If one radiator doesn't heat up but others do, try this one-minute fix first. It usually solves the problem.
With your radiator key, slowly turn the valve counterclockwise until water starts dripping out.
If you have a hot-water heat radiator that’s not heating the cause is usually trapped air, and getting rid of it is simple. At the top of your radiator, look for a small valve like the one shown in Photo 1. Use a radiator key, 1/4-in. 12-point socket, or a flat screwdriver (depending on your valve type) and slowly turn the valve counterclockwise until water starts dripping out. This will release trapped air and let hot water into the cold fins. While you’re at it, you should repeat the process with your other radiators. Bleeding the radiators will lower the pressure in your system, so you might have to slowly add water to increase the pressure. Do this by opening, then closing, the valve on the water pipe above the boiler. In fact, you may need to add water while bleeding the radiator in order to purge the air from the system. This is where a helper will save on trips up and down the stairs. If you’re unfamiliar with your system, call a pro. How much pressure you need depends on how high the water has to rise. The basic rule is 1 lb. of pressure for every 2 ft. of rise. Your gauge may read in pounds, feet or both. A basic two-story house, with the boiler and expansion tank in the basement, needs 12 to 15 lbs., or 25 to 30 ft., of pressure.
Clear the air hole in the top of the vent with a small wire or sewing needle.
Push a wire into air vent hole.
Don’t confuse a hot water system with a steam system. Steam radiators have an air vent, like the one shown, about halfway down the side. Unfortunately, many of these air vents get painted over, plugging the air hole. Clear the air hole in the top of the vent with a small wire or sewing needle. If you’re still worried about the air vents working, consult a hot water/steam heat specialist.