The programmer itself doesn't heat water. It is connected to your heating system via a cylinder stat and then the 2 port valve or mid position valve depending on what type of system you have fitted.
When there is a demand for hot water the cylinder stat contact makes and sends a voltage to the motorised valve. This valve will move to the hot water position. By doing this it sends a voltage to the boiler/pump and they run . The boiler heats your hot water and the pump pumps it around your system.
If you have a combi boiler the wiring is similar, however when you open the hot tap, the flow of water is sensed by a flow switch within the boiler and it is the movement of water that causes the boiler to ignite and heat the water
ufo8mydogs, April 2009
