A heating system, when cold, fills with air by leakage around valve stems. This air must be allowed to escape so that steam may enter. Automatic air valves may be placed on every radiator and coil. These valves are open when cold. As steam enters the systern, the air escapes ahead of the steam and finally when steam reaches the air valve, the heat of the steam expands a plug in the valve, which thus closes automatically. As the air valves often get out of order, it is a great convenience to run small 1/4 in. pipes from each radiator to the boiler-room. The engineer can then open each air pipe until steam appears. When this happens he can be certain that the coils are working properly.