While referring to the placement of the heat exchanger, There are many kinds of solar types hot water systems -pressurized closed loop, pressurized open loop, drain back, thermosyphon, and variations called direct and indirect. There are variations among the variations. One the drain version back system had been in continuous operation since the late ’70s. So here’s the question. How exactly do drain back operate? There are two temperature sensors that control a solar system. Basically, one is the Hitemp sensor on the outlet of the collectors. In the morning when the solar collector temperature rises to about 18oF hotter than the tank temperature, the controller turns the collector pump ON. Whenever picking up heat as it goes, Water is pumped from the bottom of the tank through the collectors. Whenever heating the tank continually, This process goes on as long as the collectors are at least 5oF hotter than the tank. Needless to say, the warmed water spills down the return line into the drain back tank.
Cold water goes through a copper coil heat exchanger in the solar tank and into the regular water heater, whenever someone uses hot water in the home.