This is a water pressure control valve fitted to the cold water supply to the geyser.
The purpose of the pressure control valve is to balance the water pressure of the hot and cold water supply to baths, showers and basins.
For a balanced system the cold water supply to baths, basins, showers etc must be taken off the supply between the pressure control valve and the geyser.
The pressure control valve reduces the cold water pressure to the same pressure of the hot water coming from the geyser.
Sometimes plumbers will locate the pressure control valve outside of the house where the cold water supply enters the house.
Mostly however the multi-valve will be found in close proximity to the geyser.
The pressure control of the multi-valve is designed to release excess pressure and may therefore drip when water is drawn through the valve.
Each geyser must also be fitted with a pressure control valve.
As most geysers have a maximum working pressure it is vital that the pressure is controlled in order for the geyser to have a longer life span.
If no pressure control valve has been fitted then the geyser guarantee will likely not be honoured.R394