The thermosiphon takes advantage of the fact that heated liquid, in this case a food based glycol and water solution, rises as it is heated. A thermosiphon system uses a flat-plate collector, an insulated, weatherproofed box that contains a dark absorber plate under one or more glass or plastic (polymer) covers to heat the solution within the closed system. The solar-heated glycol water solution in the collector rises through tubes and flows into a sleeve that surrounds the water to be heated in an attached storage tank. This tank must be located higher than the collector for thermosiphoning to take place, and heats the water within the storage tank via conduction. As the glycol water solution surrounding the tank cools off while heating the water, it's cooling causes it to want to go to lower, and hence returns to the solar collector to be reheated.

The system is designed for warm climates but can be used in temperate climates since another benefit of the glycol solution is that it prevents freezing.

In an ICS system, water is heated and stored in a collector, or a series of collectors, connected to the main water supply and to a conventional gas or electric hot water heater. Each time hot water is drawn from the water heater by opening a tap in the building, the supply is replaced by hot water from the collector. Simultaneously, pressure from the water main causes cold water to flow in and refill the collector.

Most contemporary ICS units are mounted in a flat insulating box with a glass or plastic cover that admits sunlight. The unit consists of a series of long tubes connected in a serpentine arrangement that holds between 30 and 50 gallons of water. The tubes are typically painted to create a dark surface that efficiently captures the sun's heat. The water in the tubes absorbs the heat collected on the tube's surface.

In a direct (open loop) system, water is circulated directly into the collector from the storage tank, typically a backup hot water heater. There are two types of active direct systems: draindown and recirculating. In both, a controller activates a pump when the temperature in the collectors is higher than the temperature in the storage tank. In the draindown system, a valve operated by the controller allows water to drain from the collector when the temperature approaches freezing. In the recirculating system heated water is pumped from the storage tank through the collectors when the temperature drops to 38°F.

In a closed-loop antifreeze-heat exchanger system, an expansion tank allows antifreeze to expand and contract as the temperature changes. A number of valves and gauges are needed to protect the equipment and keep the system working well. A pressure relief valve protects against excessive pressures in the closed loop; a spring loaded check valve guards against reverse flow at night so the collectors won't dissipate the heat from the storage tank; and an air vent helps get the air out of the closed loop (since it can block fluid flow through the system). A pressure gauge will indicate whether the system is still charged with antifreeze, and a temperature gauge will indicate how well the water is being heated. Systems that use antifreeze fluids need regular inspection (at least every two years) to verify the viability of the antifreeze solution.

A simpler indirect system, which can use distilled water or antifreeze as the heat transfer medium, automatically drains the collector whenever the circulating pump stops. It includes a valve that will purge the water in the collectors when the outdoor temperature reaches 38°F. When the temperature is higher than 38°F and the collectors are hotter than the storage tank, the valve allows the system collectors to refill and the heating operation resumes. Since the collectors will only have water in them when the pump is operating, there will be no fluids in the collector at night, or in case of power failure, that could possibly freeze or cool down and delay the startup of the system when the sun is shining. This is a simple, reliable failsafe system.