I am only going by my experience when I changed the pump on my heating system, not the same boiler as yours.
I lived in a heavy lime area and there was a build up of lime in the inlet from the header tank at a tee joint the feed into the tee was restricted halfway down the tee branch so restricting the water flow and the boiler started 'Kettling' which is also caused by build up of muck in the system.
I discovered the problem when I tried re-filling the system after fitting the new pump, it simply took ages and ages to fill so I did a check being the easiest to get at thought it might be a piece of chipboard that fell into the header tank had got stuck in the tee.
As I say I'm guessing and going by my experience so how a c/heating engineer would check that I don't know.
bob
bob, September 2005
