Bleeding a radiator is a simple DIY task to release trapped air, which can prevent radiators from heating up properly. To bleed a radiator, first, make sure your heating is turned off. Use a radiator key to turn the valve at the top of the radiator counterclockwise until you hear a hissing sound — that's the air escaping. Once water begins to leak out, tighten the valve back up, and your radiator should heat more efficiently.