Low Pressure Solar Heated Geyser

(A Non Pressure Solar heating system is illustrated in the diagram.)

The red arrows show how the Solar Radiation from the sun is absorbed within the sealed vacuum tube. Solar Radiation is effective even under cloudy conditions as sun radiation penetrates the cloud layer 

The white arrows show how a Convection Current is established within the water  in the vacuum glass tube. The hot water then flows into the insulated tank along the top of the tube drawing the cold water from the bottom of the tank down along the bottom of the tube. This cold water is again heated by the solar radiation and the process is repeated, creating the convection current in the tank as shown.

Non pressure systems are a  gravity feed system into the house.

The house must have running water. If the solar heater is fed from a reservoir or tank this supply must be higher than the solar heater unit.