This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Special valves are made for use with hot water radiators, the main advantage being a device for quick closing, a half turn being usually sufficient to open or close the valve. Connections with the radiators may be made at the top or bottom, the return pipe being always at the bottom. Only one valve is necessary to stop the flow of water through the radiator and this is put on the supply pipe, the return connection being made by a union elbow.
The ordinary "pet cock" is generally used for an air valve, but there are several forms of automatic valves which are operated by a float which allows the air to escape when the water line is lowered but closes upon the flowing in of the water.