Cold water runs through the combi boilers directly from the main water source. In order to produce heat, the gas fuel is burned. The heat is then transformed from the combustion chamber to the water through a heat exchanger.
Your thermostat would know when the temperatures drop in your house. Subsequently, it will send a signal to the combi boiler which would fire up and heat your home.
The minute the required heat is reached, your combi boiler would decrease the produced heat and keep the status quo. As a result, it will reduce your energy waste as well as keep your house warm.
A Combi boiler directs hot water to any outlet you are using such as a hot tap, a shower or a bath. When you turn off the tap, the central heating duties will be back in action.
Keep in mind that your Combi boiler directs hot water to the water outlets or the central heating, but not both at the same time.