If a hot-water radiator is warm at the bottom but cold at the top, air is trapped inside. Bleeding lets the air out so hot water can fill the whole radiator. It takes a few minutes and a cheap radiator key.
Hot water only. This applies to hot-water (hydronic) radiators with a small bleed screw near the top. Steam radiators have a bullet-shaped air vent on the side and don’t get bled this way. See steam vs hot water.
What you need
- A radiator key (hardware stores carry them) or a flathead screwdriver for slotted valves
- A rag or small cup
- A few minutes per radiator
Step by step
- Turn the heat down or off and let the radiators cool. Hot water can spray when the valve opens.
- Start on the lowest floor, farthest from the boiler, and work up.
- Hold the rag under the bleed valve and turn the key about a quarter turn counter-clockwise. You’ll hear a hiss as air escapes.
- Close it when water appears: a steady drip with no sputtering means the air is out. Don’t open the valve all the way.
- Repeat for each cold radiator.
- Check the boiler pressure gauge. Bleeding releases water, so pressure can drop. Most homes need roughly 12 to 15 psi cold. If it’s low, the system needs topping up.
- Turn the heat back on and check that the radiators heat evenly.
If the air keeps coming back
Radiators that need bleeding every week point to a problem: a small leak letting air in, a failing automatic air vent, or a boiler that isn’t holding pressure. That’s a job for boiler repair.
Not comfortable doing it yourself, or several radiators still cold? Call (312) 924-7223.