PURPOSE: To prevent hunting in the switching of the number of units by a method wherein the amount of hot water delivery of water heaters other than a water heater starting operation last is set below a decrement prescribed flow rate and the water heater starting operation last is stopped when the amount of hot water delivery of the water heater starting last operation meets requirements to approach the minimum number or the minimums necessary amount of passing water.

CONSTITUTION: The amount of passing water of a shell body of a water heater 2 starting the delivery of hot water preceding a water heater 3 by one lowers below a decrement prescribed flow rate smaller sufficiently than an increment prescribed flow rate of increment conditions and when the amount of hot water to be delivered from the water heater 3 meets requirements to approach the minimum number or the minimum necessary amount of passing water, a hot water delivery solenoid valve 93 and the water quantity adjusting valve 82 with an overspillage preventing function are closed. Then, in the water quantity adjusting valve 82 with an overspillage preventing function of the hot water heater 2, the opening thereof is adjusted from an opening for preventing the overspillage so that the amount of passing water of the shell body of the water heater 1 is K times as much as a value preset of a set flow rate of the shell body. This can eliminate fear of causing a hunting in the switching of the number of units to be switched over to a decrement operation when an incremental operation is to be made.