Pumps circulate a non-toxic, heat-transfer fluid (HTF) through the collectors and a heat exchanger. They are popular in climates prone to consistent freezing temperatures.
These systems pump heat-transfer fluids (usually an FDA approved glycol fluid) through collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Closed-loop glycol systems offer good freeze protection. However, the glycol must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures. They are generally more complicated than an open direct loop system, because either a tank with a heat exchange coil or an external heat exchanger is required. As a heat exchanger is required the collector loop will run at slightly higher temperatures than an open loop system. The collector loop needs to be pressurized(8 -12 psi).
The antifreeze may need to be recharged on a 3 to 5 year basis.