WATER HEATER

PURPOSE: To easily connect even tubes having different bores by forming the connector of a water heating circuit or a circulating circuit by a connection port in which inner and outer surfaces are threaded. CONSTITUTION: When a circulating circuit 12 is connected to a connection port 19, a cap nut 35 loosely engaged with the end of the circuit 12 is engaged with the outer threaded part 21 of the port 19, and then the circuit 12 is easily connected to the port 19 via a packing 37. Similarly, a water heating circuit is also connected to the port. Accordingly, tubes having different bores can be connected to the one port 19 to be extremely convenient. Particularly, even when a gas water heater is replaced by a petroleum water heater being purchased or vice versa, the conventional tubes can be used as they are, and no waste occurs, and handling convenience is improved. Two tubes having different bores can be dealt with the one port without necessity of differentiating the sizes of the ports of respective implements, and the execution can be easily realized at the site.