How to Bleed a Radiator

If your radiator isn't heating properly, it could be mean that you need to "bleed" it — which entails releasing trapped air from the system. Bleeding is needed to correct a problem known as an airlock, caused by excess air buildup in the system. The bleeding procedure is simple, but it's required only for hot-water radiators. If you have steam radiators, there are different solutions to poor heating.

A hot-water radiator is part of a closed loop system that circulates heated water from the boiler to each radiator in the system and back to the boiler. When the water heats up, oxygen can be released, causing a buildup of air in the closed system. The oxygen can get lodged in upper parts of piping as well as high points in a system. This condition of being airlocked or air bound results in limited flow of water and heat in affected areas. Bleeding out this air effectively restores proper function to the system.

Before you can bleed your radiator, you have to know what kind of heating system your home has, which will be either steam (which doesn't require bleeding) or hot-water. You can often determine this by simply looking at the radiators.

If your radiators are part of a steam system, you will typically see a single pipe coming into each one, and each should also have an air vent on the side. If this type of radiator is malfunctioning, thoroughly cleaning the vent may solve the problem.

On the other hand, if your radiators are part of a hot-water system, they will have two pipes to allow water to circulate through them and return to the boiler. You'll also see an electric circulation pump attached to one of them (or one or more pumps may be near the boiler). If none of your radiators are getting hot, the reason can often be traced to this pump. A pump that isn't getting power may be caused by a tripped circuit breaker or simply failure of the pump.

Hot-water radiators have a bleed plug designed for this purpose. Your bleed plug 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 nut 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.

Once you've ascertained you have a hot water heating system and you suspect it needs to bleed, here's what to do:

1. Look for the bleed plug near the top of the malfunctioning radiator.
2. Put on a glove before turning the screw to avoid getting scalded.
3. Hold a small container under the plug while you turn the plug gently counterclockwise.
4. Air will begin to escape and will keep coming out until it's all gone. At this point, water will start squirting out, and you can close the plug.

Bleeding a radiator lowers the pressure in the system. There is a pressure regulator on a hot water heating system that should maintain around 12 psi but sometimes it might fail. If this occurs after you're done bleeding the radiator, it's a good idea to add a bit of water to the boiler by turning on the water inlet for a few seconds. Watch the pressure gauge on the front of the boiler, making sure you do not over-pressurize the system. It's best to keep the pressure under 20 psi.

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 closed position. Make sure the system is indeed on and verify there is steam getting to other radiators. Next, make sure the vent is not closed and is in an upright orientation (it usually has a pointed end, which should point up). Also, confirm that the radiator valve, a knob at the base of the radiator, is fully open by turning it counterclockwise until it stops.

You can often correct a plugged air vent 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 functioning properly.