Adding an outdoor coal/wood unit to heat an outbuilding and supplement house through Rehau underground pipe. I could humbly use some help with how to tap into my radiant floor and domestic hot water system via flat plate exchangers. I had originally hoped to simply preheat the return water to the combi unit but Navien told me that won’t work on either the space heating or DHW side of the unit due to flow switches triggering the burners. How can I make this work so that the outdoor heat source will provide almost all heat but if fuel runs out or on vacation, etc. the LP gas combi unit will fire to heat the floor as well as DHW? Not as simple as I hoped I’m sure. There is a recirculating loop for DHW connected to an Aquastat on the right.