Radiators give off heat when hot water is circulated through them. This water is heated by a boiler and then pumped through the radiator system. Occasionally, when water enters the heating system, such as when the system is filled, air is also introduced. This trapped air lowers the heating efficiency of your radiators. Removing this trapped air from a radiator is called bleeding, which is done by opening a radiator valve to allow the air to escape. This is usually done with a device called a radiator key. If you misplace this device, it's still possible to bleed a radiator without a key.
Shut off the power for the boiler. This will prevent the water from circulating through the radiator while you are bleeding it. This can be done at the radiator by turning it off, or from where your central breaker panel is located, by switching off the circuit that contains the boiler.
Place newspapers or a towel on the floor, underneath the radiator.
Adjust the wrench so that it slips around the cap on the end of the radiator.
Slowly turn the wrench counterclockwise to loosen the end cap. Listen for the sound of the steam escaping. As soon as you hear that noise, stop loosening the cap and allow the air trapped inside to escape.
Turn the cap clockwise as soon as you see water starting to seep out of the radiator. Tighten an additional quarter turn with the wrench.
Open the fill valve on your radiator, if needed, to bring the system back up to pressure.