How to Let Air Out of a Radiator

Bleeder Valve on a Radiator

It is not uncommon to get air in a boiler system. This air usually works its way up to the highest points in the heat radiation system. Air in the system can corrode pipes or make strange noises such as loud knocking or the sound of water running. (Do not confuse the knocking sound with the natural ticking that occurs in many systems from the expansion of metal due to the heat.)

Bleeder Valve Location

It is relatively easy to remove air from a system through a bleeder valve that is installed in most radiators.

  • Begin by locating the highest bleeder valve in your system, or on any radiator making unusual noises. The bleeder valve can look like a hex nut with a standard head bolt in the middle of it, with a little spout coming off the bolt. In some systems there is a special head on the nut that requires a specific tool to turn. This tool can be acquired at most hardware stores.
  • Turn on the heat and let it run for about an hour. This should thoroughly heat up the system and allow for the maximum buildup of pressure.
  • Get a cup or bucket and place it under the spout on the valve to prevent any run-off from damaging flooring. Generally, the water in the system is not under high pressure and will not squirt out, but will come out in a small arc if the spout is directed horizontally.
  • Turn off the heat.
  • Turn the valve about 1/4 turn counter-clock-wise. The nut itself will not turn, only the screw inside it. It should start releasing air. It will often make a sputtering sound. Remember, the air will be warm and the water will be hot, so take precautions not to get burned.
  • When water comes out clear without sputtering for several seconds then close off the valve. Do not over-tighten as you do NOT want to strip the valve.
  • Now turn the heat back on. If the noise continues within a particular radiator it means that only some of the air was removed. Try again. [Editor's Note: I usually leave it for 24 hours just to let the air accumulate.] If there is still knocking or a draining sound, try other radiators.
  • Most systems will refill from a fresh water supply. However, some systems are closed, especially those using anti-freeze to conduct heat. If this is the case, consult the boiler owner's manual for special instructions on how to top up the system and ensure that there is sufficient pressure.

Letting air out of a radiator is a procedure most homeowners should be able to handle. Do not panic if the first attempt is not successful. Sometimes the system must be bled more than once to get rid of all problems. Also, because of leaks, the system will need to be bled periodically (but usually not more than once every year - usually only once every several years.) Boilers are an efficient way to heat a home. With proper care, a boiler and its attendant pipes and radiators should last for decades.

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