possable blockage

radiators are playing up and the header tank keeps overflowing could anyone tell me if that is a sure sign of a blockage

No, it isn't. Much more careful analysis required.

i have had the system cleaned and been told the pump is ok i seem to get one of the rads upstairs always filling with air, i know the system is old but never had problem befor my hot water is very hot but the rads are no how the plumber had problems trying to balance them now sum get hot some dont please help

header tank keeps overflowing

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Depends what you mean.
Intermittent drips outside?
Water coming out of the vent which is over the tank going back into the tank?
Something else?

i have had the system cleaned

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It depends how.

Poor circulation is due to one or more of:
knackered pump
blockage - single or distrubuted, due to sludge
valve problems
poor design
air

You need a Heating Engineer to measure what's going on. Old systems with accumulated problems are often not made good by one simple action.

many thanks for replying chris the water was flowing out side through the overflow not dripping it has only done this a few times so i switched the pump of waited a while and put it back on at a lower speed ( the plumber told me to try ) I cannot believe there would be a blockage as we had the stuff put in the tank in the loft and ran it for 3 or 4 weeks he then came back took all the rads of in the garden and cleaned them out he even put some wire way down some of the pipes to check for blockages but found nothing, the hot water in the tap is very hot, the hot water in the 2 pipes from the boiler are also hot along with the pump and a few of the rads get hot if i switch the others of, the rads that fill with air when i bleed 1 of them the water spurts out but the last one water only trickles out when i bleed it unless i open the bleed valve fully (nearly lost it when it blown out) could all this be the pump causing this even though he said it seemed fine many thanks for helping

i also have now like bubbling water in some of the rads like there isnt enough water in the rads plus the over flow went again could anyone help

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