Instantaneous gas hot water heaters or continuous flow gas hot water systems work by heating the incoming cold water using a large gas flame. This simple process allows them to provide instant hot water without the need for a hot water storage tank.
The basic configuration of a continuous flow hot water heater is a copper pipe coil wrapped around a large gas flame. This is known as a heat exchanger. The cold water comes in through the inlet pipe, travels around the heat exchanger gaining heat as its passing around the coil, then exits on the hot outlet side. This style of heating provides endless hot water for the heaters specified flow rate (i.e. 16 litres per minute).