Air vent
The radiator’s air valve, or radiator air vent or steam valve as it is sometimes called, is used only in one-pipe steam boiler systems. It is usually located at the end of the radiator opposite the supply pipe, about halfway up or higher towards the top. Many air vents are bullet-shaped, or others like the Heat-Timer Varivalve are cylindrical in shape, but they can be many different shapes and sizes. The air vent works completely independently (or should anyway).
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