Radiator cold at the top or bottom? They mean different things

Run your hand over a heated radiator that's not performing and note where it's cold — the pattern tells you the cause.

Cold at the top: air

Air rises, collects at the top, and stops hot water filling that part of the panel. The fix is bleeding — releasing the air through the small valve at the top corner. Two minutes with a bleed key; see our step-by-step guide.

Cold at the bottom or middle: sludge

Magnetite sludge — rusty deposits from inside the system — sinks and settles along the bottom, blocking flow. Bleeding won't touch it. Fixes range from removing and flushing that one radiator to a chemical cleanse of the whole system, plus a magnetic filter to stop it coming back.

One radiator cold, the rest fine?

Check its valves are open (including the plastic-capped lockshield). If a thermostatic valve head is stuck, removing the head and gently freeing the pin underneath often revives it. If the whole system heats unevenly — upstairs roasting, downstairs cool — the system needs balancing, which is an engineer job with gauges rather than guesswork.