I have (finally!) got my high tunnel greenhouse built and covered. It is a plastic, uninsulated structure, partially sunk in the earth, 16 feet long and 10 feet wide, with the aboveground height being 5 feet. I don't want to heat it through the winter, but instead want it to moderate the large daily and seasonal temperature fluctuations in Colorado. To this end, a scrap radiator connected to a tank of water sounds like a good idea: see here: https://northernhomestead.com/car-radiator-for-heating-and-cooling-a-greenhouse/
The big difference is that I plan to use water in the tank, but Polyethylene glycol antifreeze in the radiator, circulated through a coil of pex pipe in the tank.
I realize that you can't give me exact answers, but I'd like to get a ballpark idea on the following:
How big should the tank be to keep the greenhouse at 25 F when the outside temp is at 0 F? 150 cubic feet? 50 cubic feet? 300 cubic feet?