I understand what you are saying, but in my opinion I think you would have to heat the water in the ap to much to keep the greenhouse warm and constant. the more mass will give you a more even heat at a lower temp. For example: you may have to heat the ap water to 100 degrees to maintain 70-75 degrees in the greenhouse. I think if you had more water (storage barrels) at say 85 degrees your heat transfer from both the ap and barrels would be more stable. I too am concerned about efficiencies. I want the most solar storage as possible during the day. my heated water will hit the ap first then cycle thru the barrel back to the heater. I could be full of it but that's my plan. I need to maintain at least 60 deg during the winter. Do you have other thoughts