overflow on ferolli modena 102?

can anyone help , have problem with continuous leaking of water through overflow from combi boiler, wether the system is on or off water still carries on through overflow the pressure is stable at one and a half bar,and everything seems to be working ok regards heating and hot water , but worried about this continual leakage, about a litre every 15 mins , when the heating is on it comes out hot,