Central heating not working correctly

[Bmt]

I have recently developed problems with some of my central heating rads not getting hot. when the roomstat is calling for heat the boiler fires up and stays on for about a minute switches off for 2 or 3 minutes then fires up again and the underfloor pipework start banging like expansion noises for a couple of minutes. The flow pipe from the boiler gets hot but the return pipe is barely warm.The radiators near to the boiler get hot including the drop loop radiator but the others stay cold or just warm. But every now and then the system works correctly with every radiator reaching maximum temp. and all the expansion noise gone, it runs o.k for say an hour then reverts to noisy pipes and cold rads. My question is it air locks? if so why does the water pump around correctly every so often please help, I am not sure wether to have the system power flushed to clear blockages

 

[gas4you]

We really need a lot more info on your boiler and system, but,
first bleed all rads and check for air.

Then check the pump is spinning correctly, then look at any zone valves you may have on the heating.

 

[gas_man_gaz]

like gas4you says need more info, but from wat ya say sounds like the pump stickin.. check if its a myson

 

[Bmt]

Thanks for replying so quick, my boiler is a Potterton combi 80 and when I first started experiencing poor circulation I had a heating fitter fit a new pump this was 3 weeks ago I called him back as this hadn't cured the fault and he suggested that the system might need a power flush which I am reluctant to do as this costs £300 upwards. My heating system is simple 3 rads up 3 rads down the system is a sealed system which started life as a vented system 25 years ago, I had the present boiler put in 4 years ago and up until now it has been no trouble at all, I am reluctant to get it power flushed as being a sealed system I cannot see how there could be any blockages.

 

[gas4you]

New diverter valve and/or wax capsule in it.

 

[Bmt]

No new parts since new except the new pump

 

[gas4you]

Sorry, that is what I was suggesting you might need to fix it

 

[Bmt]

Thanks Dave will look into this
Brian