PURPOSE: To prevent the inflow of atmospheric air from a safety valve by charging a soluble substance into the mounting pipe of the safety valve in the hot water boiler which heats water by high-temperature steam generated through the heating of a thermal transfer medium liquid enclosed in a hermetically sealed vessel.

CONSTITUTION: In the hot water boiler in which a thermal transfer medium 2 generating condensable steam through the heating of a liquid such as water is enclosed into the hermetically sealed vessel 1 and water in a heat exchange pipe 3 is heated by the high-temperature steam generated in the vessel 1, the soluble substance (a dissolving plug) 10 is charged into the mounting pipe 8 of the safety valve 9. A soldnoid valve 7 is opened and a heater 6 is operated when a noncondensable gas is collected in the vessel 1, an exhaust valve 5 is opened when pressure in the vessel 1 reaches atmospheric pressure or higher, and said gas in the vessel 1 is discharged while set time is continued by means of a timer. The exhaust valve 5 is closed under the working of the heater 6 when pressure in the vessel 1 reaches set value or lower, the valve 5 is opened when pressure rises again, but the inflow of atmospheric air from the safety valve 9 is prevented by the soluble substance 10 during that time.