PURPOSE: To replace the hot water in a hot water passage with a new one when a water temperature in the hot water passage is decreased and to keep substantially constant the temperature of the hot water in the hot water passage in a radiator unit by a method wherein a constant temperature valve is arranged at the end part of the hot water passage arranged in the radiator unit facing toward the water drain pipe.

CONSTITUTION: Within a plug cylinder 40 at a water discharging pipe 2 of a radiation unit 4 a constant temperature valve 5 is arragned. The constant temperature valve 5 has a temperature sensing valve 52 biased downwardly with a spring 51, a first valve seat 54 provided with a valve opening 53 opened below the temperature sensing valve 52 and a second valve seat 56 having a valve opening 55 to be opened above the temperature sensing valve 52. The temperature sensing valve 52 is made such that a thermo-sensitive expansion material 58 is charged in a hollow container 57 opened downwardly, and a piston rod 59 is inserted into the opening part, the thermo-sensitive expansion material 58 is expanded or retracted in response to a surrounding temperature of the thermo-sensitive valve 52 and therby the piston rod 59 is advanced or retracted from the container 57. When the water temperature is decreased, the valve opening 55 of the second valve seat 56 is opened to discharge the water in the pipe 3 and at the same time a new hot water is taken into the pipe 3 to replace with the hot water.