Abstract: A liquid or solid fuel-fired boiler for a central heating system is disclosed comprising a combustion chamber and a heat exchanger arranged therearound and connected in a heating fluid circuit comprising an upper header and a lower header interconnected by straight, parallel spaced apart tubular elements for communication therebetween. The tubular elements are disposed along the lateral sides and the back side of the boiler and are embedded in refractory material. Cylindrical interstitial spaces are formed between the tubular elements and the surrounding refractory material to prevent stresses. Flow passages between the tubular elements and the headers are of progressively decreasing cross-sectional area from the back to the front of the boiler thus improving heating fluid flow and temperature uniformity. The lower edge of a cowl defines a throttle passage with the refractory material to control the flow of flue gases to the flue duct.