Bleeder valves:

Bleeder valve: it is also called bleed-off valve, air vent or release valve. It is usually mounted on the highest point of the system, cosmopolitan in the heating radiators and the water dividers. The air-release valves are either automatically or manually. The air may get into the pipeline when supplementing water to the heating radiators. If the air in the heating system jams, the radiators would not work. Therefore, setting up a air-release valve to bleed air in order to ensure the circulation is smooth.