I used to install those in Globe Arizona in the late 1970's. It was a simple hydronic system. The differential controller was the weak link. It was crude with lots of IC's, and was not robust. The water tank has a heat exchanger wrapped around the tank. The solar loop was a closed, pressurized loop just like a hydronic boiler. In the photo it looks like your water tank is in trouble... all the corrosion at the base.
