The invention relates to a condensing boiler (1) comprising a heating body (2) with a central combustion chamber (5) with a burner (4) and surrounded by a heat exchanger (6) consisting of a volume peripheral housing a double row of tubes (11, 12) for discharging combustion gases constituting the primary of the heat exchanger (6), tubing (11) of a first inner row communicating with the combustion chamber (5) and with tubing (12) of a second outer row connected to an exhaust stack (7), at least one outlet duct (10) and at least one return conduit (8) of coolant opening in the upper part and lower part of the peripheral volume and constituting the secondary of the heat exchanger (6).; This furnace is characterized in that it comprises an additional duct (9) of heat transfer fluid return opening between the two rows of tubes (11, 12) and in the height of the peripheral volume, or between the conduit (s) starting (10) and the conduit or the (s) return (8).
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