Unusual problem with Worcester Heatslave 15-19

[fatboy123]

Original problem was the dropping off of the hot water temperature when it had been running for 5 minutes plus. The central heating kept coming on sporadically when the heating was switched off.
I stripped down the diverter valve and noticed that it was sticking so I replaced both the body and the head. This seems to fix the problem for a fortnight, but now the problem has started reoccurring (though not as bad).
I don’t have a lot of experience of oil boilers. Does anybody have any ideas what the problem may be and how it could be fixed?
Thanks

 

[dontknowitall]

My guess is it's something to do with the water as opposed to the boiler producing the heat in the first place. Worth asking if it's been serviced in the last couple of years because if it hasn't it might have sooted up (soot = insulation).

I start with the flow pipe to see if it's creating enough heat. Then follow this to the pump. Ensure it's the same temperature on both sides of the pump and keep following the pipework to see where the heat's dropping off. Could be the paddle is stuck in the zone valve?

 

[SimonG]

When you changed the head and valve did you put inhibitor back into the heating system when you refilled it? It helps lubricate the motorised valve.

 

[kimbo]

Is the diverter valve the piston type or ball type

 

[fatboy123]

piston type

 

[kimbo]

Yes these are a constant problem I would take the piston out and make sure it moves freely up and down, also a bit of silicne grease would help.