Using a Drainback System to Heat Water
A drainback system lets you use solar power to heat water for your home. Indirect, active, closed-loop drainback systems are an excellent choice for heating water unless you receive a lot of snow and have really cold temperatures. They're the best choice in hot climates, although they're more expensive to install than ICS. The advantage is that you have no freezing danger.
When the pump is off, the fluid in the collector and feeder pipes drains back into the drainback tank. (This requires that the collector be mounted higher than the drainback tank, which may be a problem in some applications.) Fluids other than water work in the closed-loop heat exchanger circuit; however, water's best because it doesn't degrade when it gets hot (unlike anti-freeze solutions).
Most manufacturers offer complete kits for these systems, so you don't need to calculate the system parameters. These kits work very well.
Here are some installation considerations to keep in mind:
Raising the liquid up to the collectors sometimes takes a lot of force, so your pump must have sufficient head pressure: (Measure head pressure by the difference in the vertical height of your collector, and the location of the heat exchanger tank.) You can use a PV module to power the pump, but this is sometimes inadequate because there may not be enough sunlight to yield enough power. On the other hand, if there's not enough sunlight to run the pump, there's probably not enough sunlight to provide meaningful heat. PV power generally works well because you can forego the need for a controller; when there's enough sun for heat, there's enough sun for electrical pumping.
Installation must ensure the closed-loop system drains quickly and completely: Locate drainback tanks as high as possible but low enough to ensure the entire weather-exposed parts will drain back. The less distance between the drainback tank and the collector, the less head pressure the filter needs, which means less power consumption.
Pipes should never be horizontal, so ensure a minimum 15-degree slope: Because purified water is best for the closed-loop system, these systems are prone to freezing. When mounting the pipes, making sure they all have a slope so that they'll drain by force of gravity is critical. Sometimes pipes sag, and the low points retain some water, which can cause the pipes to burst in a freeze. The collector must be mounted so that it, too, will drain completely.
Don't use 90-degree bends: Instead, use two 45-degree bends staggered apart.
Larger diameter copper pipe works best: Use a minimum of 3/4 inches.