standing on the grill. Light it up and once it’s burning, you can add more. Use twigs or split wood, anything that will fit.
No smoke or flames come out the top of this box, because the draft from the heat riser pulls the fire horizontally. If smoke does come out, push the burning material into the stove a bit to heat up the riser and start the draft.