Abstract: A hot water boiler, in particular a light metal central heating boiler, having a combustion chamber and a burned gases flue above the chamber, both the chamber and the flue having a common separating wall with a common water passage, the lower part of the flue, joining the upper part of the combustion chamber, being provided with extensions from the wall for increasing the heat exchange surface thereof and providing a first heat exchanger for the burned gases, the upper part of the flue being provided with inclined ribs extending from the wall to increase the heat exchange surface thereof and forming a second, condensing heat exchanger, the ribs forming condensate collectors cooperating with condensate drains.