rocket mass heater

Rocket Mass Heater Build, Pt 2 – AWESOME!

Continuing the Build

If you have followed, then you know that Part 1 of this project covered prepping and putting in the foundation for the rocket mass heater. If you missed that, you can check it out at Rocket Mass Heater Part 1. After building the foundation, I gave it about two days to get solid enough for me to start building on top of it…then I was off and running!

I originally planed to build a 6″ system, but later decided to go with an 8″ system and I’m really glad I did. Based on the size of my greenhouse, I really needed the size. That size refers to the diameter of the burn tube, riser and duct work. I’ll explain as we go. I built an 8″ J-tube rocket mass heater. The “J” is where the combustion happens.

If you think about a J, the short side is where you feed in the wood. You start the fire at the bottom of that section, but it burns sideways toward the tall part of the J. FOR REAL…YOU READ THAT CORRECTLY! – The fire really does burn sideways, due to the draft that is created by the ductwork.

There are a variety of methods that you can use to construct a rocket mass heater, but I like to use what I have on hand. I created the combustion area out of cinder blocks, filled with cob (clay, sand and straw mixture-more about that later). I then lined them with fire bricks. This is where the fire happens.

Completing the Combustion Chamber

I completed the burn channel with fire bricks and I used pavers, stacked up and glued together with liquid nails in a large diameter circle around the far end of the burn channel. This is where the riser goes. I used a piece of stove pipe, insulated with a ceramic fiber blanket as the chimney that comes up from the burn channel (the tall side of the “J”), inside the barrel, which will sit on top of the pavers. Inside this barrel is where all of the combustion takes place.

The way it works, is the heat from the fire goes up the chimney, bounces off the barrel that is directly above the chimney (about a 2″ gap) and begins to swirl around in that cavity, thus causing more and more combustion. The super heated air then escapes through a series of ductwork that travels through a thermal mass bench.

After I got all the bricks in place, where they needed to be, I began the Loooooong process of cobbing. That is, mixing up my cob–clay, sand and straw–with my feet. I had a considerable amount of bench to construct, but I worked just about nonstop for almost a week to finish everything. (I was exhausted!!)

Got a Little Help Along the Way

Mason loves to work with momma and I love that he is learning so much as we work around the farm. One of the things that he learned is that momma’s work is not as easy as it looks!

Making Cob

The cob making process is simple, but labor intensive. I made a spot in a little hollow next to the RMH to mix it all up. I used a tarp to contain it and layered in the supplies. First clay that we dug up here on the farm, having been soaked in water. Then, sand that we had left over from another project and then straw. Then I just danced around in it, continuing to add whatever was necessary to get it to the right consistency. You want your cob to be moist, but not dripping.

The majority of my cob didn’t have a huge amount of straw in it. I used a lot of rocks in my thermal mass bench and used the cob as a filler. However, the outer layer of cob was very straw heavy. I chopped lots of straw into that mixture. The straw acts kind of like rebar in holding everything together. Something else I noticed about that mixture is that it didn’t crack as it dried like the other mix did.

Tackling the Duct Work

Once I got the combustion area built, it was time to put together the ductwork that would go from the manifold through the bench and then out a chimney.

We used basic heating ducts and taped all the joints. We have two clean-outs to access any ash that needs to be removed. One clean-out is at the first 90 degree turn and the second is it the end of the bench where the ducts turn back. I got a great tip from a fellow ‘permie’ about that turn around. My husband, who loves to work with metal, built me a metal box for the ductwork to enter on that end. That is our turn around. He made a door on the other side of it so that I can easily access it for cleaning.

After leaving the end box, the ducts come back down the bench and then exit up above. My husband manufactured an ‘exit’ for the pipes. We cut a hole in the wall for it and then sealed it up.

As an update, I will note here that we ended up adding a few feet of height to that chimney where it exits in order to get a stronger and more consistent draft. This was a really good choice.

Finishing the Rocket Mass Heater Build

From that point on, it was all me, all the time. Daily cobbing to build and finish off the bench; again, not hard, just labor intensive. I decided to use stones on the top of the bench and the fronts as well. In fact I also used a few excess tiles on the front of one side. They are all terrific thermal mass that warms up really well and then releases it into the building through the night. I still have some finish work to do on the rocket mass heater, but nothing that won’t wait for just a bit. Currently I am just thrilled that it is working and keeping my seedlings nice and warm!

Never Miss Anything!

The video above is a compilation of the process adding a little more to what I have here. I hope you enjoy and, as always, I welcome all questions and comments. If you take a minute to subscribe to Two Oaks Farm Talk and Two Oaks Farmstead Youtube Channel, you will never miss any of the terrific content! You will even find a bonus, because that is where we collide with A Life on the Farm.

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