This type of diverter valve is used by many different manufacturers and is produced by Berretta and Inter Albion in Italy. When the domestic hot water tap is opened a differential pressure is created and acts on the diaphragm. Once the differential pressure has been set up the diaphragm flexes and causes the central spindle to lift and thereby closes the port within the valve which feeds the central heating system circuit. At the same time the spindle rises through the top of the diverter valve and operates a double microswitch, which informs the boiler to operate in domestic hot water mode.
This type of diverter is used on early Vailant boilers and on the Potterton Puma. The diverter is operated on temperature, when a hot water tap is opened the 80% of the wax thermostat situated in the cold mains supply causes the wax thermostat to contract. The diverter valve drop causing the closing the central heating ports and allowing the hot water with the boiler to be directed into the calorifier where it is used to indirectly heat the domestic hot water.
This type of diverter valve uses the Venturi principle, similar to that of the instantaneous water heater. Once the hot water tap is opened the resultant flow through the Venturi creates a pressure difference to allow the diaphragm to lift and overcome the tension of the spring arrangement acting down on it. As the diaphragm lifts it also moves the spindle and valve to close the central heating ports and divert the hot water in the boiler to a calorifier to allow the production of domestic hot water. Once the tap is closed the tension of the spring forces the valve/spring assembly back to is original central heating position.