The return circulation is always connected to the bottom pipe of the stove connection, as shown in Fig. 81, in which the hot service and circulating pipe are represented by dotted lines. The side hole is simply to receive the water from the stove. There are, or should be, two holes in the top, one in the center of the head, and the other about half the radius in the direction of the side hole. The eccentric hole is for cold-water entry. The cold supply might be admitted at the bottom, but the result would be to empty the reservoir when the house supply is turned off. The cold supply is not emptied abruptly into the top of the reservoir. A delivery pipe is extended to very near the bottom, say within two or three inches, so that the water will mingle directly with the coldest portion near the bottom; where it begins its journey to the stove to be heated. The usual way is by simple open end pipe, but the end of the pipe should be plugged and holes drilled in the pipe and plug so as to form a spray delivery. This does not aid the delivery heating at all, but the spray will scour the bottom and sides adjacent when the reservoir is emptied and flushed to rinse out scale and sediment. Immediately under the upper head, the delivery pipe must have a Finch hole drilled in, so that air will enter and break the siphon, and thus avoid inadvertently emptying the reser voir when intending only to cut off the supply and drain the pipe. See Fig. "78.
The siphon hole, as it is termed, should be turned in the direction opposite the eccentric hole, which is for the hot-water exit, so that the stream of cold water which issues there when water is coming into the reservoir will not cut across and in terfere with the hot service which is always leaving the reservoir at the same time. If the delivery were placed nearest the side hole, hot water from the stove would have to pass around it in order to reach the exit. Delivering the cold through a pipe passing down through the volume of hot water is no material retardation of the heating process. The heat thus absorbed by the cold delivery is simply that much aid to the ultimate purpose. This cannot be said of the siphon-hole jet when directed across the hot exit or in its direction.
The object in putting the siphon-hole near the upper head is to avoid siphoning more water than necessary, as the waste tubes of stop and waste cocks are generally left open—not connected to drains, and often not even discharging where the waste can be left to take care of itself. Moreover, it is a waste of the stored hot water to siphon out several inches from the hottest point.
Care should be taken not to have the hot connection extend into the upper head below the inner surface, as this would form an air space which could not be filled with water, and thus annoying noise and the formation of steam would be favored, if no other consequence presented itself.
It is essential to keep the water-back or coil filled. Sometimes the supply may be off for a day or so. No water can then be drawn at the regular faucets; and extreme care should be taken not to draw too much from the sediment faucet, as this is the time when temptation to use it is hard to overcome. The reservoir full will keep the level above the side hole for weeks, if none is deliberately drawn out. The height of the water can be told by tapping on the shell, and in no case should it be allowed to fall below the side opening; neither will it do to empty the reservoir and use the fire with the back empty. Either keep water in the reservoir in cases of emergency, or remove the water heater altogether and substitute a tile back until regular water supply can be had. A reservoir can be replenished with a pail and funnel, by hand, by loosening one of the top connections.
In apartment or other houses where steam pressure is constantly maintained, the whole plumbing system is usually supplied with hot water through the medium of a reservoir provided with steam coil of brass pipe, as shown in Figs. 82 and 83. The trombone coil, illus trated in Fig. 82, can be used only on horizontal tanks; it would not drain in any other position. The water of condensation is generally wasted into the sewer, delivered to a hot well, or returned by steam trap. Steam heat in such instances takes the place of the water heater used in stoves and ranges in general domestic work.
The efficiency of a steam coil when surrounded by water is much greater than when placed in the air. A brass or copper pipe will give off about 200 thermal units per square foot of surface per hour for each degree difference in temper ature between the steam and the surrounding water. This is assum ing that the water is circulating through the heater so that it moves over the coil at a moderate velocity. The ratio of absorption decreases as the temperature of the water ap proaches that of the steam surface. In assuming the temperature of the water, take the average between that at the inlet and that at the outlet.
Example. How many square feet of heating surface will be re quired in a brass coil to beat 100 gallons of water per hour from 38' degrees to 190 degrees, with steam at 5 pounds' pressure? Water to be heated = 100 X 8.3 = 830 pounds.
Rise in temperature = 190 — 38 = 152 degrees.
Average temperature of water in contact with the coils = 190 + 114 degrees. 2 Temperature of steam at 5 pounds' pressure = 228° approximately (actually 227.964°).