To provide bath connection auxiliary piping that reduces the amount of temperature-uncontrolled water output by a water heater in current water boiling since temperature-controlled hot water is supplied from the water heater from the time hot water supply is started with respect to an instrument which supplies hot water of the water heater such as a bath connection adapter and a hot water supplier.

Piping 13 just near a bath connection adapter 4 is provided with three-way valve 7, 8 on both a bathtub return side and a bathtub feed side, and a setting of a return to a bath and a setting of a shortcut bypassing the two three-way valves are prepared. While the temperature of hot water in the piping is measured, hot water returning from a water heater 3 is not sent to a bathroom 1, but returned to the water heater from the three-way valve by making a shortcut to the three-way valve of the bathtub feed side piping while the temperature is low. Once a rise in temperature of hot water to be supplied is detected, the hot water is fed to the bathtub.