Franklin, we have a 30x70 greenhouse where I work and it has an outdoor furnace that uses wood. We made the furnace from a piece of pipe (just like a barrel stove) then surronded that with a rectangular box made from plate metal. We have a blower from a central air conditioner unit that blows air from inside the greenhouse, into the rectangular box and around the stove, and back into the greenhouse into a clear plastic tube 18" in diameter, with small holes on both sides, that hangs from the ceiling the length of the greenhouse. It can be bought from a greenhouse supplier. The far end of the tube is tied shut like a trash sack so warm air comes out the small side holes. The trick to heating it though is that we have two layers of plastic over the greenhouse with a small centrifugal fan blowing air between the layers to keep them seperated. The air space is insulation. You can tell a world of difference. In 20 degree weather you will be in a t-shirt and it is warm from one end to the other.