between the second heat exchanger 62 and the combustion chamber 66 may include in one aspect a conventional heat exchanger such as the heat exchanger 15 of the conventional gas water illustrated in fig1 or in another aspect could be another second heat exchanger as the second heat exchanger 62 as shown in fig2 to 5 in operation when the combustion chamber 66 is ignited cold water provided from an inlet 64 1 to the second heat exchanger 62 is pre heated by a gas flow 65 specifically a waste gas flow from the combustion chamber 66 the pre heated water flowing out of an outlet 64 2 of the second heat exchanger 62 is fed into the first heat exchanger 61 and then heated in the first heat exchanger 61 consequently hot water is provided from an outlet 64 3 of the first heat exchanger 61 the thermal efficiency of the gas water heater 60 is improved as compared to the conventional gas water heater illustrated in fig1 because the gas flow 65 is applied to the second heat exchanger 62 in addition to the first heat exchanger 61 resulting in less waste of heat energy the thermal efficiency of the gas water heater 60 may reach up.