How to Bleed a Radiator (Step by Step)

Hot-water and steam radiators are common in older homes in the Northeast and Midwest. Over time, air collects at the top of a hot-water radiator and blocks the flow of hot water. The tell-tale signs: the radiator is cold at the top but warm at the bottom, or you hear gurgling.

Bleeding releases the trapped air. You need a radiator key (about $3 at any hardware store) or, on many newer valves, a flat screwdriver, plus an old towel and a small container.

Step by step

1. Turn off the heating and wait

   Switch off the boiler or turn the thermostat down and let the radiators cool for at least 30 minutes. Hot water can scald.

2. Find the bleed valve

   Look for a small valve at the top corner of the radiator, usually on the opposite side from the main valve.

3. Protect the floor

   Hold a towel under the valve and keep a small container ready to catch drips.

4. Open the valve slowly

   Turn the key or screwdriver a quarter to half turn counterclockwise. You will hear a hissing sound as the air escapes.

5. Close it when water appears

   As soon as a steady trickle of water comes out instead of air, turn the valve clockwise to close it. Do not overtighten.

6. Check the boiler pressure

   Closed hot-water systems have a pressure gauge on the boiler, usually 12–15 psi when cold. If it dropped, top it up using the fill valve as described in your boiler manual.

7. Turn the heating back on and test

   After an hour, feel the radiator from top to bottom. It should now heat evenly.

Which radiator first?

In a multi-story home, start with the radiator farthest from the boiler on the lowest floor and work your way up and closer to the boiler. Air tends to rise, so the top floor often holds the most.

When bleeding does not help

• Cold at the bottom, warm at the top: sludge has built up inside. The system may need a power flush by a professional.
• Whole radiator cold: check that the valve on the side is open and the thermostatic head is not stuck.
• You need to bleed every few weeks: air is getting in somewhere, or a pump problem exists. Call a heating technician.

Evenly heated radiators warm your rooms faster, so you can keep the thermostat lower — a direct saving on your heating bill.