Closed-Loop Drain-Back Solar Hot Water Systems
Solar hot water generated by a closed-loop drain-back system is perhaps the least maintenance intensive of any active system available. These systems use distilled water as the heat-transfer fluid, which conveniently almost never requires changing.
The reason this system works with distilled water vice anti-freeze propylene glycol is because of the drain-back feature of the system. When the system is at rest (no water hot water is being used) the water that would be retained in the collector in any other system drains back into a drain-back reservoir leaving the solar heat collector empty.
The drain-back reservoir, which stores the distilled water, is installed just above the solar storage tank.
When hot water is demanded, the pump turns on circulating the distilled water from the drain-back reservoir through the collector and into the heat exchanger where it heats your potable water. When water is no longer needed the water in the collector just drains back from the collector into the reservoir tank.
This system feature requires that the collector be installed higher than the storage tank. The piping must also have an adequate amount of slope built into it to facilitate drain-back and eliminate the potential for freezing in the piping.
Drain-back systems are effective and reliable making them a great system for home in the hottest or coldest of climates. They can often go 20 years without any servicing required.
The only drawback to this system is the size of the pump required. In order to overcome the head loss of getting the water to the solar heat collector on your roof it often needs to be quite powerful.
This can be aided by strategic placement of the collector, the drain-back reservoir tank, and the solar storage tank. Some times the addition of an anti-freeze additive like glycol is necessary to overcome inconvenient piping arrangements.
Take a look at the diagram below of a solar hot water closed-loop drain-back system for a better understanding.