The wallipini used ground heat, which is about 60°F; to both heat and cool the greenhouse portion. Using solar heated collection panels, running airflow into the greenhouse will help. Running this heated air over a bed of gravel during the day, will provide heat from thermal mass at night. Sometimes, dark barrels of water absorb heat on the north wall; during the day, to radiate that heat at night. They call this passive solar collection.

Double walls are a decent insulation, you should use the on your current greenhouse!

If you can dig down several feet past your frost line, and run a tube in the trench; you can pull Earth heat out using air flow. They call this an earth battery. But a heat pump would work well too.

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