hi, i have a myson boiler approx 18 years old. has worked fine all this time, had a few spares thermocouples, thermostat 2 x controllers and 3 way motorized valve. i have just fitted a new 3 way valve (actuator only) and noticed a 15 mm dia copper pipe with a shut off valve. this pipe is between the pump and 3 way valve not sure what this is for but also should the shut off valve be fully open or closed (is it to refill the system ?) the system is an open vented sytem but i cant see where the pipe goes to