PURPOSE: To facilitate and expedite a complicated operation with high difficulty by installing a hot water space heater, and then automating a water level retention operation by air venting to a hot water circulation passage only by an operation of an air vent switch.

CONSTITUTION: An automatic water level retention controller 34 which receives an ON operation signal of an air vent switch 32 controls to open or close a water supply control valve 12, etc., of a parallel circulation passage 9, etc., having a radiator 15, etc., by a registering order by connection to a connecting terminal strip 36. A variation in a water flow after the air vent is detected by a water flow detector 30, and an end of the air vent is decided by an air vent deciding unit 35. When the air vent of the water supply control valve 39 of a last registration number is ended, it is informed by an air vent end display unit 33.