I have a very similar system and a very similar 'problem' (not sure it's a problem). Mine is fed not from a tankless, but from an outdoor wood boiler feed loop going to a plate exchanger. Over time the pressure in my system drops to zero, but will rise to 5-8 lb when it has been running for a while. I do have an expansion tank, a Spirovent which burps or hisses occasionally, and absolutely no evidence of leaks, and my loops were pressure tested before and after the concrete was poured. I'm guessing that the system somehow is still bleeding off air, and that's after three years of running it. I do use a small pump to put water in the system once in a while, and it'll bring the pressure up to around 10 psi, but it will drop down again during the heating season. I'm not sure there's a solution, but for what it's worth, the system heats my garage just fine. Constant 60°F this winter here in Vermont.
Jon Endres
Killing Trees Since 1983