I believe you are referring to the engine coolant circuit that goes through both the water heater and the helm controlled cabin heater. I believe you will find owners who recommend draining that circuit when doing a coolant change and other owners who do not. There are two valves on the front of the engine that open or close this circuit. When I did my coolant change, I chose to close those valves and only drain the engine block coolant as I was concerned with introducing air into the hot water heater/cabin heater circuit. This meant I was not replacing all the coolant, but since I do this about every three years, I was not concerned with mixing old coolant. Your 2015 D4 probably has Volvo yellow antifreeze in it. I believe that for the last year or two, you can only purchase Volvo orange antifreeze. But the orange antifreeze is advertised