Abstract: A boiler for heating a heat-transfer medium, such as water in a domestic heating system with water-carried heat. The boiler comprises a tank (2) for the radiator water; a combustion chamber (3); combustion-gas pipes (4) which extend down through the tank and which discharge at the bottom thereof in a collecting chamber (5) for combustion gases. At the upper part of the tank (2) there is arranged an outlet (7) to the radiators, while a tank-inlet (8) from the radiators is arranged at the lower part of the tank. Also arranged in the tank (2) is a reservoir (17,20) for heating tap water, whereat the reservoir-inlet (10) for tap water is connected to a fresh cold-water pipe and is located at a level which lies beneath that of the tank inlet (8) from the radiators.