I take it you mean britony combi 80, it has a 3-way valve which changes between hot water and heating, they are prone to sticking while in either position. In my experience it is usually due to the system not being flushed after installation, the fluxes after a period of time cause corrosion which jam the valve. You can check valve operation, turn off power to boiler, select water only so heating is off, turn isolation switch back on and you should hear the servo valve motor turning for a few seconds before coming to a stop.
Engineer de Gaz
September 2008