na hot water with halstead ace high?

i have had a halstead ace high fitted for about 18 months.the heating works ok but i have no hot water.I do not think the old system was flushed before the halstead was fitted.a plumber has told me that it is likely to be the thyristor valve or the flow switch.could anyone please tell me if it is likely
to be any of these parts or something completly different.
thanks