209. The principal object to be sought in designing a system of hot-water piping is to adjust and equalize the resistance in each circuit and branch, so that the hot water will flow with equal readiness to each radiator. This is accomplished by making the diameter of each pipe just sufficient to pass the desired amount of water under the head, or driving force, which is available in that particular part of the system. Artificial resistances are also introduced at some points by putting in extra elbows or bends, and valves are sometimes used for the same purpose.

The fall of temperature of the hot water, as it passes through a radiator, is usually estimated at about 20° for good practice, and 35° is regarded as the limit in any case.

## 210. Height Of Circuit

Height Of Circuit. The horizontal pipes on the upper floors of a building, and also the risers leading thereto, may be made smaller in diameter than those upon the lower floors, because the driving force which impels the water increases with the height of the circuits.

The proper size of a pipe having been determined for a given service on the first floor, the diameter for equal service on higher floors, the temperatures remaining the same, may be found by multiplying by the following factors:

 Story 2d 3d 4th 5th Factors........ .87 .80 .76 .73

No factors are given for heights above the fifth floor, or about 50 feet, because the decrease for the succeeding stories is so small that it is of little practical account.

211. Conversely, the area of heating surface that may be properly supplied by a pipe of given diameter will increase as the circuit is made higher. If the area which is known to be right for a given size of pipe on the first floor be taken as 1, the areas on the upper floors will increase in the following order:

 Story 2d 3d 4th 5th Proper area heating surface. . 1.40 1.70 1.98 2.20

## 212. Resistance Of Circuit

Resistance Of Circuit. The resistance to the flow, caused by elbows, tees, and other fittings, is considerable. The resistance in a common elbow, the ends of the pipes being left square, is about equal to the frictional resistance of a piece of straight pipe having a length equal to 100 times its diameter. If the ends of the pipes are beveled to an edge, the resistance may be reduced to 70 diameters, or even to 60 in small sizes. With a long bend having a radius of 5 diameters, the resistance falls to 10 diameters, or less.

A plain T offers about the same resistance as an elbow, and a return bend from 1 1/2 to 2 times as much. The gain made by reaming the ends of the pipe is much less in the large diameters than in the small sizes.

The actual length of a circuit is always understood to be the actual distance traveled by the water in going from, and returning to, the boiler.