Abstract: As an article of manufacture, a boiler for a heating system, a boiler-heating system combination, and a method for heating a heat-transfer medium such as water in a heating system; combustion gases are brought into heat-transfer contact with the heat-transfer medium over a large specific, heat-transfer surface; the heat-transfer medium flows convectionally in the opposite direction to the combustion gases and the gases are caused to flow downwardly and are thereby cooled to a temperature which is lower than the temperature at the outlet for the medium heated by the gases before the gases are discharged to atmosphere; the boiler is fired by oil and/or gas and the combustion chamber (16) is arranged immediately above one end of the smoke-pipes (22). The other end of the smoke-pipes (22) discharge into a collecting chamber (24) provided with an outlet (28) for the combustion gases. The collecting chamber (24) is provided with an outlet (34) for condensation from the combustion gases.