In an indirect active system, also known as “closed loop,” a simple circulation pump moves the non-toxic and food safe antifreeze solution (propylene glycol) through a closed copper pipe loop into the solar collector, through the collector’s absorber pipes, and out of the solar collector. Then, the sun-warmed heat transfer fluid flows into a double walled heat exchanger where it warms the cold water heading into a conventional hot water tank. The antifreeze solution then returns to the pump and again flows into the solar collector without ever mixing with the building’s water. Indirect systems are encouraged in climates with extended periods of below-freezing temperatures for year round solar heating.
In reality, indirect systems are somewhat more complicated than presented here. Differential thermostats, pumps, sensors, and controls are used so the simple systems illustrated work effectively and safely. However solar thermal technology is mature and proven with few maintenance requirements from the installed systems.