Radiator problem

[lsl]

We moved into a house with gas central heating last winter and realised that some of the radiators did not heat up at all or cold at bottom. Also we had to bleed them all the time. We had the system flushed out in February and all the radiators were then working fine before the central heating system was turned off for the summer. When we turned the system back on last week we had similar situation although not as severe. We bled the radiators and a couple of the radiators on first floor are still cold at bottom. The water pipes into the radiators are hot but the return pipes are cold. Can anyone help as we can't afford to have the system flushed all the time?

[gucciman]

what type of heating system do you have? is it a combi or a traditional type with a cylinder and tanks in the loft? if the radiators are not getting hot and you are having to bleed them constantly then the system may be sucking in air and needs looking at.
if you have a radiator that is completely cold and you have thermostatic radiator valves fitted then it could be that the trv is stuck. you can try taking off the head of the trv to expose the little pin, then spray some wd40 on the pin and try to loosen it by gently pushing on it with the end of a spanner or something like that, you can also try tapping the side of the valve under the pin to see if that helps.
you also need to make sure that the valve at the other end of the radiator is not closed off (very common) this will have a smooth conical cap on it which you need to remove and then you can use some small pliers or spanner to turn the valve, anti-clockwise for on, clockwise for off.

[lsl]

Thanks Gucciman. It is a traditional one.
Will follow your instruction to have a look this weekend regarding the valve.
How can I tell whether the systme is sucking in air and if so, is there anything I can do to fix it?