The operation of low pressure solar water heater is as follows, cold water enters the water tank, first the evacuated tubes are filled and then the water tank. When solar radiation begins to affect the tubes, it is absorbed by them in the form of heat and transmitted to water which raises its temperature. The hot water goes up to the tank and the cold water from the tank goes down to the pipes where it is also heated, and the process is repeated until all the water in the tank is at a high temperature. This process is called thermosiphon , or circulation by density difference.
This is called low pressure solar water heater, to be differentiated with other systems since it does not support very high water pressures unlike the others, low pressure solar water heater's maximum working pressure is: 0.05Mpa, and in turn is more economical due to its way of manufacturing. In contrast, high pressure solar water heater can withstand higher water pressures which there maximum working pressure is 0.6Mpa.