This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
53. Hot water for domestic purposes is commonly heated in the cooking range by means of a waterback, or by a coil of pipe which extends around the top edge of the firebox. A waterback is a cast-iron block provided with internal passages through which water can flow. It is made by coring out the interior at the time of casting, or by casting the block around a flat coil or loop of wrought-iron pipe, which is laid in the mold. The waterback covers one side of the firebox, and is usually tapped with two holes for 1-inch wrought-iron pipe. Waterbacks are exposed to severe internal strains, and the metal must be much thicker than is required to resist the water pressure; they are designed to resist a pressure, when cold, of 700 pounds per square inch. A firebox coil for heating water in place of a waterback, is usually made of copper tube, 3/4 inch internal diameter and about 1/8 inch thick, placed on the inner edge of the firebricks.
54. Waterbacks are sometimes overheated and weakened, or are eaten away by internal corrosion, and if an extraordinary pressure is brought to bear within them, they will explode with disastrous effect. The damage which results from the explosion of a waterback greatly exceeds that from the bursting of a pipe coil, because of the greater interior area and greater volume of contents.
The capacity of a waterback is measured by its heating surface in square inches, reckoning only that side which is exposed to the fire. About 100 square inches of external heating surface are sufficient for a 40-gallon boiler, where water is plentiful, or a 50-gallon boiler, where water is scarce.
The quantity of water heated, the time required to heat it, and the quantity of water consumed, vary in every case. The size of waterback that should be placed in a range, depends upon judgment. A large boiler requires a large waterback, and a small boiler a small one. A large boiler and a small waterback means scarcity of hot water, or plenty of lukewarm water. A large waterback and a small boiler means boiling hot water, and rumbling, snapping, and cracking noises, formed by the formation of steam.