Radiator bleeding eliminates air that has accidentally gotten into the radiator's coils. All hot water radiators must be bled on a regular basis, as the heating and cooling of water naturally releases air bubbles which remain in the coils. This air is not dangerous, but if it's not released, the radiator may not work as well as it should; it may heat unevenly or not at all. The bleeding process is relatively simple, although it can be a little messy.
How a Radiator Works
A hot water radiator works by circulating heated water in a series of metal coils or fins. As the hot water moves through fins, they heat up and warm the air that surrounds the radiator. The act of heating and cooling water within the radiator creates air inside the fins, and that air rises to the top of the fins, displacing some of the water. Since air doesn't conduct heat nearly as efficiently as water, the radiator cannot function as effectively, making it both costly and wasteful to run one with air trapped inside.
How to Bleed a Radiator
The procedure for radiator bleeding is relatively simple. Before it is done, the boiler should be turned off for safety reasons. Even if the boiler is off, the water inside may still be dangerously hot, so radiator bleeding should be performed with caution. An old rag or a small bowl should be placed under the valve to prevent water dripping on the floor.
Most radiators come with a notched fork, called a bleed key; others require the use of a flathead screwdriver or crescent wrench. There should also be a protrusion near the top of the radiator on one side, called the bleed valve. The bleed key, screwdriver, or wrench is fitted into the bleed valve and carefully turned counterclockwise slightly, usually just a 1/4 or 1/2 turn. The trapped air will start escaping with a hissing sound. When the water begins to dribble out, that's a sign that all the air has been purged out of the radiator. The bleed valve can therefore be gently tightened again.
If the bleed key is missing and the system does not accommodate a screwdriver, it may be possible to buy a new key at a hardware or or other do-it-yourself store. If a key is not available, a wrench can also usually be used to loosen the cap or screw at the end of the radiator.
Although only one radiator may be having problems, it's a good idea to bleed all radiators in a heating system at the same time. Bleeding one radiator does not remove all of the air from the system, so this can prevent other heaters from developing problems that will require the entire process to be repeated. Once the entire system has been bled, the boiler should be turned on again and the radiators should be checked after several hours to make sure they are heating correctly.
As air is released during radiator bleeding, the pressure in the system will drop. The water level may need to be adjusted through the fill valve on the boiler to increase the pressure to the level required to move the water through the system. If multiple radiators need to be bled, it may be necessary to adjust the water level between bleedings to keep the pressure high enough to force the air out. Homeowners who are not sure how much pressure their heating system needs or how to adjust it should contact a professional for assistance.
How Often Should Radiators Be Bled?
Experts on home maintenance and energy efficiency recommend that radiators be bled at least twice a year. If the radiator is used daily or must be refilled with water, it can be beneficial to bleed it more often. One that is heating unevenly — which is significantly hotter at the bottom than at the top — should be bled promptly so that the system works at its best.
If Bleeding Doesn't Solve the Problem
In some cases, it may be necessary to bleed a heating system several times before all the air is pushed out, especially if the pressure in the system has dropped. If a radiator is still not heating correctly several hours after the water level has been adjusted and the boiler has been turned back on, repeating the bleeding process may be necessary. Other problems can cause poor radiator performance, however, so if no air escapes during the second bleed, it may be necessary to call in a professional. A professional radiator inspector can determine whether sludge in the water is gumming up its circulation or if a mechanical component needs replacing.