Also called an “active” or “open loop” system, this is the type of system most often installed in the central and southern areas of Florida, and other non-freezing sunbelt climates within the United States.
In the direct system, an electronic control system  compares the temperature of a sensor  located at the solar collector  with the temperature of a sensor  located in the bottom of the hot water storage tank  (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump , that draws cold water from the bottom of the hot water storage tank and circulates it through the solar collector. Solar heated water is returned to the top of the tank. The circulating pump is very small and typically uses about the same amount of electricity as a 100-watt lightbulb. Another version of this system uses a small photovoltaic (solar electric) panel to operate a direct current (DC) circulating pump.
Advantages. The direct system typically produces the highest operating efficiency because there is no nighttime heat loss from hot water stored on the roof; nor is any efficiency lost through a heat exchange process. Potable water from the hot water storage tank is circulated directly through the collector.
Disadvantages. The only disadvantage of this system is that freeze protection is provided by circulating warm tank water through the collector. This is not a desirable method of freeze protection in climates that experience more than a day or two of freezing weather each year, because energy loss during freezing weather could be significant. Even more important, freezing weather can coincide with a power outage, preventing the pump from circulating warm water through the solar collector.
Integral collector storage systems, also called “batch” solar heaters, combine the hot water storage tank and the solar collector surface into a single component, eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system.
This type of system works best as a preheater for a conventional or tankless water heater. The cold water line that feeds the conventional water heater is diverted  and sent first through the ICS solar module . Circulation is provided by utility mains pressure. In other words, when hot water is drawn out for use from the conventional water heater , the storage tank is replenished with solar heated water instead of cold water. This allows the electric or gas heater to work substantially less.
Advantages. The biggest advantage is simplicity: the system has no pumps, no temperature sensors, no electronic controls and no heat exchanger. When combined with a tankless water heater, the system can free up five to six square feet of floor space by eliminating the conventional water heater storage tank.
Disadvantages. The biggest disadvantage is nighttime heat loss. Stored heat is lost through the glass cover plate at night, which of necessity cannot be insulated to prevent heat loss. However, this heat loss is reduced in advanced ICS systems by stretching a thin clear film just underneath the glass cover plate, which creates an insulating air gap. Also, while the greater thermal mass of stored hot water within an ICS solar module makes this type of system more freeze resistant than the direct system (above), ICS systems are not appropriate for climates that experience more than four to five freezing nights per year.
Drainback systems have a few more components, but are specifically designed to provide fail-safe operation in climates with frequent freezing during the coldest winter weather. The solar collector and control system are the same as the direct circulation system. However, an antifreeze solution is circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss, and this combined with the fact that these systems are typically installed at higher latitudes, where incoming solar radiation is reduced, makes indirect solar water heating less viable than its direct circulation cousin.
Like the direct system, an electronic control system  compares the temperature of a sensor  located at the solar collector  with the temperature of a sensor  located in the bottom of the hot water storage tank  (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump … However, in the drainback system, the fluid circulating throught the solar collector is separated from the potable water in the hot water storage tank .
Either water or a glycol solution is circulated through the solar collector and a drainback tank . When the pump stops, fluid in the solar collector “drains back” into the drainback tank, leaving the solar collector empty whenever it has no fluid circulating through it. A second circulating pump  circulates potable water from the hot water storage tank through a heat exchanger in the drainback tank.
In an alternative design, only one pump is required—in the drainback–solar collector loop. In this arrangement, the heat exchanger is typically “wrapped around” the hot water storage tank.
Advantages. The system is designed to fail-safe and drain the solar collector(s) during freezing weather, even if a power failure should occur.
Disadvantages. The heat exchanger makes this system slightly less efficient than a direct system. And as you might expect, the drainback tank, second pump and heat exchanger make this system a bit more expensive than an ICS or direct system with comparable solar collector area and hot water storage capacity. On the other hand, this system is ideally suited to climates that may experience 10 or more days of freezing weather per year.
This type of system is the most common in Japan, Israel and Australia, which have enjoyed an installed base of millions of residential and commercial systems for the last 30 years. Like the ICS system, the thermosyphon system eliminates the circulation and control system.
However, circulation is provided by the thermosyphoning principle: The hot water storage tank is located higher than the solar collector and circulation flow is induced when the coldest water in the bottom of the storage tank falls by gravity through a circulation line into the bottom of the solar collector panel, where it is heated and rises. Water heated in the solar collector panel rises into a circulation line to a high point in the water storage tank.
Advantages. Unlike the ICS system, which combines hot water storage with energy collection and so can lose heat at night through its glass cover plate, the thermosyphon system optimizes system efficiency by fully insulating a separate storage tank.
Disadvantages. The thermosyphon system’s primary drawback is appearance: The hot water storage tank must be higher than the solar collector, so it becomes a bulky protrusion on the roof. Modern thermosyphon systems place the tank on its side, along the top edge of the solar collector panel (see the photo), but while modern solar water heating collectors look like skylights, many homeowners are resistant to the idea of a tank on their roof.
An additional concern is weight: While the ICS system spreads its hot water storage over a greater roof area, the weight in a thermosyphon system storage tank is usually more concentrated. An older roof structure may not be able to support the added weight of a hot water storage tank.