On the top diagram, that hose at the very upper left hand side, see where the discharge line at the heat exchanger is, going from the top of the heat exchanger directly to the riser. That is where I would cut the line, run it to the HWH and then right back to the heat exchanger. That would appear to be the least critical hot water source.
Anyone else got any ideas? I have never hooked up a hot water heater that uses a heat exchanger mode like this one, but I'm sure it would deliver plenty hot water after running a while.
Tom