top of radiators are very hot but bottom cold

[bandicoot66]

Since putting in a new boiler and thermostatic valves on my one pipe sytem central heating and flushing out the system twice, all radiators are very hot at the top but cold at the bottom. When the old non thermostatic valves are put on, all radiatora are hot, top and bottom. I would like to use thermostatic valves. Are there many types of valeves or is it a different problem?
Please help
thanks

 

[oilman]

Ah, this is useful. Thanks for posting this, I had never thought of the problem before.

Single pipe radiators get their heat from convection currents from the main pipe. This works reasonably well using manual valves. Thermostatic valves have much smaller passages than manual valves, and this may well be stopping the water circulating fast enough to heat the whole of the radiator.

Are your rooms getting warm enough? If so, is there a problem?