The principles governing hot-water supply are naturally the same, whether it he in relation to small residence work or to the more extensive systems of large buildings.

In the latter work there is not only a much larger quantity of water to heat and distribute to various parts of the building, but, owing to the fact that there are often in these buildings hundreds of people making use of hot water, the perfect and continuous operation of the system becomes a necessity. This makes it necessary that the piping be run on correct principles, and that mains and branches be of sufficient capacity to properly perform their work. On small hot-water systems a serious error may not prevent the furnishing of hot water after a certain fashion, serving only as a matter of inconvenience to a few, while the same error on large work would produce great inconvenience to a very large number of people.

Fig. 309. - Connections for Horizontal Boiler and Tank Heater.

Hot water is generally furnished to large buildings by a special heater, such as shown in Fig. 309, located in the cellar or basement and connected to a large hot-water tank, which may be of the horizontal type, as in the above-mentioned illustration, or of the vertical type, as in Fig. 310. The horizontal tank hung from the cellar timbers is the more generally used of the two types, the principal reason being that it is less in the way than the vertical tank. As far as the connections between heater and tank are concerned, they are similar to those of the kitchen range and boiler, and therefore whatever has been stated concerning the latter applied to the former also.

Fig. 310. - Connections for Vertical Boiler and Tank Heater.

In addition to the use of water heaters for tank heating, live and exhaust steam are also used, in connection with steam coils inside the hot-water tank, as in Fig. 311. An advantage gained by this method is that during the winter season, when the heating plant is in operation, the tank may be heated by means of the coils without using the tank heater, while during the summer months the latter may be used.

Coils are often used to provide additional heating surface, when sufficient hot water cannot be secured by use of the tank heater alone.

The proper method of supporting horizontal boilers is shown in Fig. 312. This consists in the use of heavy, wrought-iron hangers at each end of the boiler. These hangers pass around the boiler, taking the weight of it, and being securely fastened to the cellar timbers.

Fig. 311. - Hot Water Boiler Heated by Steam Coil.

In Fig. 313 is shown a hot-water supply system for a large building, working under tank pressure. A tank system has the advantage of delivering water under a constant pressure, while the direct-pressure system varies considerably.

A varying pressure is not only an annoyance, but it is hard on the valves and piping. As explained in another chapter, the direct-pressure boiler is liable to siphonage, while the boiler working under tank pressure is not.

The main flow pipe in Fig. 313 is carried directly to the highest point, and thence horizontally to a second vertical line of fixtures, dropping down from this point and returning to the heater, thus forming a complete circuit.

It will be seen that the piping constantly rises to the high point where the expansion pipe is connected. When the water has traveled thus far from the heater it has begun to cool, and falls through the second vertical line back to the boiler, but being sufficiently hot to supply the fixtures on this line with hot water. The rising of the hot water and falling of the cooler water continues without interruption so long as the water is being heated, and constitutes what is known as circulation.

Fig. 312. - Supporting of Horizontal Boiler.

The expansion pipe from the high point is a necessity on the tank system of supply, not only to vent the system in case the water becomes overheated, but to relieve it of air, thus preventing air-lock, which is often the cause of the inability of the hot-water system to work properly. The attic tank is generally supplied by direct pressure. When the direct pressure is not sufficiently high in the case of high buildings, the water lift is often made use of. A description of the water lift and its action, as well as descriptions of several systems on which it is used, may be found in the following chapter.

Fig. 313. - Hot Water Supply for Large Building Under Tank Pressure.

In Fig. 314 a system of hot-water supply known as the overhead system is shown, in some respects similar to the work illustrated in the preceding figure.

This system in the present case is under direct pressure. The overhead supply system is similar in principle to the overhead heating system, and wherever it is possible to install it, may be depended upon to give fully as good results as any other system.

When such a system is under direct pressure, the expansion takes place back through the supply pipe to the boiler and thence through the street mains to the source of supply. Owing to this fact, it is not generally necessary to run an expansion pipe if it were not for venting the system of air. Such a pipe, as seen in Fig. 314, is generally connected to an expansion tank, and an overflow from the tank onto the roof.