Condensing boiler for a hybrid plant comprising two heat sources, namely said boiler and a heat pump, wherein the boiler is provided with a burner able to generate hot flue gas, a bundle of exchanging plates, an inlet and an outlet for technical liquid for a room heating apparatus of the hybrid plant, wherein the boiler is provided with a head, a bottom and a shell adapted to define an internal volume suitable for accumulating technical liquid, wherein on the head there are provided a connection for the burner, a flue gas outlet and control and actuation means of the boiler which also provides on the bottom a condensate drain, characterized in that the boiler further provides on the bottom a safety valve, and a separation frame is provided in the internal volume of the boiler, arranged to divide said internal volume into a first chamber and a second chamber, wherein the first chamber contains the bundle of exchanging plates which develop vertically like the separation frame, while the second chamber contains a flue gas chimney, wherein the separation frame is provided with, at the bottom, slots to make a liquid connection between the two chambers (51, 52) where the same technical liquid is therefore contained, wherein the exchanging plates, which are arranged to receive the hot flue gas generated by the burner, are in gaseous connection with the flue gas chimney through a lower chamber that is liquid-tight on the bottom of the boiler, wherein the boiler is suitable for allowing the circulation of technical liquid in the internal volume, in counter-current with respect to the circulation of the hot flue gas to be carried out by gaseous circulation means, from the inlet to the outlet, along a path that is provided with a descent along the second chamber, the passage through the slots and the ascent along the first chamber in contact with the exchanging plates. wherein the boiler is provided with a head, a bottom and a shell adapted to define an internal volume suitable for accumulating technical liquid, wherein on the head there are provided a connection for the burner, a flue gas outlet and control and actuation means of the boiler which also provides on the bottom a condensate drain, characterized in that the boiler further provides on the bottom a safety valve, and a separation frame is provided in the internal volume of the boiler, arranged to divide said internal volume into a first chamber and a second chamber, wherein the first chamber contains the bundle of exchanging plates which develop vertically like the separation frame, while the second chamber contains a flue gas chimney, wherein the separation frame is provided with, at the bottom, slots to make a liquid connection between the two chambers (51, 52) where the same technical liquid is therefore contained, wherein the exchanging plates, which are arranged to receive the hot flue gas generated by the burner, are in gaseous connection with the flue gas chimney through a lower chamber that is liquid-tight on the bottom of the boiler, wherein the boiler is suitable for allowing the circulation of technical liquid in the internal volume, in counter-current with respect to the circulation of the hot flue gas to be carried out by gaseous circulation means, from the inlet to the outlet, along a path that is provided with a descent along the second chamber, the passage through the slots and the ascent along the first chamber in contact with the exchanging plates.