Bleed screw
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On a home radiator unit, the bleed screw can be opened, usually by means of a key, to allow air to escape from the unit. Bleed screws are also found on some pump types fulfilling a similar purpose.
They are most often located at the top of the radiator on the side where the inflow pipe is. The screw itself, usually a hexagonal or square knob, is inside a small round protrusion.
The key looks similar to that used to wind a clock. It is inserted into the protrusion, mates with the bleed screw and turns it. Opening the bleed screw then allows air which has risen to the top of the radiator system (the top of the radiator itself) to escape and new water to take its place. Removing the air and allowing water to displace it makes the radiator work more efficiently since it fills completely whereas before it did not.
Automotive cooling systems can also have a bleed screw. This usually takes the form of a bolt with a hole through the middle that is threaded into a hole on the vehicle's cylinder head. This hole goes into the water jacket of the cylinder head. When the bleed screw is loosened, antifreeze is added to the vehicle's cooling system and air is pushed out the hole through the center of the bleed screw. When liquid begins to flow out the screw is tightened down and the passage to the center hole is closed. Bleed screws are not common on cars today and are only necessary when design of a vehicle's cooling system results in areas where air can be trapped in the system.
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