In the overhead system, as in the system of Fig. 313, the main flow pipe runs as directly as possible to the top floor, and then is run horizontally, drop risers being taken out wherever required, these pipes entering a main return in the basement, which carries the return water back to the boiler, the drop risers thus doing the work of flow and return. This system probably provides more evenly heated water than most other systems of supply.

A third hot-water supply system for large buildings is shown in Fig. 315, this system differing considerably from those already shown. The main flow pipe is carried from the boiler to such points as vertical risers are to be supplied, the main pitching upward from the boiler. These risers should connect at the top into a main horizontal line, to which a main vertical return connects, through which the water is carried back to the boiler. An automatic air valve should be placed at the highest point on the piping, to relieve the system of air-lock. On each vertical line, as shown, there should be a valve, by means of which the circulation of hot water may be equalized, in order to prevent different parts of the system from taking too large a share of hot water, to the detriment of other parts of the system.

Fig. 314.   Overhead System of Hot Water Supply for Office or Apartment Building.

Fig. 314. - Overhead System of Hot Water Supply for Office or Apartment Building.

Fig. 315.   Hot Water Supply for Office or Apartment Building.

Fig. 315. - Hot-Water Supply for Office or Apartment Building.

Fig. 316.   Section of Double Boiler.

Fig. 316. - Section of Double Boiler.

In selecting the proper system of hot-water supply for a large building, as much judgment is needed as in the selection of the proper system of heating.

While one system under certain circumstances and conditions will perforin excellent service, another will fail entirely. If the plumber is well grounded in the principles of circulation, and a man of practical experience, he will usually be able to determine what system of supply under the given conditions will give the best results.

Fig. 317.   Hot Water Supply from Double Boiler.

Fig. 317. - Hot Water Supply from Double Boiler.

The double boiler is a device which finds an important application in the hot-water supply systems of many high buildings in the large cities. It is very seldom used, however, in any but the largest cities.

In Fig. 316 is shown a sectional view of the double boiler, from which it will be seen that it consists of two boilers, one inside the other. The outer boiler is heated in the usual manner, and the inner boiler is heated by contact with the heated water of the outer boiler. The outer boiler is under direct pressure, and the inner boiler under tank pressure. It will be readily considered that in high buildings, running up many stories, the upper floors are sometimes above the height at which water under city pressure can reach. Even though this condition is not permanent, it often happens that during certain times of day the pressure which at other times is sufficient is so reduced that it cannot force water to the higher floors. It is under these conditions that the double boiler is made use of, as shown in Fig. 317.

Fig. 318.   Simple Form of Cut off.

Fig. 318. - Simple Form of Cut-off.

The inner boiler supplies hot water to the upper floors under tank pressure, and the outer boiler supplies the lower floors under street pressure.

The attic storage tank must generally be supplied by means of a pump or water lift.

Fig. 319.   Automatic Cut off.

Fig. 319. - Automatic Cut-off.

The cross connection between the two boilers allows both to be fed with street pressure when the latter is high enough to reach the upper floors. At night when the pressure is high, this course may be adopted, thus saving the expense of pumping to the tank.

In Fig. 318 is shown a simple form of cut-off, a device often employed on double boiler work for the purpose of delivering into the distributing pipes a supply of water under either tank pressure or direct pressure, as the case may be.

The successful operation of this cut-off is entirely dependent on the attention and caution of the attendant in the opening and closing of the proper valves, and the occasional lack of care has resulted in casting this device away for automatic cut-offs, such as the one of Fig. 319.

In the latter, each of the four valves is rigidly attached to a system of levers, operated by a handle, the throwing of which either opens or closes the proper valves, thus obviating any mistakes.

The same work that is accomplished by the double boiler may be performed by two single boilers connected in the proper manner, as seen in Fig. 320.

In order to run the two boilers under different pressures the water front of each must be entirely separate from that of the other. Separate coils in the same heater may be used for this purpose. Special heaters having two separate heating surfaces may also be used. The general connections for this system are quite similar to those of Fig. 317. One boiler supplies the upper floors, and is therefore under tank pressure, while the other supplies the lower floors under street pressure.

It may be stated that double-boiler work is taken up by the author in his work entitled "Modern Plumbing Illustrated."

A very common difficulty that is encountered in many sections of the country, especially in the West, is the accumulation of lime in the range connections and piping. This trouble is serious enough in connection with the procuring of hot-water supplies for residences and dwellings, but in the case of larger work, such as public or semipublic buildings, its existence results in much greater annoyance.

In some localities the only natural supply of water that can be obtained is hard water, strongly impregnated with lime, which it takes up in its passage through the soil. The depositing of lime takes place principally in hot-water pipes, although it also occurs in the cold-water pipes. When the lime has once begun to accumulate on the interior of a pipe, the accumulation increases rapidly, and often in a comparatively short time the pipe will become entirely filled. This condition is not only a source of much annoyance, but a source also of danger.

Many attempts have been made to clear pipes thus filled with various substances, but even though sometimes partially successful, such a cure does not produce permanent results. It is usually more satisfactory to replace the filled pipe with new pipe rather than to attempt to clean out the lime deposit.

Fig. 320.   Two Single Boilers Doing Work of Double Boiler.

Fig. 320. - Two Single Boilers Doing Work of Double Boiler.

Acids act on such deposits, as also caustic soda, but they constitute a harsh remedy, capable of doing harm as well as effecting the desired remedy.

Special patented devices for attachment to the range connection may be procured, which are claimed to prevent the choking of the pipes with lime.

It would seem, however, that these various devices and recipes were not along the proper lines, and that a much better plan would be to prevent the entrance of the impregnated water into the range connection, where most of the trouble from accumulation of lime occurs. Fig. 321 illustrates a method of this nature. According to this plan, two attic tanks are used, a small tank for rain water and larger tank for the hard water of the natural supply. It will be noted that no range boiler is used.

A down supply line from the rain-water tank is connected directly to the water front, the flow pipe from which is carried up to the hard-water tank and connected to a heating coil located in the latter. After passing through this coil, the water passes back to the rain-water tank. This constitutes a complete system of circulation for the heating of the contents of the hard-water tank by means of the coil, the work being done by water free from lime, and thus avoiding the deposit of lime in the range connections. From the hard-water storage tank a line of hot-water supply is taken down to the various fixtures. It is an excellent plan to place blocks of limestone on the heating coil in the hard-water storage tank, as the lime in the water will deposit on such blocks in preference to the piping or sides of the tank. The use of these blocks will therefore aid greatly in keeping the tank and piping free of lime. If limestone is not at hand, oyster or clam shells will do the work. Shells placed in a tea kettle in which hard water is used will prevent the accumulation of lime inside.

While the method just described is the most effective one that has yet been suggested, there are two objections connected with its use that should be mentioned. In the first place, if the hard-water storage tank is open it cannot be made to heat to so high a temperature as the range boiler,' owing to the escape of heat from the exposed surface. In the second place, more or less steam must inevitably escape into the air, and in many cases this feature would be objectionable. Without doubt, however, the tank could in most cases be heated to a sufficiently high temperature for all ordinary purposes.

Fig. 321.   Method of Preventing Deposit of Lime in Range Connections.

Fig. 321. - Method of Preventing Deposit of Lime in Range Connections.

There is no reason, however, why this tank cannot be sealed over, with a vent taken through the roof, which would in a large measure free it of its objectionable features.

Another method of preventing lime deposit is to use a range boiler having a coil, the water front being connected direct to the coil, but not otherwise connected to the boiler. The water front should be supplied with rain water, and from the high point on the flow pipe an expansion pipe should be carried to the tank.