Reading a radiator's cold spot, bleeding it safely, and telling trapped air apart from settled sludge.
A cold upper section
Gas can collect at the top of a radiator and stop that part filling with hot water. So a cool strip along the top, above a warmer lower panel, points to trapped air or other gas. Check the radiator itself: the pattern tells you what to check next, not what the fault is.
Safe bleeding
Switch the heating off and let the radiator cool. Protect the area round it from water, open the bleed valve gently as instructed, and close it once the gas is out and water runs steadily. Hot water can scald, and the system is still under pressure with the pump off. On a sealed system, check the cold pressure afterwards and top up only as the filling procedure says.
Check the radiator heats properly again. If gas keeps coming back, investigate: air getting in, filling problems or corrosion. Bleeding it again will not tell you which.
A cold lower section
Sludge settling in the bottom can block the lower passages, but poor circulation can also leave the bottom cold. Check the valve positions, the flow to that branch and the water condition before choosing a cleaning method. Bleeding removes gas, not sludge, and a cold bottom is not, on its own, a reason to power flush.
Black or dirty water
Black iron-oxide sludge shows there is corrosion in the circuit. It does not tell you the inhibitor level now, or whether the sludge formed before the present treatment went in. Test the water and inspect the circulation. If it needs cleaning, use a method that suits the equipment and its condition, then check the result and restore the inhibitor level and filtration. Find out why the system keeps needing fresh water: every top-up brings in more oxygen.