Radiators hot only at top and sides

[reo49]

Most of my double panel radiators are hot [normal heat] around the sides and the top, leaving a large area cold or some are luke warm in a very large area in the centre. Two single panel radiators are heating as normal.

Had a pipe [outside] to the boiler repaired during the summer as it had burst.Plumber did not drain system down to do this but his mate did something in the loft before they started which seemed to be important before they could begin also at the same time had a valve replaced on boiler [oil fired] at top right as it was leaking [think this may be some type of pressure valve?]
Afterwards they bled some radiators as I have done now, no air just water.
Not blaming the plumber as he is a family friend, just wanting some advice and giving a bit of history.
Did not know rads were not heating correctly until now [winter]
Any help would be greatly appreciated.

 

[ChrisR]

Problem is SLUDGE - do a search

 

[reo49]

Many thanks for such a quick reply. I have been searching through and did notice the SLUDGE problem. I thought it might not be sludge because it is such a regular pattern in all the radiators [except the two single panel radiators which are heating evenly all over] - the hot part of the radiatiors is a 4 inch margin/band along the top and down each side.

 

[reo49]

Does sluge happen suddenly because the radiators were working before the repairs were carried out. There had been no problem/history with the radiators until now and 8 double radiators are all affected in the same pattern.