PURPOSE: To prevent the capacity reduction in a warm water circuit for a bath at the simultaneous use time of space heating and the bath by a method wherein the discharge pipe at the outlet side of a reheat exchanger and the return pipe at the inlet side of a hot water storage tank are connected and a variable by-pass circuit where a water valve is controlled variably by the circulating warm water quantity to a space heating load is equipped.

CONSTITUTION: Upstream of the radiators 5 of one boiler with two water paths device, a variable by-pass circuit 18 connecting a discharge pipe 15 with o return pipe 16 is installed and on the way a water valve 14 controlled variably by the circulating water quantity to the radiators 5 is installed. By this constitution, the flow quantity for space heating outside the radiators is zero (even a set of radiator is not operated), the opening at the circuit 18 is fully opened and the necessary circulating water quantity corresponding to the capacity of the warm water circuit B for a bath flows in the flow path of a reheat exchanger 9-the circuit 18. When the flow quantity outside the radiators is much (three radiators are used), the opening at the circuit 18 is brought in a closing direction and the flow path is changed to the exchanger 9-the radiators 5, however, at the reheat exchanger more water quantity than the necessary circulating water quantity to the circuit B flows and the capacity of the warm water circuit for the bath is secured.