I have a Vaillant VCW 25 Combi Boiler. The CH is working but when the hot tap is turned on, the burners fire-up and remain burning until the demand for hot water is terminated but the water out of the tap is only 'luke warm'. The diaphragm has been changed by a plumber but it had no effect. He now suspects the DHW heat exchanger, but can't guarantee it. Is there any test I can do to prove that it is the heat exchanger? Hot water going into Heat exchanger about 70 deg, and dropping to about 60. The cold water is going in at about 6 deg and I am getting about 25deg out.