How to Bleed a Radiator

A radiator along a white wall under a window.
  • 1 hours
  • Beginner
  • $10-30
What You'll Need
Radiator key
What You'll Need
Radiator key

If you have radiators, chances are you have a fairly old home. This means that people who know about radiators and how to take care of them are become rarer all the time. It is an old technology, but in truth, nearly as efficient as today’s more common hot air furnaces with the proper maintenance.

That starts with the boiler, be it oil, or gas. Check with the manufacturer or a licensed technician on proper maintenance of the boiler. But bleeding the radiators can be done easily by anyone.

What Does it Mean to Bleed a Radiator?

Radiators contain water heated by the boiler. The metal radiator is designed not to retain the heat very well. The heat “radiates” from the metal into the air and thus heats the home. This only works when there is nothing blocking the water from traveling through the pipes.

Occasionally, air pockets will develop that do just that. Fortunately, each individual radiator has a valve on one end which can be opened to allow the air to escape. The system then automatically fills the radiator with hot water.

Why Do I Need to Bleed My Radiators?

Your radiator will not do its job if water can't circulate through the pipes, which is what happens when air gets into them. This can be due to the water supply being cut off for repairs, or if there is insufficient water pressure for any reason that prevents the water from filling the pipes.

If this sort of thing happens frequently, you may want to have your radiators inspected and serviced.

When Do I Need to Bleed My Radiators?

After work on the system, if it’s been drained and refilled, and once a year before the cold season begins. While it is rare that the system needs bleeding annually, it is a good diagnostic tool to determine how well the boiler is working, which is explained below.

How Do I Bleed My Radiators?

If you don't have a radiator key, buy one. They are inexpensive, and are more likely to be found at "mom and pop" hardware stores than at the big chains.

Have a small cup handy to catch any discharge before you begin. With the cup under the valve and a cloth around the key, simply insert the key into the valve and turn it counter-clockwise. If you see water coming out, close the valve by turning the key clockwise. That radiator is fine and you can proceed to the next one. The water may look dirty; this is normal.

You may hear air rushing out. Hold the valve open and position the cup underneath it. It can take some time depending on how much air is in the pipes. The air may have an odor. Continue holding it open until water comes out, then close the valve and move on. If no water comes out, and there doesn’t seem to be air coming out either, there is not sufficient water pressure from the boiler, which may need attention.

Once you're finished letting bleeding the radiator, check your boiler to see if it's pressure has changed due to the water you had to let out. If so, follow instructions on the boiler or from a local professional to raise the pressure back to its normal range.

    With the proper attention, radiators can be very reliable and can provide more than sufficient heat for your home. They can also be handy to dry wet mittens or gloves, to warm towels or pajamas while you shower, or to provide a warm perch for your cat.