The subject of cold-water supply, which has already been considered, is a comparatively simple matter. On the other hand, the subject of hot-water supply involves numerous difficulties which do not enter into the former branch of work. Indeed, the proper installation of the hot-water supply system is often a most perplexing problem, one demanding a knowledge of the theory which underlies the action of hot water, and the application also of knowledge which comes by experience. In this branch of plumbing construction, more than in any other, practical experience counts for a great deal. It is the employment of a knowledge of the theory of the subject in combination with the practical knowledge that comes by experience, that produces the perfect system of hot-water supply.

Many a system of hot-water supply produces results which, to the person unacquainted with the subject, would seem to be all that is required of it, whereas, if the same system had been properly constructed, it would have been able to do better work, and a greater amount of it. A great deal of the supply work that is installed, is like all other construction work - it will answer the purpose, while at the same time it is not capable of performing its work as it should be performed. As in the case of the drainage and vent systems, the perfect operation of the hot-water supply system depends very largely on the observance of the small points which, to the uninitiated, would often appear to be of small consequence. By the observance of these small points, one workman will install a system which will produce excellent results, while another workman, careless in observing these same points, will construct a like system from which only poor or fair results are to be obtained.

The principle of circulation underlies all hot-water supply work, as well as all heating systems, whether hot water, steam, or hot air, and a thorough understanding of this principle is necessary to an understanding of either subject. For the purpose of this explanation, it is necessary to understand something of what is known as the molecular theory. According to this theory, all bodies, regardless of their composition, are made up of molecules or particles, these particles being so minute, and of such numbers, that they cannot be estimated. These particles are in constant motion or vibration, the path which each molecule traverses being so small as to be immeasurable. When heat is applied to some bodies, metals, for instance, these particles are set in more violent vibration, and the result, as is well known, is the expansion of the body. If heat continues to be applied of sufficient intensity, the vibrations of the molecules become so rapid and so violent that they refuse to hold together, and separate from the main body, that is, the body melts. Thus it will be seen that by means of the molecular theory, the action of expansion and contraction, the melting of metals and many other phenomena may be explained. Indeed, it is by means of this theory that innumerable actions and operations may be explained.

The molecular theory may be applied much more extensively than the above, but the simple statement as given, will be sufficient for the present purpose. As a means of illustration, suppose that a block of ice be subjected to heat. In its original form, the ice is a solid, compact mass. As the heat is applied, however, according to the above explanation, the particles or molecules of the ice begin to vibrate more rapidly, finally refusing to hold together longer, and the ice melts and forms water. If heat continues to be applied to the water its temperature rises and its molecules expand. The vibration of these molecules finally reaches such a stage that it is stronger than atmospheric pressure, and in their expanded state, being lighter than the air, they rise into the air in the form of steam. Thus, by the application of heat, the original block of ice has been changed to water, which is a fluid, and the water in turn has been changed to steam, which is a vapor. Each of these actions is an example of the action of circulation.

Now, when this same principle is applied to the range boiler, as shown in Fig. 268, the action is the following:

Fig. 268. - Connections for Range Boiler.

Cold water enters the water front of the range, from the boiler, and in passing through the water front is heated. When heated, as already seen, the particles of water become expanded, and therefore lighter than the colder particles, with the result that they rise, as indicated in the illustration by arrow heads.

After entering the boiler, the heated water will continue to rise to the highest possible point, or until it has become cooled.

Because of the tendency of hot water to rise, the hottest water will be found at the top of the boiler. While such is the action of hot water, a reverse action occurs in the case of cold water. The particles of cold water being unexpanded, are much heavier than those of the hot water, and their natural tendency is to fall to the lowest points. Therefore, the coldest water will be found at the bottom of the range boiler. In order, also, that this natural tendency may be favored as far as possible, the supply of cold water to the range boiler is carried through a pipe directly to the bottom of the boiler, instead of being connected to the top.

Inasmuch, also, as the storage of hot water is at the top of the boiler, rather than at the bottom, the pipe supplying fixtures with hot water should always be connected at the top of the boiler. A connection at any other point would give entirely unsatisfactory results.

The construction of a successful hot-water supply system, then, depends upon the running of pipes of proper sizes in such a manner as to provide the easiest and most natural path for the hot water. Having now seen that the hot-water supply depends upon the circulation of hot water, it will be readily understood that the same principle underlies certain other actions. For instance, hot-water heating also depends upon the circulation of hot water; steam-heating upon the circulation of steam; hot-air heating upon the circulation of air, and ventilation upon the circulation of air. The action of the local vent, moreover, depends upon the circulation of hot air.