Combi boilers are designed to heat water up instantly for domestic as well as provide heat into the central heating system. Upon a signal from the room thermostat for the boiler to fire up the central heating system. The water starts flowing passing through a narrow gap in the pipe creating a pressure difference and causes the gas valve to open. Gas is ignited in the main burner. The heat exchanger holds a limited amount of water, which is rapidly heated up and circulated around the boiler through the heat exchanger. When the water temperature reaches 60c the thermostatic element expands, the hot water system valve closes, the heating system valve opens, which allows water to flow to the central heating system.