PURPOSE: To obtain a stable air warming of a high reliability by circulating water between a heat accumulating equipment of vessel type and a heat exchanger with a fan.
CONSTITUTION: A circulated water fed from the vessel type heat accumulating equipment 3 to a ground heat exchanger 5 via the second pipe 9 with the pump 6 and circulated to the heat accumulating equipment 3 via the first pipe 7 is heat- exchanged compulsively during the daytime by a relatively hot air within the plant house 1 and the fan 4. Thus warmed water is then stored in the heat accumulating equipment 3. During the night, the warm water stored in the equipment 3 flows therefrom through a water pipe 13 having the pump 6 to the ground heat exchanger 5 located near the ceiling of the plant house 1, in which it is heat-exchanged between the cool air within the plant house 1. Thus cooled water then returns to the equipment 3 through a return pipe 14. Thus the air warming of the plant house 1 is carried out during the night.