Short definition

Bleeding a radiator means releasing trapped air from the top of a hydronic radiator using a small bleed key. Air pockets at the top stop hot water from filling the upper section — the classic “cold at top, hot at bottom” symptom. The fix takes five minutes and one inexpensive tool.

What it is

A hydronic heating system runs hot water through pipes into radiators. Air dissolved in the makeup water (or introduced during refill) rises and collects at the highest points — typically the top of each radiator, where there’s a small bleed valve. Once enough air pools there, hot water can no longer reach the upper section, and that part of the radiator stays cold.

The bleed valve is a small square or hex fitting at the top corner of the radiator. A radiator bleed key (a four-sided brass key matching the valve) or a flat-blade screwdriver opens it a quarter turn. Air hisses out; once water trickles, you close the valve.

The procedure:

  1. Turn off the boiler or set the thermostat below current room temp so the system isn’t actively pumping.
  2. Hold a towel and small container beneath the bleed valve.
  3. Open the valve a quarter turn counterclockwise. Listen for the hiss.
  4. Close as soon as a steady trickle of water comes out.
  5. Check the boiler pressure gauge. If it dropped below ~1 bar (15 psi) cold, top up via the filling loop to 1.0–1.2 bar.

That’s it. Most homeowners can do every radiator in a house in twenty minutes.

Why it matters to a homeowner

Bleeding is the single most common DIY task on a hydronic-heated home, and it’s almost always the right first step on a radiator that isn’t heating evenly. A plumber charging $150–$200 to bleed three radiators is doing a job you can do yourself with a $5 key.

Two warnings worth knowing.

If you keep bleeding the same radiator and air keeps coming back, the gas isn’t air — it’s hydrogen, produced by electrolytic corrosion inside the system. That’s a sign of magnetite sludge buildup and depleted corrosion inhibitor. The fix is a system power flush and a fresh inhibitor dose, not more bleeding.

Second, a bleed valve that won’t seat after opening — keeps weeping water — usually means the valve seat is corroded. Drain the affected radiator section, replace the bleed valve (cheap and threaded), and refill.

When you’ll encounter this term

  • One radiator stays cold while others heat fine.
  • A radiator is hot at the bottom, cold at the top.
  • The boiler ran out of pressure and was just refilled — air entered the system.
  • A plumber’s invoice line item: “bleed system, top up pressure.”

Common failure modes

  • Bleed valve seized. Common on legacy WA radiators (1920s–60s). Penetrating oil and a careful turn — too much torque snaps the valve.
  • Bleed valve won’t reseat. Continuous water seep after closing. Replace the bleed valve.
  • Air keeps coming back. Hydrogen from electrolytic corrosion, not real air. Power flush + corrosion inhibitor.
  • Black water at the bleed. Magnetite sludge in the system. Schedule a flush.

Washington note

Hydronic heating is concentrated in older WA neighborhoods — Capitol Hill, First Hill, Pioneer Square, Queen Anne, parts of West Seattle, Tacoma’s North End, older Bellevue, much of pre-war Spokane. Many of these homes still run cast-iron radiators on 1950s–80s gas or oil boilers. Bleeding a single radiator is the most-Googled hydronic question among WA homeowners in fall when the heat first comes back on after summer.

If your home runs hydronic and you’ve never bled the radiators, schedule a thirty-minute walk-around in early October. Cold-at-top is a fall pattern; bleeding before the deep heating season starts means you’re not chasing it during a cold snap.