How Drain-Back Solar Thermal Systems Work

Drain-back solar thermal system diagram

1: In a drain back system, water is used as a fluid heat collection medium. Distilled water not only provides excellent heat transfer, but also does not allow for minerals to deposit.

Water circulates through solar collectors, capturing and storing solar heat energy when the solar collector is exposed to sunlight.

2: When the system is not actively collecting heat, the circulation water is drained out of the collectors and piping, into an insulated reservoir tank. This prevents the water from freezing in colder weather conditions.

3: As the sun comes out and heats the collector panels again, a differentially-controlled pump engages. The pump moves the water out of the reservoir tank and back through the collectors, so that more solar heat can be collected.

The circulating heat-collection fluid in this system operates in a non-pressurized closed loop, only moving between the collectors and a heat exchanger. The heat exchanger transfers heat to potable water in the tank. Nominal operation noise is evident in non-pressurized systems.

Drain-back systems require less maintenance compared to other closed-loop systems; because water is the heat transfer liquid, it does not require periodic changing.