My solar thermal drainback system worked beautifully and saved thousands of dollars over the course of many years. I simply followed the instructions of a Home Power article, which are now available for free online.
I dedicated two smaller panels know that the hot tub water would eventually corrode them, but for me in this application is was not a problem. I also had a lot of roof space to work and the drainback feature worked fine.
I had an electric heater/pump on a timer that would turn on around 6pm and run for 2-3 hours. The solar hot water was technically a "pre-heat system," augmented by a conventional backup. The solar got so hot during the day the only thing preventing it from overheating was a differential controller, which worked like a charm.
Don't expect the solar hot water system to provide 100% of your heat unless you have the capacity and want to only soak during the day.
Drainback insures non-freezing.
My monthly bill for the hot tub averaged about $7, compared to what would have been over $100 to heat conventionally. When the pump and heater turned on at 6pm, the heater would turn on for a few minutes and then turn off because it has a sensor that prevents overheating...so the only draw was for the pump.
