Central heating problem

[ArchieBoy]

Hello all,
I would appreciate some help in diagnosing a problem with a central heating system which uses a Potterton Performer 24 combi boiler (about six years old). Until recently the system had worked fine for both hot water and heating with the temperature controls always turned nearly to maximum, but now when central heating is selected the water quickly overheats accompanied by banging, overpressure and the thermostat tripping. By turning the central heating temperature control down to about halfway I can keep the boiler operating but there seems very little flow around the heating circuit - the upstairs radiators slowly get warm but the downstairs radiators stay cold. As it appears that the boiler works OK in hot water mode I thought maybe a blockage in the heating pipework was the trouble, but I have flushed out the pipework and radiators without seeing any significant sludge or flow restriction. Has anyone else had this problem and if so how did you fix it? Is there anything in particular you could suggest that might affect the heating like this but not the hot water operation?
Thanks

 

[gas4you]

DV could be stuck on HW.

 

[diyhandyman]

or a faulty / stuck pump possibly

 

[gas4you]

As the boiler is working ok in HW mode, I doubt it is the pump.

 

[methaneman75]

it could be the pump if it does not have a plate heat exchanger, cant remember if this model has a twin pass ex, if it has ,most of these types only use the pump on ch, any idea dave?

 

[gas4you]

This has a plate HE. I have fitted several of them in the past.

The 'i' version also had the pre-heat 'tank' incorporated into the exp vessel.

 

[methaneman75]

ok cheers mate, that rules that out