How a drainback solar hot water heating system works

A drainback system is a kind of closed-loop system used to preheat water. The systems consist primarily of a water tank (11), a smaller tank that stores heat-transfer fluid (14), a solar panel array(3) to collect heat from the Sun, and a heat transfer system to transfer heat to the water tank. A heat-transfer fluid circulates through the system.

How it works:

• The solar collector panels(3) are heated by the Sun's rays;
• The heat transfers to the transfer-fluid;
• The pump (13) transfers the heated water to the drainback tank (14);
• The pump (8) circulates water through the heat-exchanger (12), transfering the collected heat to the potable water tank (11).
• The heated water is stored in the water tank until it is used --such as when hot water pours from a faucet etc.

The fluid only circulates through the system when the panels are collecting a usable amount of heat from the sun's rays. The heat transfer loop is only filled with the transfer fluid when the pump operates. At other times such as at night, the pipe loop is full of air. When working properly, this configuration prevents the freezing of the heat transfer fluid.

The transfer fluid generally depends on the pump type. Stainless-steel and bronze pumps generally use distilled water, whereas iron pumps use a transfer fluid that contains a corriosion inhibitor to reduce oxidation or corrosion damage (rusting) to the inside of the pipes and pump. Some regions regulate the use of corrosion inhibitors.