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Large 8in Batchbox Rocket Mass Heater for my shop

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A couple years ago I randomly watched a video on YouTube about rocket stoves and rocket mass heaters. I was Immediately Intrigued by the concept and wanted to know more.
I started consuming all the info I could on the subject and after 2 weeks I put together my first rocket stove with junk metal I had laying around.

This first Iteration worked pretty well, but I kept reading how it was necessary to keep the heat in the system to have the most efficient burn possible. I also have read that metal as a heat riser was not the best choice. so i continued my research and was driven toward that idea of a batch box for my second iteration because it didn't take as much attention or constantly fed until the mass was properly charged.

I went back and forth on design and knew that my decisions would ultimately be made in the midst of the build.

I sourced the materials I needed from Smith-Sharp Fire Brick Supply (2129 Broadway St NE, Minneapolis, MN 55413) (612-331-1345)

I ordered 4 - 8in Calcium Silicate heat riser cylinders used in kilns and boilers (these had to be ordered and took about 2 weeks to arrive)
1 inch thick roll of flexible calcium Silicate insulation
2boxes 2in rigid calcium silicate boards at 2'x4'
and 2 tubs of Sairset High temp mortar.

Note!: if you are using Calcium Silicate insulation remember to wear safety glasses, gloves, a respirator and long sleeves. you'll be itching for days if you don't.

after all the materials were purchased, I had to settle on my final design for the stove itself. I had gone through a few different designs and finally settled on a stove that would utilize 3 55 gallon drums.
If you don't have any 55 gal. drums laying around they can usually be sourced from Facebook Marketplace or Craigslist for relatively cheap.
I was able to source my barrels from a local spray foam installer who just wanted to get rid of them.

I wanted to join 2 of the barrels at 90 degrees and weld them together. this proved difficult for me to do using the cut and try method.
So back to the internet and found a spreadsheet that would calculate the saddle cut and then i was able to transpose that info on to sheets of paper joined together with tape.
I think it took 7 sheets to fit the whole cut pattern.
I then began to transpose the numbers on to the paper that would eventually give me the pattern i was striving for.
total time to do this was around 6 hours.
I then copied the pattern on to a piece of cardboard that would be my final cut guide.

so now Finaly I had everything I needed to get started.
I burned my barrels to remove the paint. gleaned them up with a wire wheel and flap disc on the grinder.

I traced the pattern on to the barrel and did my cut.
I then had to take my grinder and adjust the cut in a few spots to get the best fit.
I cleaned up my cut barrel along the edge so the metal was shiny as well as the area of the barrel i was going weld to.

when your welding thin metal like 55 Gal. drums you have to use a low amperage as to not burn right through.
I have a stick welder so i used thin 6011 rods in my construction.

I began to tack the 2 barrels together. when doing this you want tack one side and then the opposite side then move 90 degrees do your tack then move to the opposite side and tack. when you have your 4 tacks in place  you can start to tack the rest of it together. if you put too much weld on one side while doing this it will cause this metal to distort and may cause problems later on. once i had my first pass completed it looked pretty rough so i laid another layer of weld on top to clean it all up and make sure there were no holes.

i then figured out what i wanted my box size to be that the heat riser would eventually sit on I believe that outside dimensions were 15 wide by 18 tall by 21 long. I cut an opening in the portion of the barrel that would eventually be the vertical barrel in my stove design.

now it was time to start insulating the interior of the stove and building my box.  I wanted to be sure to insulate the bottom of vertical barrel to give me a surface to adhere the fire box. i them built my box using the sairset mortar paying special attention to sealing the inside and outside of the box to prevent any air leaks.

I then used the 1in roll of insulation to insulate the inside of the barrel and applied a layer of mortar to the inside of the blanket to add some rigidity

Finally I made sure my system wouldn't leak around the outside of the firebox with more mortar.

i then added a simple plate steel door with welded hinges. as well as a simple mechanism to allow me to adjust my airflow and a secondary air intake.

I cut a hole out of the back of the barrel for exhaust and connection to the thermal mass.

I moved my stove in to place and added my heat riser. put the barrel on top and used fireplace mortar that comes in a tube and applied with a caulk gun to seam where the barrels meet

heat riser is 2.5inches from the top of the barrel.

my heat bench consists of roughly 40' of 8 inch stove pipe with rocks and sand on the bottom and rock and clay on top, i didn't bother with making cob because my bench is made out of wood and lined with thin steel i have laying around. I haven't had any instances of the wood frame getting too hot and i have had the barrel up to 700 degrees.
I also added a damper and direct connection to the flue to make it easier to get the stove to a good draw before slowly transitioning to the thermal mass.

this stove will heat my shop from 30 degrees to 70 degrees in an hour. I haven't done much testing on the bench as of yet because I just finished it and it's still relatively warm outside.

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Posts: 1795
Location: Kingman, Arizona
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Hi Jake and a big warm welcome to the Rocket Mass Heater forum!

I can see that you have been a very industrious man, pulling together all kinds of bits and bobs to make yourself a shop dragon.
Thank you for the detailed descriptions of how you accomplished your work and the things that you've learned along the way.
Looks like you have quite a few twists and turns in your exhaust pipe as it travels through your mass. Very good that you installed a bypass to overcome a slow start when everything is cold.
Could we get a picture of the inside of your stove?
Creativity is allowing yourself to make mistakes; art is knowing which ones to keep. Keep this tiny ad:
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