This section is from the book "American Plumbing Practice", by The Engineering Record. Also available from Amazon: Plumbing: A working manual of American plumbing practice.
In looking over the plumbing of a New York building in 1891 a condition of affairs was observed which may, in more than one case, account for a large water bill, and may in others account for want of warm water in the boiler at times when the fires are good and when plenty of hot water is expected.
In the case in question we found the air and vent pipe a brought in a goose-neck fashion b over the edge of the tank, and down into the top of the standing overflow pipe c as shown. We asked why the pipe was brought close down to the overflow, and were informed "that it was to insure the keeping of water in the trap under the tank," and were told " that when the water got very warm in the hot-water pipe and circulation, that it would run over and add water to the trap." This may be true, and if the goose-neck is not carried sufficiently high above the water in the tank there is no doubt of it, but it did not seem to strike the man who designed it that should the density of water in the hot-water pipe from the boiler decrease, by being warmed, until it reached the top of the goose-neck and run over, then the water, as it run down the short leg of the goose-neck, would balance an equal height of water in the rising pipe, and that this syphon might then run continually, taking water from the tank and actually discharging it at a higher level into the overflow pipe, just as long as heat was applied to the boiler, or until the water in the tank fell so low as not to be able to balance a column of warm water equal to the height of the open end of the gooseneck. To make this plain, assume the height of water from the water-back to the top of the water in the tank to be 60 feet, and its temperature to be, say, 400 Fahr. This will exert a pressure of 26.11 pounds per square inch in the water-back. Consider, again.
From this it is evident that unless a difference of fully 2 feet 4 inches exists between the level of the water in the tank and the lower end of the goose-
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