Radiator not heating up warm on top cold at bottom

Dependent upon the age / condition of the radiator, the rad requires removing and flushing through as the problem indicates a build up of sludge , hence why the rad is cold at the bottom and the heat is being transferred to the top via convection. Alternatively if cost effective just replace the rad which will resolve the issue

If your radiators are cold at the bottom, but at least a few are warm at the top, that’s a good sign. This radiator problem is caused by central heating sludge. You can rectify this issue by either Flushing out the system with special chemicals to remove sludge, fitting a scale reducer and a boiler filter, in some cases sometimes it’s better to replace the radiator or simply just balance your central heating system could work.

You’ve got a build up of sludge if it’s in more than one radiator I would suggest getting tour central heating power flushed if it’s just one or two radiators then remove them and attach hose pipe and flush them, can use chemicals. Also when you put the radiator/radiators back on make sure you put some inhibitor in the system this will help prevent this happening in the future. The sludge is basically your pipes and radiators rusting from inside due to not enough/no inhibitor in the system. I’d get a plumber to do it for you, taking radiators off if you don’t know what your doing can get very wet. While he’s there get him to install a magnetic filter so that any sludge can be caught.