Home radiator systems work by circulating hot water heated by a boiler, or in some modern systems, a hot water heater. This water circulates through a closed system of piping, usually propelled by a circulating pump. There shouldn't be air in the pipes, but it gets in so frequently that radiator manufacturers equip radiators with valves to allow it to escape. Periodically opening these valves -- usually called bleeder valves -- keeps radiators hot.
Radiators are part of a hydronic heating system; these systems are especially common in the northeastern United States, but you can find them all across the country. In a hydronic system, electricity, gas or oil fuels the boiler that supplies the hot water. Older systems rely on the natural tendency of hot water to rise to circulate water, but newer systems have circulating pumps, so the heating elements warm up more quickly. Because the pipes form a closed system, they could rupture when water is heated. To prevent this, the system includes an expansion tank, which is a small air-filled tank that gives the pressurized water someplace to go.
Air in the Radiators
When air enters a radiator system, it has a natural tendency to rise to the highest point, which is the coils of the radiators on the highest floor of the house. As a result, these radiators won't heat properly because the air is preventing water from moving through the coils. Even if they still are working satisfactorily, they may make banging and knocking sounds. These sounds tend to occur when you turn the system on and off, and add to the knocking caused by expansion of the metal from which the radiator is made.
Using the Bleeder Valves
When a radiator feels colder at the top than it does at the bottom, it's a sign air is trapped inside it, and it needs bleeding. To open the bleeder valve, located near the top of the coil, you may need a special radiator key, available at most hardware stores -- but a flat-head screwdriver often works. The heat must be on for the valve to work. When you open it, steam will come out until all the air is gone, then water will follow, so you need to position a small bucket under the valve. Be cautious of the steam. When the air is gone and only water comes out, you can close the valve.
In order to keep radiators working properly, you should bleed them at least twice a year. If you do this, there shouldn't be much air to release when you open the valve, and you'll be able to close it almost immediately. Some radiators come with self-bleeding valves. They open as soon as they detect air and close again when water begins to flow. If you aren't around to regularly maintain your radiators, you can retrofit one of these valves to any existing radiator.
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