Drainback solar hot water systems are not as flexible as pressurized systems, but still work very well in both residential and business applications. They are very efficient if they are planned and installed carefully.

These systems work well in most warm climates but do not have the freeze protection of a pressurized solar hot water system. They cannot survive in regions with long or harsh winters because they usually employ simple water as a solar fluid, which makes them susceptible to freeze damage. In regions where cold temperatures are rare but possible, a thin antifreeze mixture is used to avoid disaster. If your location experiences few or no freezing periods throughout the year, a drainback system is a good choice.

Drainback systems are indirect, closed, active systems. Although many use simple water as the solar fluid, the system keeps the fluid in a separate, sealed solar loop and heats the domestic water supply via a heat exchanger. At least one pump is used to keep things running effectively.

Unlike pressurized systems, drainback solar thermal installations empty themselves when the domestic supply reaches the required temperature. This means that the solar fluid never lies stagnant in the solar loop – it's either there and pumping around or gone – and so has an extended life.

Provided the local climate allows their use, drainback systems are ideal for installations with extended idle periods, such as holiday homes, and commercial applications where the heating load varies a great deal. They are a good choice for both residential and commercial water heating and space heating systems.