PURPOSE: To permit to keep the blow-off temperature of the heating machine at the same temperature by a method wherein the flow amount of hot-water is changed by a temperature sensor with the gradient of change same as the gradient of the change of an airflow amount caused by a fan by following it.

CONSTITUTION: The fan 6 is controlled steplessly by the temperature sensor 1 through an amplifier 2 and a gate control circuit 3 within the ventilating capacity thereof. On the other hand, a proportional control valve, changing the flow amount of hot-water, is provided on the way of the pipeline for the warm-air heating machine and the flow amount of the hot-water is changed with the substantially same gradient of the change of the airflow amount of the fan 6 by following it. When a set value A is set in the temperature sensor 1 and the heating machine is operated from a low room temperature under this condition, the fan 6 is operated in the capacity of 100% and the ventilating amount thereof is changed as shown by a dotted line in the diagram. The hot- water flows through the proportional control valve under the flow amount of 100% like as the fan and it is changed as shown by a full line in the diagram. When the room temperature is increased, the airflow amount of the fan 6 is lowered, the flow amount of the proportional control valve is decreased at the same time with the same gradient as the fan 6 and the flow amount is choked.