If you prefer to go even lower tech, check out compost heating: https://www.happysprout.com/outdoor-living/compost-heating-greenhouse/ https://www.happysprout.

If you prefer to go even lower tech, check out compost heating: https://www.happysprout.com/outdoor-living/compost-heating-greenhouse/ https://www.happysprout.com/outdoor-living/compost-heating-g... Unless you have pigs fed with swill, you'll easily produce enough compost to survive the winter in warmth.