How to Bleed Radiators That Stay Cold at the Top: Step-by-Step
When winter arrives and you switch on central heating, you may notice that some radiators are warm at the bottom but completely cold across the top, or make gurgling running-water sounds. Trapped air pockets prevent hot water from circulating fully. Bleeding your radiators takes just 3 minutes per unit.
Tools Required for Radiator Bleeding
1. A standard square radiator bleed key (or a flathead screwdriver).
2. A small plastic cup or bowl to catch water drips.
3. A dry towel or rag to wipe the valve.
Step 1: Turn Off Heating and Let the System Cool
Always bleed radiators with the central heating turned OFF and the boiler circulation pump stopped. Bleeding while the pump runs can draw more air into the circuit or spray scalding hot water.
Step 2: Open the Air Bleed Valve Slowly
1. Locate the bleed valve on the top side corner of the radiator opposite the thermostatic valve.
2. Hold your collection cup under the valve nozzle.
3. Insert the bleed key and turn it counterclockwise a quarter-turn. You will hear a steady hissing sound of escaping air.
4. Keep the valve open until the hissing stops and a smooth, steady stream of water trickles out.
5. Tighten the key clockwise to close the valve securely (do not over-tighten).
Step 3: Repressurize the Boiler to 1.5 Bar
Bleeding radiators releases air and volume, lowering your overall system pressure. Head to your gas boiler, check the pressure gauge, open the bottom filling loop valve until the needle reaches 1.2 to 1.5 bar, and close it tightly.
FAQ
In what order should I bleed household radiators?
Start with the radiator closest to the boiler and work outward along the circuit toward the furthest unit. In multi-story houses, bleed ground floor radiators first before moving upstairs.
What does it mean if black, foul-smelling water comes out?
Black water indicates magnetite sludge and internal iron oxide corrosion in the pipes. If radiators have cold spots in the middle or bottom, the system requires a chemical power flush.