Sooo. In my initial plan for the studio we are building, I had planned to use a small wood-burning stove that I picked up on Craigslist about two years ago for a hundred bucks. The title of this post is the question Greg asked me when I arrived home with it in the back of the truck.
Unfortunately, once I had laid everything out on paper when we were contemplating the studio, and I took into consideration the very large clearance from combustible materials that is required for a cast iron wood stove, I realized it was going to eat up a whole lot more floor space than I thought. I haven't solved this dilemma completely, but I'm looking at a few options, including a direct-vent gas (propane in my case) fireplace that recesses into the wall and has a low profile on the inside (most of the stove bumps out of the wall to the outside), and does not require a chimney. I also found a very small Vermont Castings cast iron gas stove that is only 13 inches deep and can sit as close as three inches from the wall since the combustion chamber is insulated. This would eat up very little floor space.
Anyway, I'm still working on this issue and cost is a factor. We'll probably wait until next fall to deal with the heat issue because we can limp along with a small electric heater for the time being, except when the temps drop into the single digits. I just can't store anything out there that will freeze until we solve this issue.
Anyway, I have this old second-hand, wood-burning stove that looks exactly like this... It's a Vogelzang Deluxe Boxwood Stove and that's the image from their website. Mine is buried under a bunch of stuff in the barn at the moment. I figured I'd hang onto it and eventually use it to heat a greenhouse since it turned out not to be ideal for my studio. It's a nice little utilitarian stove, no window, just a cast iron box with a flue. It's pretty cute and you can even cook on it in a pinch. It's perfect to heat a greenhouse if you have a way to move the air around the space. There's even a little gadget that you can add between sections of the stove pipe that will capture much of the heat going up the stove pipe and send it out into the room rather than having it escape to the outside. It makes the stove much more efficient. Anyway, the problem with cast iron stoves is that they get super hot, and once the fire goes out they lose heat pretty quickly. There's just not enough thermal mass to really retain a lot of heat for very long.
Or so I thought... check this out!
This is from Kiko Denzer's webpage. He's the guy I got the original idea from, for Sunny the clay oven. Look at that thing! He's basically created a "clay hat" for his stove and inside this hat is a serpentine tunnel that winds around and around inside that thing. The clay heats up and becomes basically a radiator and the winding design inside there is basically a big heat exchanger, with lots of surface area to soak up the heat. After the clay absorbs the heat traveling through the tunnel, the stove pipe exits "the hat" and the smoke and some of the hot air goes on up out of the roof, leaving most of the heat behind in the Hat. The hat slowly radiates the heat into the room, just like my clay oven radiates heat back into the baking chamber. Genius! (although I do have some concerns about the ability of those spindly legs to support what has to be several hundred pounds of clay. Looks a tiny bit precarious.
Anyway, it sparked an idea about how to heat a future greenhouse, using a stove I already have, and the abundant supply of oak firewood we have accumulated over time.
I could really extend my growing season if I could put a little bit of supplemental heat into a tunnel or lean-to greenhouse, particularly if I use twin-wall polycarbonate or a double layer of poly film. I'd start by taking the legs off the little Vogelzang stove (they're bolted on), and right smack in the middle of the greenhouse, I'd dig down several inches and put a scrap of foam insulation in the hole about 4 x 4 feet square. Then, I'd pour gravel on top of the foam insulation until it was level with the ground. Then, I'd basically build something similar to Sunny, but embed this stove inside the mud dome, leaving the front of the stove exposed, for access to load the wood, build the fire, and clean out ashes. Access to the damper is also in the front part of the stove. You could make a hearth out of the clay too so you could sit by it and warm up when working in the greenhouse in the winter, and even build out to the side, a chamber that could hold your firewood. The heat from the clay would probably get warm enough to help dry the firewood so it burns better, with less smoke, and it would also give you a place to store several days' worth of wood without having it piled up all over the place. I'd cover the stove pipe with a thick tube of clay up to about six feet or so, then stick another damper in the stove pipe at that height, to help draw the air upward, then add that little gadget to capture residual heat that is still in the stove pipe, and then vent it out through the roof. Basically, the stove becomes the firebox for a big masonry heater, and all that clay (probably 12 or more inches thick) would absorb the heat through the walls of the stove, and slowly release it into the greenhouse. You could really make this a massive thermal storage device and really get a lot of bang for your buck with each fire.
I think you could still do what Kiko did and form a small chamber above the firebox that you could use as an oven, and get double duty from this contraption. Not sure what you would want to cook in a greenhouse but, whatever... Although, you certainly could bake bread in there when it is too cold outside to use Sunny.
It would look like a big adobe fireplace in a pueblo except for the cast iron door. By enclosing the stove inside a big envelope of clay/mud, I think you would eliminate any issues of minimum clearance to the wall, and with the exception of the door, none of the surface should get hot enough to actually burn you. The thermal mass of the clay structure should help avoid the extreme temperature swings that you get with a typical wood stove.
One thing you'd need to keep track of is the humidity level in the greenhouse. As a rule, you want a lot of humidity in a greenhouse, and the hotter the air temperature, the more water it holds. This is a good thing when trying to raise plants. However, raising the air temp in the greenhouse way above the outside temperature is going to cause a lot of condensation on the walls/ceiling of the house. This water is going to come from somewhere so you would need to make sure your planting beds are not drying out too much, or figure out a way to recapture the condensation as it runs down the walls of the greenhouse. Hmmm, surely there's a solution for this. People grow food in heated greenhouses at latitudes above 50 degrees so this is not a unique problem. It might be something as simple as having an open tank of water as your humidity source, which slowly evaporates, providing the necessary water for the surrounding air, and then capturing the condensation along the bottom of the greenhouse walls and feeding it back to the drip irrigation system. This could help prevent the soil from drying out. Sounds complicated but I'll research this issue and figure out what other people have done.
This might be a fun project to do when my folks come up sometime. It would be much more challenging than building Sunny, which took the better part of a weekend. My dad would have a blast doing this with us. Based on the amount of clay, sand and straw I used when I built Sunny, a clay heater of this size would probably require at least a full pickup bed each of sand and clay, and probably not as many bales of straw. The straw strengthens the mix but too much straw creates an insulating effect, trapping the heat inside the stove, which was perfect for the clay oven but not what I would want for this heater. I would want the heat moving through the clay walls and out into the room. I'd have to mess around a bit and figure out how little straw I can get away with and still have a strong structure, particularly the chimney part.
Best part about this project is the cost. I already have the hundred-dollar stove. Local clay is free or almost free (I probably can use the tractor to dig some really decent clay out of my pond that will be easy to work with) and coarse sand is a few bucks a ton. Literally. I just need to go to the sand pit and get it. The rest is just manual labor. As for fuel, we have numerous dead trees on our property that need to be dealt with anyway. Much of the wood is useable lumber but the limbs and so forth are perfect for firing this little stove, and we have an almost unlimited supply of them.
If I eventually do this, the greenhouse should be stay comfortable in all but the worst part of winter, and I ought to be able to grow all kinds of cool season veggies well into December or perhaps... even... dare I say it? January????? Eliot Coleman would be proud!