Pump-driven systems may use water or propylene glycol as the heat transfer fluid. As in thermosiphon systems, discussed earlier, using water as the heat transfer fluid allows one to create an open-loop or direct system. Although open-loop systems have their advantages and may, therefore, seem desirable, there is a major challenge that must be overcome in colder climates: freezing.
Freezing is a problem because when water freezes, it expands, and not just a little bit. It expands quite a lot. Water freezing inside a copper pipe can create enough force to crack it wide open.
Solar designers solved this problem in two ways. The first is a draindown system. This type of system is not recommended by most of the folks I’ve talked to, so I won’t discuss it here.
There is a viable alternative to the draindown system, a simpler trustworthy cousin, the drainback system (Figure 3-10). “Although similar in name to the draindown system,” writes Ken Olson, an expert on solar hot water systems, “the drainback system is far different and much more reliable.”
Image caption: Fig. 3-10
This is a drainback system, generally recommended by solar installers over the draindown system.
Drainback systems rely on gravity to drain water from the pipes and panels when the circulating pump stops. When the sun stops warming the panels, a sensor signals the circulating pump to cease operation. Water flows out of the system into a storage tank.
Gravity drainback systems are simpler than draindown systems because they have no electronic sensors and motorized valves, which dramatically reduces the possibility of system failure and deep freeze. Remember: in active solar systems simplicity reigns supreme. The fewer the parts, the less likely the system is to malfunction — and the less maintenance will be required.
Gravity drainback systems can be used in all climates and are less expensive and easier to maintain than other active systems. According to Greg Pahl, author of Natural Home Heating, direct-circulation, gravity-drainback systems are “considered by many people to be one of the simplest and best systems to install.” They not only eliminate mechanical parts that can fail, they do not require a heat exchanger and propylene glycol heat exchange fluid which, as noted earlier, deteriorates over time and must be replaced every five years or so. Although a better option, these systems need to be designed and installed correctly to ensure complete drainage when the pump stops.
Pros and Cons of Drainback Systems
As just noted, drainback systems are not only simpler and more reliable than draindown systems, they’re a little less costly. They can be used in cold climates and operate without propylene glycol. On the downside, these systems require the largest pumps of all solar hot water systems in use today.