Abstract: A boiler for hot-water heating which is of rectangular cross section in vertical and horizontal planes and which comprises only two horizontal headers located on diagonally opposite sides of the fire chamber and connected by a first array of spaced apart parallel bent tubes forming the roof and one longitudinal side wall of the boiler chamber and with a second array of spaced apart bent tubes forming the other side wall and floor of the chamber. Additional tubes can be provided along the front and rear walls and slotted openings toward the rear of the boiler form passages traversed by the exhaust gas. The water return is connected to the lower header while the hot water supply pipe is connected to the upper header.