New extension, some rads cold...

Right here goes....

Had a new extension built but I think the plumber fed the two new upstairs rads from downstairs...not 100% about this but there's no holes in the walls upstairs, etc.

Old system
Now. The old upstairs rads work fine. Downstairs, one rad, where the feed from upstairs goes down is very hot. There are two other rads downstairs - both big ones. One is warm to mildly hot and one is cold and barely heats up and is the biggest one in the house.

I tried to balance them all, but with mixed results. I've no problems with the old ones upstairs, they all heat up. But if I turn off one completely (both valves to - minus) then the ones downstairs seems to heat up. I did an experiment and minused all of them apart from the two new ones upstairs (there are no new ones downstairs). Now one heated up perfectly but the other was cold, but I felt it getting warmish. I then put the big old one to + downstairs and the pipe going to it is hot, but the rad is warmish at the top but cold elsewhere....when i did this, the new ones upstairs went cold.

Any ideas, I reckon it could be a weak pump unable to handle the two new ones, or is it something else. Balancing seems a pain as I really dont know which ones are closest to the water tank..

I havent got a combi, but a water tank in the closet of one room.

what did the plumber who fitted them say or do about it?

Nothing....he did a runner when it was done because the contractor didn't pay him. I did all the remaining plumbing work (i.e. leaking toilets, fitting shower, etc.).

The rads do work tho. The new on in the bathroom was very hot, but only because most of the others were turned off. I turned the one in my sons room back on and the new bathroom one went just warm...