The device, making a heat-exchange smooth and controlling the inlet flow of the first combustion air according to the temperature of exhaust gas for improving thermal efficiency, comprises: the second heat exchanger (21A) set on a external heat exchanger (21), a heat exchange pipe (25) for offering warm water to the second heat exchanger (21A), an internal heat exchanger connected to the external heat exchanger (21), a combustion residue drain pipe connected between combustion chamber (12) and the lower part of the internal exchanger (22), and a exhaust blowing motor (32) controlling the quantity of exhaust gas according to the pressure of the combustion chamber (12).
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