Closed-Loop Active System

This system circulates heat-transfer fluids (usually a glycol-water antifreeze mixture) through the solar water heater. Heat exchangers transfer the heat from the fluid to the water that is stored in tanks. Double-walled heat exchangers or twin coil solar tanks prevent contamination of household water.

Some standards require double walls when the heat-transfer fluid is anything other than household water. Closed-loop glycol systems are popular in areas subject to extended below freezing temperatures because of the glycol freeze resistance.

However, glycol antifreeze systems are more expensive to purchase and install and the glycol must be checked and changed every few years, depending on glycol quality and system temperatures
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