The combi boiler takes water directly from the mains supply. When you turn on the hot tap, the gas burner inside the boiler quickly heats up the water, delivering a very fast hot water supply to your rooms in your home. This approach of heating water is just one of the great features of the combi boiler. They also have a small area of water that they permanently keep heating, ready to distribute to your taps and showers. All this is based around belivery hot water at a very quick rate to your home.