Bleed screws have numerous applications, including on household central heating radiators. Bleed screws also abound on cars, where they are used on the engine itself, plus the radiator and the braking system.
Basically, if a car has a closed system that uses fluid to operate, it will also have a bleed screw to allow you to purge air from the system after, say a fluid change.
For example, if there’s air in a cooling system, the bleed screw can be opened and further coolant added. The air will exit through the bleed screw.
Once fluid comes out of the bleed screw, the system has been bled and the bleed screw can be retightened.
A similar process applies when bleeding a braking system, except this time someone must pump the brake pedal until bubbles stop appearing through the bleed screw. Dedicated brake bleeding systems make this much easier to see.
Once again, when bubbles stop appearing, tighten the screw, top up the fluid and the system is bled.
One other use of a bleed screw in a car is in the clutch system. This is most often hydraulically operated, so having air in the system would not allow the clutch to disengage properly when the clutch pedal is depressed.
In this system, the bleed screw is normally found on the slave cylinder in the engine bay, and again allows the removal of air from the system, making it work more efficiently.
In the domestic application, a bleed screw can be found on every radiator on your house.
These are typically operated via a square-ended ‘key’ that allows you to open up the system and allow the air that has accumulated at the top of the radiator to bleed out.
The boiler in your house merely heats up the water in the central heating system, so bleeding out the air will stop you having cold spots on your radiators, rendering them much more efficient.