As most of you know, I'm test driving a greenhouse this year. I just knew everything in there would freeze when the temps fell below freezing for an extended period. Boy! Was I pleasantly surprised! It has been as low as 4 degrees and remained below freezing for days now. I have a small "milk house' heater on the floor set to just below 40. It does not run all the time but the temp in there has not dropped below 40 yet! A couple of times on sunny days it has risen high enough for the vent fans to come on and they are set to 85. I am totally pleased. Look how happy the herbs are.
I am real pleased with my max/min themomator that I purchased from Charley's Greenhouse Supply . I've moved it around to different zones to be sure there were no cold spots. I'm going to buy another one this spring - one for shelf level and one for floor level. This picture was taken today with an outside temp of 31 and no sun in site!