Topping off or bleeding trapped air from the top of a Stelrad radiator to restore even heat distribution is a straightforward maintenance procedure requiring a radiator key and a small cloth or container. First, turn off your central heating system and allow the radiators to cool completely to avoid any risk of scalding from hot water. Locate the bleed valve square socket usually situated on the upper corner of the radiator. Insert the radiator key, hold a cloth beneath the valve, and slowly turn the key counter-clockwise by about a quarter to a half turn until you hear a hissing sound of escaping air. Once the air clears and a steady stream of water begins to emerge, immediately turn the key clockwise to close and tighten the valve securely.