If your home has a hot water heating system and one of the radiators won't get hot, the problem is usually an airlock—or more appropriately, a steam-lock. The radiator is part of a closed system, and when you turn it on and the water gets hot, some of it can turn to steam. The steam gets lodged in upper parts of pipe loops and prevents water from circulating. This is a common problem, so every radiator has a bleed plug to allow you to release the steam. If your radiators are part of a steam heating system, which is different from a hot water heating system, you won't find bleed plugs, but you will see an air vent on the side. If the radiator is malfunctioning, cleaning that vent may solve the problem.
Do I Have a Steam or Hot Water System?
Before you can fix your radiator, you have to know what kind of heating system your home has. You can often determine this by looking at the radiators. If they are part of a steam system, you'll see a single pipe coming into each one, and each should also have an air vent on the side. Radiators that are part of a hot water system have two pipes to allow water to circulate through them and return to the boiler. If you need more evidence that yours is a hot water system, look at the pipes connected to the boiler. You'll see an electric circulation pump attached to one of them. If none of your radiators is getting hot, the reason can often be traced to this pump. It either isn't getting power, which could caused by a tripped circuit breaker, or it is broken.
Procedure for Bleeding a Radiator
Once you've ascertained you have a hot water heating system, look for the bleed plug near the top of the malfunctioning radiator. It may have a wing nut that you can turn with your fingers, a regular nut that requires a wrench, a slotted head that you can turn with a flathead screwdriver or a shape that requires use of a special key. If the radiator is old, this plug may need a squirt of lubricating fluid before you'll be able to turn it.
Put on a glove before turning the screw to avoid getting scalded. Hold a small container under the plug while you turn the plug gently counterclockwise. Stream will begin to escape and will keep spraying out until it's all gone. At his point, water will start squirting out, and you can close the plug.
Top off the boiler. Bleeding a radiator lowers the pressure in the system, so after you're done, it's a good idea to add a bit of water to the boiler by turning on the water inlet for a few seconds. You may need to do this while you're bleeding the radiator to purge all the steam out of the system, and if so, you'll need a friend to do it. Use your cell phones to stay in touch and synchronize the operation.
Air Vent Trouble
If you have a steam system and one of the radiators isn't getting hot, one of the reasons may be that the air vent is stuck in the open position. If so, you'll probably feel steam coming from the vent, which is located on the side of the radiator. You can often correct this by turning off the system, letting everything cool off, unscrewing the vent and soaking it overnight in vinegar. Vinegar dissolves the hard water deposits that are likely preventing the vent from closing.