Drain Back Systems for Solar Hot Water Heaters

A solar hot water heater reduces the amount of electricity required to make hot water in your home, sometimes eliminating it all together. Instead, energy from the sun is harnessed to heat the water which is then stored in a tank. There are a number of different types of solar hot water heater appropriate for use in the region in which you live.

Because of climate differences, there is no universal type. Solar hot water heaters come in two main varieties: active and passive. After that, they are distinguished by the method of water circulation they feature and the type of solar collector they use. One system good for use in climates where freezing occurs is called a drain back system. In short, a drain back solar system is an active, direct or indirect means of water heating. To explain that, further clarification is needed.

Active vs. Passive Solar Hot Water Heaters

The basic distinction to be found between solar hot water heaters is active and passive. An active system uses electric pumps and automatic controls to force water into a solar collector. A passive system does not. It relies on the physical process in which water rises when heated. Passive systems generally require less upkeep because they have fewer parts, but they can also be less efficient. Active systems will use some electricity in order to run the pumps and controls, but it pales in comparison to the amount used by a traditional water heater.

Direct vs. Indirect Circulation

The next distinction to be made is direct vs. indirect circulation. A direct system, also called an open-loop system, circulates the water for use directly through the solar heater and back into the storage tank. They are often found in climates where freezing temperatures only seldom occur. The water is held in the solar collector in batches where it is heated and then distributed to the tank. An indirect system, on the other hand, circulates a heat transfer liquid through the solar collector. Once heated, this is drained into the tank and into a heat exchanger. From there, the heat gathered is transferred to the potable water used in the house. Indirect systems work well in colder climates because there is no water to freeze. Direct systems can also work in colder climates, but they must have insulated pipes or be drained. Here, at last, is the drain back system.

Drain Back Solar Hot Water System

A drain back system can be either direct or indirect. That is, it may pump the water for use directly into the solar collector or it may use a heat transfer liquid. Either way, the process is the same. Water is pumped into the solar collector where it is heated. With the aid of gravity, it drains back into the storage tank to be used. A sensor on the solar collector can detect when no further heat comes from the sun at which point the water is allowed to drain. Because no water stays in the collector after nightfall, it is protected against freezing. A closed-loop or indirect system recirculates water through the solar collector, back into the heat exchanger to heat potable water. This type is desirable in colder climates because it is better to separate the heat collection function from that of heat storage.

A drain back solar hot water system is most often an active, closed-loop or indirect means of heating potable water with solar energy. By allowing heated water to drain back into the storage tank after heat sensors detect no further solar radiation, the system is protected from the threat of freezing in cold weather. A drain back system is one type of solar hot water heating, but it can be one of the more complicated means of doing so.