My 12x12 greenhouse has a solar collector loop charged with glycol that runs through a 400 gal. heatsink. The water in it gets pumped into the insulated thermal mass of the floor with a DC pump on a solar panel. For supplementary heat, I disassembled a three-element radiant patio heater and mounted the 500W lamps on the ceiling on a thermostat set at 45°F.
For severe cold (it can go to -40°) I have a small propane heater with an outdoor tank.
