PURPOSE: To reduce the fluctuation of the level of water in a condensed water head tank and prevent excessive flow amount of water returning into a hot well by a method wherein the operation of a clean-up motor valve is controlled by inputting the signal of flow amount of the system or the level of water in the head tank.

CONSTITUTION: The water level LHH of the head tank is a level whereat a difference between the flow amount of the system and the passing flow amount of the clean-up motor valve is corresponding to a predetermined value W0. When the water level of the head tank becomes higher than the level LHH, the signal becomes "1" and clean-up motor valve opening approval signal 59 is outputted through an OR circuit 70 and a signal hold circuit 71. On the other hand, the hold circuit 71 is effective only when a hold excluding signal 76 is "1" while the holding thereof is released and the signal 59 becomes "0" or the opening operation of the clean- up motor valve is stopped when the signal 76 becomes "0". When a signal 73 becomes "1" under a condition that one set of condensed water purifying pump is operated by the circuit 51 and the opening degree of the clean-up motor valve is less than a predetermined value l1, the hold excluding signal 76 is approved and the hold is permitted. When two sets of condensed water purifying pumps are operated by the circuit 54 and the opening degree of the clean-up valve is less than another predetermined value l2, the hold may also be permitted.