Posted: 22/08/2017 at 06:40
What are you planning on growing that will require a fixed heat source?
Heating is such a vague term; for me I heat my greenhouse only when it snows heavily at night by lighting a church candle and leaving it to burn in there to keep the worst of the frost out. For others they may require a +5'C temperature throughout the winter.
I can't help with the frost line but I'm sure someone more with knowledge than me (or good on Google at least) will be along soon.