Outdoor Thermal Mass Wood Stove Project (Not a Boiler)

Hi everyone! This is my first post here on Permies, though I've been following Paul Wheaton and the Wiseners for a while on info to heat my studio. I'm a full-time artist (I paint portraits in acrylic) and God recently blessed my family with a home in the country after living for 16 years in the city. We have some partially wooded acreage and a 17' x 25' outbuilding I'm using for a studio.

We moved in April, so I don't know what winter will be like in this building. But in Wisconsin, it gets mighty cold. There is a small propane heater, but I don't want to be slave to propane. My insurance company will not allow me to have a wood stove inside the building, but I can have one outside.

I wanted to build a rocket mass heater, but it's not practical outside, because most of the heat would be lost from the riser, and you would need a huge enclosure to capture it.

So, instead, I'm opting to enclose a normal steel wood stove in cob. Fortunately, I have a lot of clay on my land, and sand/gravel backfill that I can mix together. My church got rid of a furnace a couple of months ago, and gave me their double walled-stove pipe. We also built a pavilion and I was able to get the extra steel siding they had from that project too. I plan on insulating around the cob and putting siding around it. I had to buy some cinderblocks for the base, but I also got some free decorative blocks on Craigslist to keep costs down too.

The project is well underway, and I wanted to share it with you for your thoughts and maybe it would be of help to someone else.

I bought the wood stove off of Craigslist for $10. It had a lot of surface rust, but otherwise is in excellent condition. I painted it with several layers of stove paint and added firebrick to the inside. It has a jacket outside and an opening for a blower. My plan is to pipe the heat in using ducting and forced air. I also put some copper tubing within the cob so that if I want to extract heat and do in-floor radiant heat in a small portion of my studio, I can. I would run pex tubing inside my studio and put an antifreeze/ water mix in and pump it through. I'm hoping the stone/ cob mass will retain a lot of heat that can be absorbed into the tubing and maybe into the air duct as well.

I'll post a link to my video later (once I make it) :) Sorry, the images are not posted in chronological order. I still need to insulate this, install the 20' stove flue, and put siding around it and connect the air duct to my studio. What do you guys think? Thanks for any feedback!

Very nice!
Are you planning on recirculating the air to be heated, or starting with fresh winter air every time?
What kind of insulation are you planning on using?
You're bringing heated air in past your combustion chamber?
If so, maybe get CO and smoke detectors for inside the building, in case there is a leak.

Thanks for the kind words, William!

I am planning on bringing in fresh air from the outside. There is a 4" square opening on the bottom of the wood stove for air entry into the jacket. I used some aluminum dryer duct (not the flex kind with plastic, but solid aluminum.) Using aluminum flashing, I created a square duct and connected it to the round pipe and then directly to the opening. Finally, I surrounded it all with sand. Hopefully, that preheats the air a bit before entering into the jacket of the stove.

The pipe is turned downward and away from any potential smoke back and is covered with aluminum screen to keep any critters out. :) This opening is on the left side of the stove, as you look at the combustion area in the front.

Good idea on getting a CO detector. Even with fresh air coming in, if there was a leak, it would quickly become poisonous. The stove has a jacket surrounding the combustion chamber with about 3 inches of gap all the way around. If there is a leak, I'll have to go to plan B. (not sure what that would be, yet. :) )

For insulation, I plan to use fiberglass, layering it to R-30 on the sides and R-60 on the top.

Is the air intake just for combustion air, or general feed into the jacket? Will any air return from the studio to the jacket to be reheated?

I hope the enclosed stove works for you. If you find it isn't as efficient as you want, you could always build a rocket core in the shell and use that to heat it - the heating effect would work exactly the same as with the wood stove. A batch box would work better than a J-tube for this application.

What would your insurance company think of a mass (not the combustion core) that happened to be right up against the studio wall? If you made the mass tall, wide and thin, and insulated the top and exterior faces heavily, you could get the mass radiator effect without the system being in the space. Of course, you would remove any insulation from the studio wall where the mass was, and ideally make the wall thin like one layer of metal siding.

Hi Glenn,

Great thoughts. To answer your question, the air intake is just for the jacket that supplies hot air through the duct to my studio. The air intake for the combustion, at this point, is just the vent in the metal woodstove. I might put an insulated door in front of that door, with an opening for air. Or I might insulate the stove door itself. Not sure.

I think they wanted the stove to be three feet away from my studio building. They didn't give me a precise set of rules, just guidelines.

Your idea of a rocket mass heater that was butted up against the building, I believe, would work in that scenario. It would be a lot harder if it had to sit away from the studio.

I hope this will work as is. I love, love, the idea of a rocket mass heater--the concept of burning wood more efficiently. I tried a coiled exhaust system, but it ended up making the draw so poor that smoke was pouring out of the front of the stove. So I changed it for a typical run of stovepipe, although enclosed in cob. This will unfortunately still give off smoke and go through a normal amount of wood, but I am hoping the mass will even out the heat output a bit in between burns.

Thanks for your ideas, Glenn!