PURPOSE: To facilitate the air purge at the time of a test run by stopping the circulation pump so as to prevent its racing without pumping and filling the expansion tank with water when the water level in the expansion dropped, and stopping the water supply and starting the circulation pump when the water level became high.

CONSTITUTION: Upon turning on the switch 11, the water filler valve 5 opens to fill the expansion tank 1 with water if the expansion tank 1 is empty and the low water level sensor 8 is off. As the water level rises high enough to turn on the high water level sensor 6, the water filler valve 5 is closed to stop the water supply, and, at the same time, the circulation pump 2 is started to purge the air from the circulation lines. The water level in the expansion tank 1 then drops significantly enough to turn off the low water level sensor 8, whereupon the circulation pump 2 is stopped and the water filler valve 5 opened to fill in the water. By repeating such process, the air is purged from the circulation lines. By turning on the switch 11 this way, the water supply and air purge are repeated to automatically complete the air purge from the circulation lines.