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Homemade wood stove/masonry hybrid

 
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Hi permies, just thought I would share my homemade wood stove/masonry heater hybrid I just completed. I built a steel wood insert, mounted it on a 32”x32” piece of 1/4” plate. The insert is surrounded with 36 concrete half blocks (8x8x8). I cut and modified many of the blocks so that the flue gases pass through every single one of the 36 blocks before leaving the chimney so as to shed as much heat as possible before exhausting. Based on some rough calculations I estimate the whole setup to weigh somewhere between 800-1000 lbs. unfortunately it is summer time so I haven’t had the chance to test it in cold weather yet. Yesterday morning I split up one 12” round log about 16 inches long and lit the stove. That was at 7:55 am. The fire burnt out in approximately 1.5 hours. I designed the stove to run like a rocket stove or masonry heater in the sense that there is no smoldering, it runs wide open until it goes out. I then shut the chimney damper and closed the air intake. At this time the hottest portion of the concrete was 350F. I checked on it throughout the day seeing how the temperature was dropping. The last I checked on it was at 7:30 in the evening, 11.5 hours after the fire was lit, and at that point the hottest portion of the concrete was still at 105F. Needless to say I was very impressed. I know I won’t see those kind of numbers when the temperature outside drops but I am excited to try it out when the time comes. I know it will not be as efficient as a rocket mass heater or a full sized masonry heater, but the idea is that I wanted sometime smaller that would fit well almost anywhere. The whole thing measures 32” wide, 32” deep and 38” high. I am thinking of purchasing some 1/16” sheet steel and surrounding the whole thing with a steel shell. Any ideas how that may affect the stove performance? Mainly I was to do this so the whole thing looks better. I will also eventually put in a well built door with a window to see the fire through. Thanks for reading! Any input would be great on how I may improve things further.
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Justin Hadden
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Here is a rough layout of the gas path, it’s hard to show but the gases exit below the two blocks second in line from the back. You also can’t see it but the gases wrap round the back of the stove as well before turning around to make their way up subsequent levels. The stove is surprisingly not too bad to get started. From the time I start lighting it is about two minutes with not much more smokeback than a regular wood stove. After about two minutes the draft is good and away it goes.
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gardener
Posts: 3647
Location: latitude 47 N.W. montana zone 6A
980
cat pig rocket stoves
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Hi Justin;
I merged your two posts into one.  
Nice looking hybrid you have built there! Thanks for sharing!
I know the feeling of building a new stove and then its too warm out to test fire it!
We will hope for a cool rainy day soon in your neck of the woods, so you can let her rip!Or as we say in the rocket world... take your Dragon up for a test flight!
I'm curious how well your cinder blocks will hold up over a winters worth of burning.
 
thomas rubino
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Location: latitude 47 N.W. montana zone 6A
980
cat pig rocket stoves
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Hey Justin;   I would hold off surrounding those blocks with plate until you see how hot they get while being open.
Cover them and they might really heat up.
With a short test flight you had concrete up to 350 F ... get it above 500 - 600 F  and things will start to degrade.
I suggest running what you have (when you can) and see how it holds up.
If it is doing super, than tack a skin around it and see what happens.

One possible issue I see might be the inner metal warping as it gets too hot?
Experiment and see!
 
Justin Hadden
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Hi Thomas, yeah I was worried about the base of the top blocks as they’re right in contact with the top of the stove. I’ve fired it about 3 times now, the only problem seems to be that the cement I used to seal between blocks keeps cracking. The other thought I had about putting a thin skin around it is that I could put the blocks in dry and make sure the outer skin is sealed. Hopefully they will hold up to the heat. I was thinking maybe I could take the top blocks off and put a coating of furnace cement between the top of the steel stove and the base of the top blocks as that would be the hottest portion.
 
Justin Hadden
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Also, I haven’t lined the inside of the firebox with firebricks yet, but I plan to in the future, as that should help protect the hottest portions of the metal stove from excess heat.
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