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 (21-25) 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 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 (21-25) 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 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 . <EOS>