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How do you bleed a hot-water radiator?

Updated August 2026

15–20 minutes · Easy DIY

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A radiator that is hot at the bottom and cold at the top has air trapped inside, and letting it out takes about 2 minutes per radiator with a $5 radiator key. Air rises to the top of the panel and blocks hot water from filling it, so the room never warms while the boiler burns fuel anyway. A whole house runs 15 to 20 minutes. Do it at the start of every heating season, and again any time one room falls behind the rest.

What you need

Before you start: Water in a hydronic system runs 140 to 180°F and leaves a bleed valve as a hot jet, so work with the boiler off and the radiator cooled, wear gloves, and keep your face clear of the valve. Never open the air vent on a steam radiator: that system runs under pressure and the vent releases live steam. If a bleed screw is seized or the square is rounded off, stop before you snap it, because a broken valve means draining the loop to replace it. Rusty brown water instead of clear points to corrosion inside the system.

Steps

  1. Confirm you have hot-water radiators, not steam. A hot-water radiator has pipes at both ends and a small bleed screw in one top corner. A steam radiator has one pipe and a bell-shaped air vent on the side, and that vent is never opened by hand.
  2. Run the heat 10 to 15 minutes so every radiator warms and the air collects at the tops, then shut the boiler off and let the system settle 15 to 20 minutes.
  3. Write down the boiler pressure gauge reading before you start. Cold, most residential systems sit at 12 to 15 psi.
  4. Start at the radiator nearest the boiler on the lowest floor and work outward and upward, finishing on the top floor. That order keeps you from pulling air back into rooms you already did.
  5. Lay the towel down, hold the cup under the bleed valve, fit the key, and turn counterclockwise a quarter to a half turn. No further.
  6. Listen for the hiss. When it becomes a steady dribble of water, close the valve snugly and stop. The brass square strips easily, so hand tight is tight enough.
  7. Repeat at every radiator, then recheck the gauge. Bleeding drops system pressure, so top up to 12 to 15 psi at the fill valve if it fell below 12.
  8. Fire the heat back up and feel the tops of the radiators 20 minutes later. Evenly warm top to bottom means the air is gone.

Or hire it out

Few heating techs will come out only to bleed radiators, so you would pay a $100 to $200 minimum service call for 20 minutes of work. It is normally folded into a $150 to $300 annual boiler tune-up. A radiator key costs about $5 at any hardware store, which makes this one of the better DIY trades in the house.

Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for radiator bleeding sets itself.

Log this task, get the next reminder

Frequently asked questions

Which radiators need bleeding?

Feel each one while the heat runs. Hot at the bottom and cold across the top is the classic trapped-air pattern, and a gurgle or trickling sound confirms it. A radiator that is cold everywhere is a different problem: a closed valve, a stuck thermostatic head, a circulator that is not moving water, or sludge in the bottom. Why a radiator will not heat up sorts those causes by likelihood.

I bled it and the bottom is still cold. What now?

Cold at the bottom with a hot top is the reverse pattern, and it points at sludge rather than air. Iron oxide settles in the bottom channels and blocks flow, and no amount of bleeding touches it. A power flush or chemical clean runs $400 to $800 for a system, and an inhibitor afterward slows the return. Check the lockshield valve at the far end too, since an unbalanced loop starves the last radiator on it.

How often should radiators be bled?

Once at the start of each heating season is the baseline, plus any time a room stops keeping up. Needing it every few weeks is a signal, not a chore: air is entering through a small leak, a failing automatic air vent, or a waterlogged expansion tank, and that fresh oxygen accelerates corrosion inside the radiators. How often to bleed radiators and the radiator maintenance checklist cover the rest.

Do I need to add water to the boiler afterward?

Usually a little. Every cup of air you release is replaced by water, and on a house with eight or ten radiators the gauge can drop a couple of psi. Bring it back to 12 to 15 psi cold at the fill valve, then close that valve rather than leaving it open. Below about 12 psi the upper floors will not fill properly. Cast iron radiators themselves often last 50 years or more, so the pressure habit protects them.

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