Quick answer
If a radiator is cool at the top and warm below, trapped air may be the cause. Follow the boiler and radiator instructions, use the correct key and check system pressure afterwards.
Before you begin
Radiators can be hot and water may escape from the bleed point. Turn the heating off and let radiators cool. Find the manufacturer's instructions for your boiler, as sealed systems may need pressure checking after bleeding.
Use a suitable radiator key and cloth. Do not use pliers on the bleed screw or force a damaged fitting. If you see corrosion, a leak or a loose valve, stop and call a professional.
Bleed the radiator
- 1Turn off the heating and wait until the radiator has cooled.
- 2Place a cloth and small container beneath the bleed valve, usually near the top corner of the radiator.
- 3Fit the key and turn the screw slowly a small amount. Air may hiss; keep your face and hands clear of the outlet.
- 4When air stops and a steady trickle of water appears, close the screw gently. Do not overtighten it, then wipe the area dry.
Check pressure and operation
Check the boiler instructions and pressure display. Bleeding can lower pressure in a sealed system. Only top it up using the model-specific user instructions; stop if the filling controls are unclear or the pressure does not behave as expected.
Turn the heating back on and check whether the radiator warms evenly. If it remains cold, makes persistent noises or loses pressure repeatedly, the cause may not be trapped air. Do not open the boiler casing. Gas work must be carried out by a Gas Safe registered engineer.
When to call a plumber
- The bleed screw is damaged, stuck or leaking.
- The radiator stays cold after bleeding.
- System pressure repeatedly falls or the boiler shows a fault.
- You find corrosion, dampness or water around a radiator valve.