About twelve years ago I installed a solar heating system in a pool. The buildings nearby, a garage and a pool house, had aluminum roofs. I attached copper lines to the inside surfaces of the roof, going in the parallel direction of the studs. All was fed uphill to a central pipe going along the top of the ridge board. Feed was supplied by a Bell and Gossett SLC-25 circulator pump. I had a bleed line at the top of the ridge to drain the system. You just opened a valve on the wall and air was introduced to the top. In one minute everything was drained for freezing weather. Closing the valve and turning on the pump reloaded the system. I had a jog button installed to force maximum pump output. I only had 18 feet of head and it took all it had to push the bubbles from the system. A thermal differential was installed to vary the pump speed. After sundown the unit would shut off.
The setup for Appalachia was almost too good. The water in the pool was vary warm for most of the season. In October they were still swimming with only solar heat. It more than doubled the swimming season for this area.
Truth be told; I also installed a ten section National solid fuel boiler in the garage, the next year. I installed a dual heat exchanger setup for domestic water and one to heat the pool. The boiler heated the garage, house, barn, and the remaining waste heat was shunted into the pool. After this the solar unit only was used when the boiler was down. This meant it was only used from May through September, from then on.
The solar system had the added affect of lowering the temperature in the buildings. It is amazing how much cooler a building is when the roof is cooled. The water evaporation got extreme. Warm water in cool dry weather blows away like someone punched a hole in the bottom of the pool.