Navien + NaviCirc + tank water heater = can it work?

[Swrobel]

We recently purchased a 3-story duplex, and each unit had its own tank water heater, leading us to believe that the plumbing was separate for each unit. The heater on the 3rd floor of the upper unit (ours) was due for replacement, so we had our plumber replace it with a tankless Navien NPE-210A2. Times to get hot water to some of the taps in our unit were unacceptably high (>1minute), and there's no return line, so our plumber suggested installing a NaviCirc under the farthest faucet in our unit, but that didn't improve the situation at all, even at that faucet. He did some troubleshooting and eventually tried turning off the Navien and lo and behold, we still had hot water in our taps, which he said indicated a crossover with the plumbing of the lower unit that allows its tank water heater for the lower (rental) unit in the garage to provide our unit hot water. He didn't seem to have a straightforward solution to this problem and said he needed time to think it over.

Does anyone have any ideas here about what the most straightforward solution to this problem is? We don't need instant hot water, but would certainly like to get it more quickly than we are currently.

As an aside, I suppose it's possible that the NaviCirc we received was bad, and I could get another one and try it out. Is it at all possible that the Navien + NaviCirc would work in this configuration, with the other hot water heater in the loop? The tank heater has an expansion tank, if that's relevant.

 

[Fitter30]

You can see the flow rate through heater under basic information even when the recirculation is on. Since the heater on the 1st floor is supplying hw system has a cross over this has to be investigated because even if there is flow through the navien who knows where its going.

 

[Sponsor]

 

[Breplum]

I do know that LADWP has a variety of disenfection methods depending on the area.
Up here in the north of the state, they add chloramines and that wreaks havoc on the o-rings in the check valve, to the point that they can bulge enough to cause leak-through bypass. So identify if the check valve is ok.
Which, depending upon the setting of the recirc lever inside the NPE A units, may or may not be your bypass problem.
Easy to check, just close the in and out bypass valves at the Navien unit. Then turn on hot water faucet anywhere and see if the flow stops completely. If it doesn't stop completely, then you have bypass problem in the house. Isolate the navcirc to confirm it is not at fault.

 

[Swrobel]

As an update, I installed a Navicirc under the primary bathroom sink on the 3rd floor and it seems to be working great.