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Close-Loop Drainback Systems

The problem with glycol, which has to be changed once every other year at least, can be eliminated by using a closed-loop drainback system. In these solar hot water systems, distilled water is used in the solar loop rather than glycol. To ensure that the water never freezes, the cold water supply arm to the collector is never left filled with standing water.

There are two sensors in this system on the closed loop. The first is where hot water comes out of the solar collector and the second is where cool water leaves the solar storage tank. The collector sensor shows water that is 11 degrees Celsius above that of the tank sensor, a pump activates to pump cool water into the collector, thus also moving warm water back into the tank. When the collector is filled with cool water, the pump shuts off and any water in the cool feed line to the collector drains back into the storage reservoir. The result is that only the collector is filled with standing water at any time and thus the risk of freezing (other than at night) is exceptionally low.

Other than the closed-loop operation, everything about drainback systems is the same as a glycol system. Drainbacks tend to be much easier to maintain and less costly because they do require glycol be changed every year or two. In some cases, glycol is added to provide an extra measure against freezing. In these cases, the glycol does need to be changed, but because the system is not pressurized like glycol systems are, it is a simple, do-it-yourself process and thus much cheaper. Compared to open-loop systems, drainback systems are less maintenance because they contain distilled water and thus are not subject to lime and calcium buildup.

The only drawbacks to drainback systems are their reduced efficiency due to their closed loop design and their need for a larger pump in most cases to circulate water to the collector, which must be the highest point in the system.

As a side note, drainback systems can often be combined with limited photovoltaic installations to create a 100% self-sufficient water heating system. Because the pump operates infrequently, its power use is relatively limited and can be supplied by a single battery in most cases.
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