The gas water heater is very similar in construction to the electric water heater. Here instead of electric supply, there is a gas supply. Instead of heating elements, there is a burner and burner control. The burner is used for heating the water in gas water heaters.The working principle is based on law of convection. The cold water always remains at the bottom as it is denser than the hot water. As the water gets heated up it expands and moves to the top of the tank. If you look at the construction, we can see that the outlet tube is always smaller than the dip tube (inlet tube). This is to ensure that cold water is not supplied and only hot water is supplied through the tube. There is also a flue exhaust in gas type water heater which acts as heat exchanger as well as is used in exhausting the combustion gasses.