This bleed key fits most radiators and comes with a detailed guide on how to bleed the air from your radiators and make your heating system more efficient.
The radiator system in a house is water-based. The heating and cooling of water naturally releases air bubbles which rise to the top of the radiator and displace the water. A couple of inches of air at the top of your radiator could be costing you more money for less heat. Check if your radiators stay cold at the top when warm lower down. If you hear gurgling noises from your radiators when they are heating up or cooling down, it also could be due to a build-up of air.