Abstract: A cast metal heat exchanger (e.g. of iron or aluminum) for a hot water boiler having a plurality of flueways and waterways and which is rectangular in plan view down onto the flueways with the waterways extending parallel to the minor rectangular axis. A method of manufacturing a cast metal waterway section of cuboid volume less than 4500 mls for a heat exchanger (which comprises casting the metal into a mould with the waterway core vertically disposed. A heat exchanger comprising a plurality of such sections can be formed integrally by vertically casting such a plurality simultaneously in one mould.