=Water-filled Radiators.=--Radiators frequently fill with water and are noisy because of the position of the valve. This may be true in any gravity system but particularly so in radiators having a single pipe. When the valve of a single-pipe radiator is opened a very small amount, the entering steam is immediately condensed but the water cannot escape because the incoming steam entirely fills the opening. Under this condition, the radiator may entirely fill with water. If the valve is then opened wide, the imprisoned water has an opportunity to escape while the steam is entering, but the entering steam and escaping water sets up a water-hammer that sometimes is terrific and lasts until the water is discharged from the radiator. The same condition may exist in a two-pipe system, if the steam valve is slightly opened while the escape valve is closed, but in a well-designed system the radiator will be immediately emptied when both valves are open. =Air Vents.=--All radiators must be provided with air vents. The vent is placed near the top of the last loop of the radiator, at the end opposite from the entering steam, as indicated in Figs. 2, 3, 6, etc. The object of the vent is to allow the air to escape from the radiator as the steam enters. Steam will not diffuse with the air and, therefore, cannot enter the radiator until the air is discharged. The air vent may be a simple cock such as is shown in Fig. 10, that must be opened by hand when the steam is turned on, to allow the air to escape, and closed when the steam appears at the vent; or it may be an automatic vent, that opens when the radiator cools and closes automatically when the radiator is filled with steam. There are many makes of air vents of both hand-regulating and automatic types; of the former, Fig. 10 furnishes a common example. The part _A_, in the figure, is threaded and screws tightly into a hole made to receive it in the end loop of the radiator. The part _B_ is a screw-plug that closes the passage _C_, leading to the inside of the radiator. When the steam is turned on, the vent must be opened until the air is discharged, after which it is closed by the hand-wheel _D_. [Illustration: FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG. 10.--A common form of air vent for radiators. FIG. 11.--An inexpensive automatic radiator air vent. FIG. 12.--Monash No. 16 automatic air vent. FIG. 13.--The Allen float, radiator air vent.]