I think Navien strongly recommends that your combi is piped primary/secondary, which mean you need two pumps, the one in the unit itself and an external circulator pump. That system allows the two loops to work independently of each other, such that the demand of the radiators in the house on the secondary loop does not affect the flow through the heat exchanger which is driven by the internal pump. That way you are always assured that the temperature and flow through the combi itself remains within the operating parameters. (For a lot more information, Google the term "closely spaced tees" and it will explain how a P/S system works.) I think some people, if they're only operating one zone, have opted to go direct and simply use the internal pump as a system circulator, but if your system is multi-zone, you're asking for trouble. Not an expert, but that's what I've been able to glean about the process.