PURPOSE: To prevent some dusts containing compound of low melting point from being adhered to a seat surface of the hot gas valve by a method wherein a hot gas valve for a hot air furnace for refuse or dust melting furnace is opened or closed under such a condition as having scarce existence of gas flow.

CONSTITUTION: Hot air furnace 7 for a melting furnace 1 is provided with a hot air gas valve 15, a waste gas chimney valve 16, a cold air transporting valve 17 and a pre-heated hot air valve 18. Heating of air and air blowing are changed over under a specified order or sequence so as to close or shut off the gas or air. In case that the operation is changed from the air blowing to the air heating, at first the valve 17 is closed, then the valve 18 is kept under its closed condition, the valve 16 is released for a little while to remove air therefrom, the valve 15 is gradually opened while the valve 16 is kept closed, and the valve 16 is gradually opened while the valve 15 is fully opened. When the valve 15 is closed, the valve 16 is gradually closed and the valve 15 is gradually closed while the valve 16 is fully closed. In case that the heating is changed over to air blowing operation, the valve 16 is fully closed, thereafter the valves 17 and 18 are gradually opened. Under this above- mentioned operation, the valve 15 shows scarce pressure difference between the upstream and downstream thereof.