To prevent local boiling of hot water in a piping at the interior of a heat- exchanger for hot water supply during re-combustion or individual combustion by a method wherein a fuel supply chamber extending from an auxiliary nozzle burner port to a nozzle group is arranged independently from a plurality of fuel supply chambers extending from the main flame burner port to the nozzle group.

A manifold 4 comprises a first fuel feed chamber 42; and an independent second fuel supply chamber 44. A hole 46 to feed fuel to a main flame burner port and a small hole 1 to feed fuel to an auxiliary flame burner port are formed in a fuel supply chamber 44, a hole 48 to feed fuel to the auxiliary flame burner port is formed. Air-fuel mixture having surplus air is fed to a main flame burner port, air-fuel mixture having surplus fuel is fed to an auxiliary flame burner port is fed to an auxiliary flame burner port, and thick and thin fuel concentration combustion is performed. Solenoid valves 21, 23, 25, 27, and 20 are switched, and by regulating the opening of a proportional valve 28, proportional combustion control is effected. During re-combustion or individual combustion, the solenoid valves 21, 23, 25, and 27 are closed, only the solenoid valve 20 is opened, and combustion is performed only by the auxiliary flame burner port of all-burner.