Solar Water Heating: Overview

15 Gal Drainback tank on top of an 80 Gal solar storage tank

What Does “Drainback” Mean?

A drainback system allows water to circulate through the collectors without the risk of freezing. When the sun is not shining, the water in the collectors will drain back into an indoor reservoir tank . The drainback system can also offer collector protection in extremely hot environments. At very high temperatures, glycol breaks down into acidic by-products that can corrode the collectors’ copper tubing. Drainback systems can be set to stop circulating the fluid through the collectors when a high temperature set-point is reached, allowing it to drain down into the reservoir tank, preventing overheating. If your climate is truly freeze-safe, you can use distilled water instead of glycol. Water is more efficient at collecting heat, and distilled water won’t leave mineral deposits in the collector tubing, as tap water can do. In hot climates that experience no freezing, drainback systems may be your best choice.

AC or PV Powered?

All solar water heating systems, except thermosiphon systems, require electric pumps to circulate the water or heat-exchange fluid. Pressurized closed-loop systems can often be circulated by a small pump, which could be powered by AC or DC sources. The main advantage of a DC pump is that it can be powered by a small 12 VDC solar electric (PV) module. Complete systems for AC or DC/PV operation are available. Another advantage to a DC/PV system is that even if the utility grid goes down, leaving you without AC power, the pump can still run and you will still have hot water as long as it’s sunny! Open-loop and drainback systems usually need larger pumps to push the unpressurized fluid upwards to the collectors--that will usually require a stronger AC pump.