My greenhouse is heated in the same way as subartic's. As I am growing mostly orchids and tropicals, its pretty humid so I always have several small fans running to keep the air circulating. I try to keep the winter temperature above 55. My system works well enough so I only need an emergency backup when the temps are approaching 0. I use a cheap utility heater with a forced air fan sitting in the middle of the floor on the dry end of the greenhouse.
Most heaters now have a tip over shut off feature but you should always be using one with a grounding prong.
