A leaking radiator can almost always be repaired without replacing the unit and without calling a plumber.
Step 1: Find the Exact Leak
Dry the entire radiator with a towel. Place white kitchen paper along the floor under the radiator. Turn the heating on and wait 15-20 minutes. The paper shows exactly where water is dripping from.
Fix 1: Leaking Bleed Valve
Close the bleed valve fully by turning clockwise with a radiator bleed key. If it still leaks, wrap the thread with PTFE tape and retighten.
Fix 2: Leaking Thermostatic Valve
Tighten the gland nut half a turn with a wrench. If it still leaks, close both valves on the radiator, unscrew the gland nut, wrap PTFE tape around the spindle, and reassemble.
Fix 3: Pinhole Leak in Radiator Body
Pour radiator leak sealer directly into the system through the feed tank. Run the heating for several hours to circulate it. It seals pinhole leaks permanently from the inside.
Fix 4: Leaking Pipe Connection
Turn off the heating. Disconnect the leaking joint, clean the threads, wrap with PTFE tape, and reconnect firmly.
Preventing Future Leaks
Add central heating inhibitor to your system every year to prevent the internal corrosion that causes pinhole leaks.