when dhw on rads heat up changed heat exchanger problem still there hot water takes longer to warm up

dave, October 2016

That will be because the heated water is being shared between the radiators and the hot water storage tank when it should not be being shared. Consider that the diverter valve is stuck in its mid position. You can override it manually.

I just guess - I have no experience or qualification here.
The heat exchanger is not involved in where the hot water is sent to.

Good Luck..., October 2016