Trapped air can cause a hot-water or steam radiator to stay cold at the top. Releasing the air restores efficiency. On a hot-water unit, let the air out through the bleed valve, a square nut located near the top of the unit. Using either a special valve key or a flathead screwdriver (depending on your model), open the valve a quarter turn until you hear hissing. When water begins bubbling out, close the valve again. Steam units trap air if the valve's air vent is clogged with paint or dirt (or not pointing up). With the heat off, clean out the little hole at the top of the valve with a wire, or just replace the whole thing. But with either system, warns This Old House plumbing and heating expert Richard Trethewey, "be careful with those valves. That radiator is way older than you are, and if those threads break, you've got a big problem." For a stubborn valve, he recommends holding the outside nut with a crescent wrench as you turn the valve. You can also use WD-40, but not too much elbow grease: Better to leave a bubble than to bust a radiator.