Indirect (closed loop) solar glycol systems use propylene glycol as the heat transfer fluid in the solar array to overcome issues of freezing (glycol can tolerate – 60F in high concentrations) and eliminate scaling. The glycol is separated from the potable water by means of a heat exchanger that transfers the energy from the array into the storage tank.

Indirect solar water drainback systems are also a indirect design that use a heat exchanger to separate the collector loop from the potable water. Instead of relying on glycol for freeze protection, this system allows all the water in the array to drain back from the array and into a storage vessel in the solar loop when the pump is turned off. Because the array completely drains, water can be used in the solar loop, which avoids the need for double-wall heat exchangers demanded in some localities.