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 water which stands in them, and is allowed to become cold, is a fertile cause of water-hammer as well as dripping air valves. When the steam is turned into the cold radiator it is apt to gather up this water, together with the freshly condensed water, and drive it into the back end, clogging up the passages and causing water-hammer and dripping air valves, to the great discomfort of those in proximity.

Air valves.—In regard to air valves there is but little to be said. They are a necessity to every radiator; without them any radiator would soon become dead by being filled with air. Automatic air valves have almost entirely superseded the old-time hand air valves. They are made with a composition disc, with a high coefficient of expansion, which is arranged to close the valve as soon as the hot steam comes in contact with it. They are also provided with a screw attachment by which the valve opening can be adjusted after the valves are in place. The disadvantage of the automatic air valve is that when steam is turned on, the entire radiator will become heated. For this reason the author prefers the plain hand air valve on radiators in his own rooms, as the amount of the radiator heated can be very accurately regulated by means of them, especially when they are connected on a one-pipe system. The automatic air valve, however, takes the circulation in the radiator entirely out of the hands of persons who are not acquainted with their principles, and in the case of indirects is a necessity.

Air valves are generally placed about 18 inches from the bottom of the last loop. Theoretically the best location would be near the bottom of this loop, but if there is danger of much water in the radiator they are safer near the top. The author has cured several cases of a dripping air valve by tapping it into the extreme top of the loop. When so placed the last loop will sometimes remain partially air bound on one side, but otherwise they are quite effective in this position. The trouble referred to, however, is much more often due to improper piping than to anything in the design of the radiator.