PURPOSE: To shorten the rising time of a heating temperature effectively by a method wherein the feeding amount of a fuel to a boiler at the time of initiation of heating is increased and the boiler is controled to have a maximum hot water temperature based on the temperature of circulating hot water for the floor heating.

CONSTITUTION: After a power source switch 18 of a controler 3 is turned ON, a burner 4a of the boiler 1 is ignited through an ignition timer 17 so as to heat the boiler 1 up to the maximum hot water temperature quickly with the aid of a combustion control circuit 11 while the hot water is circulated through the boiler by a circulation pump 6 so that the rising time of the heating temperature is shortened. Next, when a detecting element 8 detects that the hot water temperature reaches the maximum temperaure, a command signal is transmitted to a hot water temp. detecting circuit 10 through a feedback from a memory circuit 12 resulting from the output of the detecting circuit 10 while an AND signal due to the feedback is inputted to an AND circuit so that the amount of fuel supplied to the burner 4a is reduced and the combustion of the burner 4a is thereafter so controled as to keep the hot water temperature in a suitable range.