Plumbing cross over more pronounced after Navien tankless water heater install?

[GinoDivx]

Hi all, I decided to ask for some advice, after spending quite a few hours troubleshooting myself. I am a homeowner, with very basic plumbing knowledge, but I understand general principles
We replaced our aging 50gal power vent gas water heater with tankless Navien tankless water heater (top of the line 240A2 model) about 1.5 months ago. Basic plumbing remained the same, the house is ~15 years old, and we have a dedicated hot water return line. Connections seemed to be made correctly, and external recirculation seems to be working fine.
Immediately we noticed a significant drop in temperature, and maybe even the pressure in the main bathroom shower. Especially if next to the shower sink faucet is on. Water becomes lukewarm in the shower, until the faucet is turned off again.
I called Navien a few times, and we went through the diagnostics procedures. They at first suggested replacing the check valve, which they sent to me, and I replaced it. No change.
After the latest session, they suggested that our house has pluming cross over, and tankless water heater made the issue much more pronounced, compare to regular water heaters with more consistent flow and water pressure.
So, I went out on the quest to find cross over. I shut off hot water coming from tankless water heater, and went on, to open all hot faucet handles and showers.
There was eventually no water coming out of any opened hot faucets and showers, except for two:
- Jacuzzi hot tub had a steady stream of cold water (not as much as from cold faucet though). I opened the service door and I could see a mixing valve, and check valves. By adjusting mixing valve, I could change the amount of cold water coming out of hot faucet side somewhat, but I could never completely shut it off. Don't know if this is by design, or this is the problem.
- Shower in question in the main bathroom also had some cold water still flowing, even when turned all the way to the hot (110F I believe). It also has a thermostatic mixing valve, so I am not sure if this is also by design.
Now that I have described the situation, I wonder if someone smarter than me may have some thoughts.
Thanks a lot in advance!

 

[breplum]

Thermostatic shower or tub valves have check valves and that is where would look to the problem being.

 

[GinoDivx]

Thank you. Based on your reply, should I assume that I should not have ANY cold water coming out of the shower when I turn it all the way to the hot? This is with hot water shut off at the heater of course. Thanks!

 

[breplum]

With cold water shut off at the WH, there should be no water flowing at any hot tap.

 

[GinoDivx]

Thank you, now I will need to talk to a real professional

 

[Twowaxhack]

I’d replace the cartridge in the shower faucet having the issue.

I don’t believe you have a cross over flowing enough that would cause your problem, but you might.

Just a wild guess.

Try turning off circulation and isolating the recirc line with the cut off valve for it. Then check to see if you can recreate the problem.

Also try running a lower volume of hot water at the shower OR turn the tankless to 115 degrees and see if you still get temp swings.