Why is my radiator cold at the top but warm at the bottom?

Cold at the top with warmth at the bottom is the classic sign of trapped air. Air rises to the highest point in the radiator and displaces water, so the upper section cannot heat. Bleeding it releases the air and lets water fill the space.

The process is simple. Turn the heating off and let the system cool, so you are not releasing hot water. Locate the bleed valve, usually a small square fitting at one upper end, and hold a cloth and small container beneath it. Turn the key slowly anticlockwise until you hear a hiss, wait for the hiss to stop and a steady dribble of water to appear, then close it. Do not open it fully.

After bleeding, check the system pressure on the boiler gauge. Releasing air lowers pressure, and if it drops below the normal range the boiler may not fire. Topping up via the filling loop restores it, and the correct range is usually marked on the gauge.

If a radiator needs bleeding repeatedly, that indicates air is entering the system, which is not normal. Common causes are a leak drawing air in, a failing automatic air vent, or corrosion inside the system generating hydrogen. This is worth investigating rather than bleeding endlessly.

A different pattern points elsewhere. Cold at the bottom with a warm top usually means sludge, meaning corrosion debris settled in the base, and that needs flushing rather than bleeding. Cold across the whole radiator while others are hot points at a valve, either a closed one or a stuck thermostatic valve pin, which can often be freed by removing the head and gently working the pin.