Here is quick drawing of the previous set up. The new set up has the tankless in place of the two conventional units. Nothing else has changed besides some piping adjustments to move the unit over to the wall.
So here goes. I previously had two 35 gallon tanks in series, feeding household domestic water needs a well as 2 radiant heat zones. . The hot side of the first fed my 2 zones of radiant heat as well as the input for the second tank. The return line from the heat zones, along with the main clod...