Radiators cold at bottom

[Just4Fun]

A minor mystery with our heating system. A couple of our radiators were hot at the top but cold at the bottom. Not an unusual problem I guess. Normally I would think this would indicate sludge in the rads but I am puzzled because:

1. These rads are upstairs and I would have expected sludge to accumulate downstairs rather than upstairs.
2. These rads did need to be bled a little, and this seems to have fixed the problem. I would have thought air in the rads would make them cold at the top rather than at the bottom. Am I wrong about that, and if so, why?

 

[Sachakins]

Sludge settles each time the system is off. Once settled it coagulation. When system comes on again, that sludge is harder to displace, thus buibing up wherever it lays.

Radiators are designed so as water enters, the initial path takes heated water to the top of radiator first.
The air then prevents it circulating so the heated water flows across the top.
Air is compressible an acts as a bung against the water as pressure rises.
, hence preventing water circulating correctly to the bottom.