Why would a radiator be hot at the top and cold at the bottom?

Hereof, what does it mean when radiators are cold at the bottom?

A radiator that is cold at the bottom is usually caused by a build up of sludge in the system. As the sludge is heavier than water, it settles at the bottom which prevents water and heat flow to the bottom of the radiator.

Also, why is only half my radiator hot? Check for Trapped Air and Bleed Your Radiators If just one (or a few) of your radiators aren't heating up, the most common reason for this is trapped air. If you've just turned the heating back on after the summer, air can become trapped in your radiators, causing them to be warm at the bottom but cold at the top.

Accordingly, should radiators be cooler at the bottom?

This is because in convection heating systems the hot water rises to the top naturally forcing cooler water back down to the bottom where it is reheated in a cycle. And, to a certain extent, all radiators should be cooler at the bottom than at the top, but they shouldn't have cold patches.

Why is my radiator cold in the middle?

The middle of the radiator is cold Potentially caused by a build-up of debris or sludge obstructing the bottom part of the radiator, cold patches you find in the middle can be fixed by cleaning your radiator thoroughly. In some cases your radiator will need replacing.