When not in use, a hot-water radiator will fill with air, and that can cause problems when you switch it on after months of inactivity. Before turning on your system for the season, bleed the lines of any air. To do so, switch on the heat and the radiator. Locate the vent at the top of the radiator, unscrew it, and let the hot steam escape. Once the trapped air is gone, water will begin to drip out of the radiator. Catch the water with a cup, and screw the vent cap back into place.