24 CDI Water Problem?

I have a 24 CDI. The hot water is only running luke warm. I have taken off the microswitch on the diverter valve and switched it manually, the boiler fires up as it should and the temp dial is set to max. The heating seems to be working normally Ie. rads are hot.
I think the diverter diaphragm needs replacing also as it is not consistent lately, could it be linked?
Thanks in advance.