All solar water heaters do the same thing. They collect heat from sunlight and irradiation through the solar collector and transfer that heat to water which is then stored in a vessel (the geyser or tank). The solar collector is generally a flat plate collector or an evacuated tube collector. More basic systems may be a coil of black pipe within a box or similar. The levels of efficiency are determined by quality and size of the collector. Although there are over 140 solar water 'high pressure' systems tested and passed by the South African Bureau of Standards (SABS), and more that have not passed the rigorous testing process, they all set out to do the same thing. That is, to heat water from the sun and to store the hot water.
High Pressure and Low Pressure Systems
The pressure of the water determines the type of overall system you require. If 'high pressure' water comes out of your tap, you will need a 'high pressure' solar water heating system. If the water flows with no pressure, only using gravity, it will generally be a 'low pressure' system. Most homes with an existing electric geyser will require a 'high pressure' solar water heating system. Lower income homes that have no existing hot water boiler are generally fitted with 'low pressure' solar water heaters.
High Pressure Types
For the consumer the variety of choices to be made may be confusing. The first question is whether the homeowner is happy to have the complete solar water heating system sitting on their roof. This will include the tank and the solar collector, frame and accessories. If the answer is "NO" a split system will be required, where the tank is inside the home (generally in the roof) and only the solar collector is on the roof. Aesthetically this may be more attractive, and permissible by some governing bodies in developments, while also having the tank on the roof may not be.
There are numerous configurations within high pressure solar water heating systems. Indirect and direct, split and integral, pumped or thermo-syphon are the main ones, but unless one is interested in the inner workings of an engine, their relevance is not that important. The important questions, other than cost and efficiency, is whether the system has electrical back up (most do), and whether the system can operate in a non-coastal area, and is therefore freeze resistant.