How to Bleed a Radiator

Air trapped inside a radiator creates cold spots that rob you of heat and drive up your energy bills. The fix takes five minutes and requires exactly one tool. A properly bled radiator heats evenly from top to bottom, cycles efficiently, and stops making that knocking sound that wakes you up at 3 AM. This is foundational hydronic heating maintenance that every homeowner should know. The air gets in through tiny leaks, from refilling the system, or just from dissolved gases coming out of solution over time. Left alone, that air pocket grows, your radiator works harder for less heat, and your boiler runs longer than it should. Bleeding resets everything.

1. Cool the System First. Switch off your boiler or furnace at the thermostat and wait 30-60 minutes for the system to cool completely. Hot water under pressure can spray out and burn you. Touch the radiator — if it's cool enough to keep your hand on comfortably, you're ready to work.
2. Find the Bleed Valve. Find the small square-socketed valve at the top corner of the radiator, usually on the side opposite the supply pipe. It looks like a small brass or chrome nipple with a square socket in the center. Place your towel on the floor directly below this valve.
3. Insert and Position. Slide the square end of your radiator key onto the valve socket. It should fit snugly without wiggling. Hold your container or cup directly under the valve with one hand while keeping the key in place with the other.
4. Release Trapped Air. Turn the key counterclockwise no more than a quarter turn. You'll hear air hissing out immediately. Keep the valve barely cracked — you want a controlled release, not a full opening. Hold your position and let the air escape completely.
5. Confirm Air is Gone. Keep the valve cracked until the hissing stops and water begins to dribble out steadily. The moment you see a solid stream of water with no air bubbles, stop. This means all trapped air has escaped.
6. Seal the Valve. Turn the key clockwise until snug. Don't overtighten — you're seating a small valve, not building a rocket. The valve should close with firm finger pressure on the key. Wipe away any dripped water with your towel.
7. Monitor System Pressure. Go to your boiler and check the pressure gauge. It should read between 12-15 PSI for most residential systems. If it's dropped below 10 PSI after bleeding multiple radiators, you'll need to add water through the fill valve until pressure returns to normal range.
8. Verify Even Heat. Turn your heating system back on and wait 20 minutes. Feel the radiator from bottom to top — it should heat evenly without cold spots. Listen for gurgling or knocking sounds, which mean you need to bleed again or check other radiators in the system.