PROBLEM TO BE SOLVED: To solve the problem that, for example, about two hours are required for raising 2 deg.C controlling temperature when the temperature of a circulating water which becomes the heat source at the starting time of heating is lowered to about 20 deg.C near to room temperature although the room temperature is required to be raised to the controlling temperature at the heating time at early morning when heating the greenhouse for protected horticulture, and utilizing the heat source water for heating, and the heating time is required to be shorten because the shorter the heating time, the better. SOLUTION: This method comprises starting warming up as dummy run of heating at the time tL×A hours before the starting time ts of the heating in the early morning, and controlling the three-way valve so that the temperature of the circulating water of the heat source may be raised to about 40 deg.C which is a warming up temperature of the water.
