I got started building a rocket mass heater this week. There are several basic designs and a lot of information on the internet about rocket mass heaters (RMH). Its hard to distinguish the good advice from the bad advice. There seems to be a theme that I’ve run across which is that the first RMH that people build doesn’t work and undoubtedly they have to modify and try again. Will this happen to me? Only time will tell….stay tuned.
I read and used the design concepts from the book Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build by Ianto Evans and Leslie Jackson. What I consider to be the meat of the 100 page book is a diagram listing dimensions and pointers to build your own RMH. There is also an example sketch design of a RMH. I didn’t follow the design exactly in the book for one reason or another…mostly because there were some materials I just couldn’t find or didn’t want to buy.
Before I show you my design I should point out the different parts of the RMH. Below is a cross-sectional view of a rocket mass heater with the burn tunnel, heat riser, and heat exchange barrel identified. For more information on how the RMH works, see my post Rocket Mass Heaters – what in the world is that?
The feed tube is where wood is put in to burn. The burn tunnel is where the flame from the wood travels before making a right turn up into the heat riser. The heat exchange barrel is placed over the heat riser and keeps the gas and smoke contained. The heat exchange barrel also conducts heat out into the room.
Phase 1 of my design included the construction of the heat riser and the burn tunnel. The heat riser and burn tunnel are often made from fire brick being stacked together like legos.
I didn’t have any fire brick, and didn’t know where to get any either. I’m sure if I looked hard enough I could find a place that sells them. The heat riser doesn’t have to be fire brick and is often made from a smaller and a larger diameter pipe with insulation between the two. The insulation helps keep the heat riser at a very high temperature which is important for the RMH to work properly.
Instead of going out and buying some steel pipe, I started to look for things that I could use that were easy to come by. Most rocket stove designs that I have seen, the inside diameter of the heat riser is either 6-inch or 8-inch. In the Evans and Jackson book, they recommend using an 8-inch inside diameter RMH when using a standard 55-gal steel heat exchange barrel.
I have a bunch of old empty #10 tin cans. The inside diameter is 6-inches. I decided to use these for the inside of the heat riser. Its a little smaller than what Evan and Jackson recommend (since I’ll be using a 55 gal heat exchange barrel), but I thought I would give it a chance anyway.
I cut the tops and bottoms off the No. 10 tin cans (with a can opener) then duct taped them together to make a pipe.
Some may question why I would use duct tape since it is sure to disintegrate in the flames and heat. I expect this will happen, and that is okay because of what I’ll be using for insulation. I got my idea for insulation from watching this youtube video that uses a mixture of perlite and concrete to make a highly insulated but solid rocket stove that should handle high temperatures. The mixture that I used was 80% vermiculite to 20% portland concrete (by volume). Here is another video example of a heat riser that used used stove pipe and rockwool. The interior stove pipe was completely disintegrated in the heat, and the rockwool was melted.
For the outside of the heat riser I used aluminum heating duct. Since the duct is sold in pieces that are snapped together to form a tube, I was able to connect a 10-inch pipe and an 8-inch pipe together to form a single pipe that is 18-inches in diameter. The inside diameter of the 55 gal heat exchange barrel is 22.5-inches, so I should have enough space between the outside of the heat riser and the inside of the heat exchange barrel to allow the smoke/gas to run down the outside of the heat riser. Evan and Jackson recommend having about a 1.5-inch gap between the outside of the heat riser and the inside of the heat exchange barrel.
When I bought the two ducts I didn’t realize that the metal thickness was different between the two, this prevented the two pipes from snapping together very well. Nothing a little duct tape and bailing wire wouldn’t fix.
In the picture above, the heat riser is turned upside down from what its final resting location will be. Notice that I cut half of the heat riser a little shorter than the other half. I did this so that half of the heat riser would ‘sit’ on the burn tunnel. The height of the heat riser is set to allow for a 2-inch gap between the top of the heat riser and the interior of the 55-gal heat exchange barrel.
I added just enough water to the vermiculite/cement mix so that it looked like wet sand and would start to somewhat stick together. I then dumped the mixture between the heat riser interior pipe (6-inch) and heat riser exterior pipe (18-inch).
I plan to wait 3-weeks for the concrete mixture to cure. I estimate the heat riser weighs about 100 lbs. The vermiculite makes it a lot lighter than it would otherwise be.
An important aspect to keep in mind is the cross-sectional area where the gas/fire/smoke flows. From what I’ve read, the burn tunnel and heat riser cross sectional area need to be the same. The cross sectional area around the outside of the heat riser and the exhaust need to be the same or a little larger than the cross-sectional area of the burn tunnel. Since the heat riser is 6-inches in diameter, the cross-sectional area is 28.3 in^2 (pi*(D^2)/4). I decided to make a rectangular burn tunnel using the same mixture of vermiculite and Portland cement. I made a particle board form that was 4-inches tall x 7-inches wide since the cross-sectional area is 28 in^2 and is the same as the heat riser cross-sectional area.
I made the length of the burn tunnel about 12-inches. This allows the burn tunnel to run from the center of the heat riser to the outside of the 55-gal heat exchange barrel.
I used bricks as forms for the vermiculite/Portland cement mixture.
I plan to leave the wooden form in place while the concrete mixture cures. Once it is set, I’ll burn the wood out of it, and this should leave a burn chamber that is 4-inches x 7-inches.
Note that I’m pouring this mixture indoors, since the weather outside right now is freezing. I eventually want to move this RMH outside in a greenhouse. The next phase will involve constructing the pedistal for the 55-gal heat exchange barrel and the exhaust. I plan to bury the burn tunnel under the ground, so that the bottom of the heat exchange barrel sits level with the ground. I plan to surround the exhaust piping from the RMH in a sand lined trench along inside of the greenhouse. The sand will act as the mass to slowly release heat over time.
The next phase will need to be moved outdoors to the final position of the RMH. Stay tuned!
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