it is a worcester combination boiler and it seems to be an intermittent fault.
Have you checked the pressure on the boiler is between 1 2 bar. If it is then you will have a problem with either the plate to plate heat exchanger, a faulty thermistor or the diverter valve is passing. Have a look on corgi`s website for an engineer near you
I assume your boiler provides both heat and hot water...so you have a coil in the boiler that gives you hot water? This sounds like a control problem. There is an aquastat that senses when the boiler calls for hot water. It may be malfunctioning. It is best to have your boiler serviced by a reputable and licensed plumbing contractor.
