Bleed radiators

Release trapped air from hot water radiators using a radiator key to restore even heating and eliminate cold spots.

Why it matters#

Air bubbles trapped in your hot water heating system rise to the top of radiators, preventing hot water from circulating properly. This creates cold spots, makes radiators take longer to heat up, causes gurgling noises, and forces your boiler to work harder—increasing energy costs. Regular bleeding keeps your heating system efficient.

What you'll need#

Tools#

• Radiator key (or flat-head screwdriver for some valves)
• Small bowl or container

Materials#

• Old towel or rag

Safety gear#

• Protective gloves (water may be hot)

Steps#

1. Turn Off Your Heating System#

Before bleeding, turn off your heating system completely. Use the thermostat to set it to its lowest setting, or flip the emergency shutoff switch (often near basement stairs). This prevents new air from being drawn into the system while you work and allows water to cool enough for safe handling.

2. Identify Radiators That Need Bleeding#

Check each radiator for signs of trapped air: cold spots at the top while the bottom is warm, radiators that heat unevenly or take longer than usual to warm up, or gurgling/bubbling sounds. Even if some radiators seem fine, it's good practice to bleed them all once a year.

3. Locate the Bleed Valve#

Find the bleed valve on each radiator—it's a small valve typically located at the top corner on one end. It looks like a small square or slotted fitting. Some require a radiator key (available at hardware stores for a few dollars), while others can be opened with a flat-head screwdriver.

4. Prepare for Water#

Place your towel on the floor beneath the valve and position your bowl directly under it to catch any dripping water. Keep an extra rag handy. If your system has been running recently, the water inside may be hot, so have your gloves ready.

5. Open the Bleed Valve#

Insert your radiator key or screwdriver into the valve. Slowly turn counterclockwise about a quarter to half turn—no more. You'll hear a hissing sound as trapped air escapes. This is exactly what you want. Don't open the valve too far or water will gush out when the air finishes escaping.

6. Wait for Water to Flow#

Keep the valve open until the hissing stops and a steady stream of water appears. This indicates all the air has been released. Be prepared—the transition from air to water happens quickly, and the water may be hot if your system was running recently.

7. Close the Valve#

As soon as you see steady water flow (not sputtering or air-mixed), quickly turn the valve clockwise to close it. Tighten firmly but don't over-tighten, which can damage the valve. Wipe away any water that dripped out.

8. Repeat and Check Boiler Pressure#

Work through all radiators in your home, starting with the one farthest from your boiler and working your way back. After bleeding, check your boiler pressure gauge—it may have dropped. Most systems should read 12-15 psi (or about 1.5 bar). If low, use the filling loop to add water until pressure is correct.

Pro tips#

• Bleed your radiators annually before heating season starts—fall is the ideal time for this maintenance.
• If a bleed valve is painted over, use a stiff wire to clear paint from the opening and acetone to soften stubborn paint around the screw before attempting to open.
• Start with upstairs radiators and work down, or with the radiator farthest from the boiler—this prevents displaced air from one radiator moving to another.

Warnings#

• Never bleed steam radiators—they don't have water inside and bleeding them won't solve heating issues; steam systems have air vents that work automatically.
• If water continues flowing after you've closed the valve, the valve itself may be damaged; consult a plumber to avoid water damage.

When to call a pro#

If you've bled all radiators but still have cold spots, there may be sludge buildup or a blocked valve requiring professional power flushing. If a radiator remains completely cold, or if your boiler pressure keeps dropping after you refill it, call a heating technician—you may have a leak in the system.