Image by SomnusDe via Wikimedia CommonsIn the most basic configuration, a reservoir tank is placed on the drain-side of the solar collector. When the temperature of the water in the solar collector is higher then the tank temperature, a controller turns on the water pump. As a result, colder water is pumped into the collector, which preheats it before it is returned to the storage tank. When the pump is off, gravity drains the water back into the reserve tank.
The Primary Advantages of Drainback Systems
The biggest advantage of drainback systems is that when they are not operating, all of the fluid drains back into the reserve tank so that the solar collectors will not freeze. Usually, portable water is used as the transfer fluid because it has better heat transfer characteristics than anti-freeze systems. Unlike pressurized glycol systems where the pump runs continuously, drainback systems only turn the pump on when there is a need for it. As a result, drainback systems use less electricity.
Also, drainback units will not overheat unlike anti-freeze systems, which tend to do so. In general, drainback units are cheaper, require less maintenance, and are more reliable than other solar designs. Lastly, they do not require installation of an expansion tank, which can be very expensive.
One of the biggest concerns with drainback designs is that they require careful installation of the piping in both the drain and input loops so that all of the fluid will drain into the reservoir. Also, drainback units can be noisier than other solar designs. In addition, since the transfer fluid must be elevated to roof level, a high capacity pump must be used. These pumps can be somewhat expensive and will use more electricity than you might expect. Lastly, a pipe with a larger diameter is recommended in most scenarios, which can increase the cost of a drainback system.
Where Drainback Systems Make Sense
Deciding which solar hot water design is right for you should ideally be determined by the climate in which you live. In temperate climates where the temperature rarely drops below freezing, passive solar designs are a good choice. In regions where temperatures are moderate, closed loop designs using pressurized anti-freeze are probably the best option. In areas where there are a significant number of days below freezing, drainback systems should definitely be used. In extremely cold climates, anti-freeze can be added to offer another level of protection so that your unit works properly and provides a reliable source of supplemental hot water. To decide if a drainback system is right for you, contact a professional in your area.