Cold at the bottom of a radiator is almost always sludge — iron oxide settled inside the radiator where the water sits still. Cold at the top is usually trapped air that needs bleeding out. Radiators that stay cold entirely can be a stuck thermostatic valve, a closed lockshield, poor circulation from an aged pump, or a system that's badly balanced so the hot water is all going to one side of the house. We diagnose the actual cause before suggesting work. A lot of jobs that get quoted as 'you need new radiators' are really a balancing issue, a stuck valve, or a power flush away from fixing properly.