A radiator that is cold at the top but warm below often contains trapped air. Other causes are possible if the pattern persists after bleeding.
Bleed Keys for Trapped Air in Heating Panels
When the top of a radiator stays cold while the bottom warms, trapped air may be the cause. A radiator key opens the bleed valve just enough to let that air escape. MacBlair includes this small maintenance tool in its plumbing and heating range.
Before buying, check the bleed screw profile. Many panels use a square spindle, but not every vent is identical. Keep a cloth and small container ready. Turn the heating off, allow the radiator to cool, open the vent gently and close it once water appears. Do not remove the screw.
On a sealed system, bleeding can lower pressure. Check the boiler instructions before topping up and investigate repeated air build-up rather than bleeding indefinitely. System cleanliness and corrosion control also matter; see heating inhibitors and cleaners and the wider boiler range when planning maintenance.
Radiator Key FAQ's
No. Turn it off and let the radiator cool so hot water does not spray from the vent. Follow the boiler or radiator instructions.
No. Many vents have a square spindle, but shapes and sizes vary. Check the vent before buying.
Open it gradually, only enough for air to escape. Close it as soon as a steady trickle of water appears; do not remove the screw.
Repeated air can point to water loss, corrosion or another system fault. Have the installation checked rather than continually bleeding it.