Dave Hastings recently sent me some photos of the system he built for heating his greenhouse bench at Waxwing, which is an off-grid farm in Nova Scotia:
Dave explains: "Like I said, we are off-grid, so when we were planning a seedling greenhouse we couldn't go with a standard hoophouse with two layers of inflated plastic. To run the small fan needed to inflate the plastic 24/7 would have required us to double the number of solar panels we'd need to run everything in our house. So I built a wood framed structure covered in polycarbonate panels."
"To heat the greenhouse I built a seedling bench along the north wall. The front of the bench is a row of cinder blocks painted black. Between the cinder blocks and the wall I filled in with rocks almost to the top. I then laid 6 inch salvaged stovepipe on top of the rocks running the length of the greenhouse. I poured concrete around and on top of the stovepipe to make the top of the bench. I was told that concrete conducts heat the best of the common high thermal mass materials (sand, rock, brick). I wanted to make sure that the heat from the stovepipe not only warmed the thermal mass, but also moved through the thermal mass to heat the seedlings on top. There is about an inch of concrete on top of the stovepipe. The stovepipe goes out the far wall and then up the side of the greenhouse (you can see it in one of the pictures). I had an old new mac/wood chief stove rusting on the farm and put it to use. It works great in this case because the stovepipe exits the stove on the side, straight into the seedling bench (see the close up photo). Also, those old stoves can't really be damped down, so normally lots of heat is lost straight up through the stovepipe. We capture a lot of that heat in the seedling bench. So far there have been no issues with draw through the long stretch (16 ft) of horizontal stovepipe."
"At night I drop down a sheet of greenhouse plastic over the bench to create a greenhouse within a greenhouse. Even when the nights are below freezing, one fire at 8pm or so keeps the space under the plastic above 15 C until morning. I can hold 22 seedling trays on the bench and the shelf above it. It works well to have two different temperature zones in the greenhouse. On the bench is warmer and great of hot crops getting started. Off the bench in the rest of the greenhouse is cooler at night and good for onions and such."