For hot water baseboards, air bleed means removing any trapped air from the heating system. Heating water causes it to release dissolved air into the units. Over time, the air builds up, causing noise and uneven heating when the boiler operates. In some cases, the air lock becomes so bad that it completely blocks water circulation, according to SF Gate.
Each radiator or baseboard unit has a small, cylindrical fitting sticking up from its supply line. The valve is approximately 1 inch in height and has a screw at the top with a small nozzle on the side. Some bleed valves are located out of sight behind the shroud or end cover.
Turning the screw in a counterclockwise direction opens the valve, allowing air and some water to escape. To bleed the unit completely, the valve should remain open until a steady stream of water escapes. When valves do not release air or water, corrosion is likely closing off the opening, requiring cleaning or replacement of the valve.
Upon completion of the bleeding procedure, turning the screw in a clockwise direction closes the valve. Upon removal of the air from all units, owners should check lines for correct operation and replace any covers that were removed.