I have fitted newer modern radiators to replace the cast iron radiators in two of our bedrooms. However, these are not warming properly at all, even though I have bled the air out completely - they initially fill with hot water then, but the heat disappears so I can only assume the water is not flowing through properly.
These have isolation valves and TRVs fitted, so I wonder if the smaller bore of these compared to the larger bore of the old radiators and fitting is constricting the flow? I have removed the top sections of the TRVs on both the flow and lockshield ends, i.e. they are fully open and the pin moves freely to open position, but this does not make any difference.
I have also tried closing all the other radiators in the flat to "force" the hot water through the new radiators, to no effect - perhaps because we are on a communal system feeding all the flats in our block this kind of balancing would not work anyway, ie. as the other radiators in other flats on the circuit would be fully open?
I also tried disconnecting the pipe from the valve on both the flow and return sides to make sure water flows freely and there are no blockages. When I disconnect from either end, the water flow through the radiator and out the other end into my bucket freely - and its hot. Its just when I have both ends connected, the radiator slowly loses heat (again, even after I have checked for air and bled it).
Running out of ideas (am replacing the TRVs with standard valves next to see if that helps), so advice would be appreciated.