All modern systems of plumbing include a hot-water supply to the various sinks, bowls, bathtubs and laundry-tubs throughout the house.

Fig. 13 shows the usual arrangement of a kitchen boiler and water-back with the necessary pipe connections. The boiler is commonly made of copper and supported upon a cast-iron base. It may be located in the kitchen near the range, or may be concealed in a nearby closet. The "water-back," so called, is a special casting placed so as to form one side of the fire box in the range. The cold-water supply pipe to the boiler usually enters at the top and is carried down to a point near the bottom, as shown by the dotted lines. Connection is made between the bottom of the boiler and the lower chamber of the water-back. The upper chamber is connected at a point about one-third of the way up in the side of the boiler, as shown. The circulation of water through the boiler and supply pipes is the same as already described for hot-water-heating systems. The range fire in contact with the water-back heats the water within it, which causes it to rise through the pipe connected with the upper chamber and flow into the boiler or tank; in the meantime cooler water flows in at the lower connection to take its place, and the circulation thus set up is constant as long as there is a fire in the range.

Fig. 10.

Fig. 11.

Fig. 12.

Fig. 13.

The "boiler," so called, is not a heater, but only a storage tank. As the water becomes heated it rises to the top of the tank and is carried to the different fixtures in the building through a pipe or pipes connected at this point. The cold-water supply pipe is connected with the house tank so that the pressure in the boiler is that due to the height of the tank above it. When any of the hot-water faucets are open, the pressure of the cold water in the supply pipe forces out the hot water at the top of the boilers and rushes in to take its place. There is no connection between the circulation through the water-back and the pressure in the cold-water supply pipe. The circulation is due only to the difference in temperature between the water in the pipe leading from the top of the water-back and the water in the lower part of the boiler, and difference in elevation of the connections with the boiler. The nearer the top of the boiler the discharge from the water-back is connected, the more rapid will be the circulation and the greater the quantity of water which will be heated in a given time. The cold-water supply simply furnishes a pressure to force the hot water through the pipes to the different fixtures, and replaces any water that is drawn from the boiler.

Care should always be taken to have the pipes between the water back and the boiler free from sediment or any other obstruction. If the water-back from any cause should become shut off from the boiler, an explosion would be likely to occur if there was a hot fire in the range. Freezing of the pipes is sometimes a cause of accident. The sediment which accumulates more or, less rapidly should be regularly blown off through the blow-off cock provided for this purpose at the bottom of the boiler. The best time for doing this is in the morning, before the fire is started. The device shown in Fig. 14 is intended to prevent the sediment from collecting in the pipes or from being drawn into the water-back, making the water roily when a large amount is drawn off at one time. It consists of a small cylinder or chamber connected to the bottom of the boiler in such a way that the sediment will fall into it and not be disturbed by the circulation of the water through the pipes.

Fig. 14.