[Gypsy61]

Do you have livestock? You can make a large circle with cattle panels and coil copper pipe in it (outside of the green house), and fill it with manure and compost materials to make a hot compost pile. Then run some Pex under the beds. Run into a holding tank and from the holding tank back to the pile.

I saw someone used a hot pile method to heat shower water through a whole winter for people volunteering on their farm, years ago in a YouTube video. I think recirculating water would prob help keep lines from freezing for your application.