Could this be a problem caused by the boiler or is it with the wiring?

I have a glow worm ultimate boiler. The boiler is 12 years old and appears to be functioning perfectly. The problem i'm experiencing with the system is having changed 3 way valve. Heating and hot water is fine, but when calling hot water only the heating still comes on but radiators are very hot. The water is ok. could this be a problem caused by the boiler or is it with the external system wiring?