The branches should have an upward pitch from main; also, the radiator connections are made the same as for the regular twopipe system.

We have found it excellent practice on work of any considerable size to increase the size of the radiators somewhat, that are connected on the last two sides of the circuit. The water in the main being cooled somewhat before it reaches this part of the system, it is necessary to provide more radiation in order that all portions of the work will heat evenly. The sizes of the branches may be somewhat smaller nearest the boiler than those toward the end of the main. It will be found that this system of piping will prove most efficient and acceptable when properly proportioned and erected.

The systems of low-pressure hot-water work we have described and illustrated are the principal forms of this class of work. There are several modifications of each, which it is not necessary to describe as the same general principles of piping, etc., prevail. Having detailed the character of this work, it is well that we should understand the principles which underlie it, and will therefore treat briefly on the cause of hot-water circulation.

Why Water Circulates

In answering the question-What causes circulation?—we say that unquestionably it is heat which causes the water to circulate in a hot-water heating system. When heating by hot water first came into general use in the United States, the writer was taught that

water, being heated, became lighter and when confined in a heating system would ascend to the top and circulate through the piping and radiators. This statement was a gross error, although we believed it at the time, and as we have heard the same statement made many times since, it is undoubtedly a very common error. As a matter of fact, hot water will move only when there is a cooler and heavier body of water displacing it and forcing it upward, and were it not for the difference in temperature between the flow and return pipes of a hot-water heating system, there would be no circulation at all.

Hot water, as it cools, becomes compact and outweighs the warmer water in the heater, causing it to rise in the system and circulate through the piping and radiators, the difference in the mean temperature of the water as it ascends and descends in the system keeping the circulation constant. The higher the water in the system, the more rapid the circulation, or, stated in another form, the greater the height of the return pipe (in which the cooler water is descending), the more energy and push against the warmer water in the heater and consequently the more rapid the circulation. The height of the flow riser (the ascending water) makes no difference in the rapidity of the circulation of the water in the apparatus, except as the height of the return is increased. The velocity of the flow of water in a heating apparatus depends upon the difference in weight of the ascending and descending columns of water, with due allowance made for friction. There are several methods of determining theoretically this velocity. However, as this book is written only from a practical standpoint, we shall not burden our readers with a discussion of these theories.

CHAPTER XIV

Pressure Systems of Hot-water Work

THE high-pressure system of hot-water heating is not, as a rule, practiced in this country. In England we find it used for various purposes, such as laundry dryers, bake ovens, enameling, etc., the apparatus carrying from 250 to 350 degrees temperature. The piping used is small in diameter and extra strong, or extra heavy in weight. The fittings used are also much heavier than it is our custom to use on heating work. This system was designed and used originally by Mr. A. M. Perkins, of London, Eng., and is known as the "Perkins System."

Pressure work as practiced in this country (closed-tank system), consists of sealing the outlets of the expansion tank, thus putting the apparatus under pressure, a safety valve being used on the overflow at the tank to regulate the same. On ordinary work it is seldom that a pressure exceeding ten pounds is employed, the water in the apparatus at this pressure having a temperature of about 240 degrees. This style of work is probably used in greenhouses more frequently than in any other manner, and among its advantages are the use of less radiation, a less volume of water in the apparatus and a more quickly controlled apparatus. For use in heating dwellings or apartments it is objectionable because of the element of danger connected with its operation. Should the safety valve at the expansion tank become inoperative from any cause, an explosion would be the probable result.

We have known heating contractors to use this method when they find that too little radiation had been installed to give the temperature required, and frequently to adopt this seeming remedy without giving notice to or obtaining the consent of the owner of the property, which involves not only a dishonest, but a very dangerous practice as well.

The following table gives the temperatures at which water will boil at various pressures (atmospheric), with the equivalent head in feet:

When it is necessary to place both boiler and radiator on the same floor, as is shown by Fig. 135 in the previous chapter, it is sometimes advantageous to put the work under a moderate pressure in order to quicken and maintain a more positive circulation throughout the system.

On certain work of this character it is sometimes impossible to run the overhead piping sufficiently high to admit of a free circulation through all of the radiators, those farthest from the heater not working as well as those placed nearer the heater. This is readily remedied by placing the system under sufficient pressure to maintain a free circulation in all parts of the apparatus.

Expansion-tank Connections

The expansion-tank connections for pressure work may be made in the same manner as for the open-tank system. The opening in the tank used for air vent is plugged and the safety valve, which is usually of the lever variety, is placed on the overflow pipe at a point near the tank.

Where a vertical tank is used, the connections should be made as shown by Fig. 139. Where a horizontal tank is used, the con

FIG. 139.-Expansion tank with safety valve.

nections should be made as shown by Fig. 140. We show on this illustration the use of a vacuum valve. When the safety valve

FIG. 140.-Expansion tank with safety and vacuum valves.