after looking again at your diagram, it seems something is amiss. Did your return line from your zones originally run back into your propane unit?
My setup has my original install of the propane system and zone as a completely closed loop. My install of my OWB is also a closed loop and the two systems are "connected" by the heat exchanger.
However, on my original propane/infloor heat system, the return line from the zones came back into the propane furnace. On your diagram, it's not clear that is the case. According to my understanding of your diagram, both the propane furnace and the OWB feeds a common manifold that feeds your zones. Then, the return line from your zones flow back to the OWB. The feed loop on your propane unit comes from where? As long as your propane furnace has cool water coming back from the zones, it will fire...does that make sense?