Abstract: A hot water boiler, e.g. for central heating, has two castings which contain water passages and are connected together to enclose a combustion chamber and above it a flue. From the flue, the burned gases pass downwardly through passages outside the castings into which ribs on the castings project. In order to improve thermal efficiency by achieving considerable condensation of water vapor on the burned gases, the castings are located on a sump into which the downward passages for burned gas open and which has means for removing condensate collecting in it. A blower is provided to extract the burned gas from the sump.