Combi boilers provide hot water without a hot water cylinder or a cold-water tank. This makes it an efficient system as it reduces the amount of wasted energy in comparison with traditional models that require a hot water tank. It takes water from the mains and heats it up as required and is on standby when water is not in demand. To produce hot water, cold water from the mains is circulated over a highly-efficient heat exchanger which transfers the heat to the cold water from the burnt gas and delivers the water to the taps as required. Since hot water is produced as needed, it is not necessary to turn on the heating when the water is not running.