Shower gradually cools, gets hotter when sink is on. Navien tankless whole house HWH

Question

Our shower and a few other fixtures are behaving strangely. When I get into the shower it gets hot quickly and then gradually cools. But, I found that if someone else turns on a nearby sink (but not a distant one) with just the hot water then the shower gets nice and hot -- although then the sink isn't very hot. This is reversible, i.e. they can turn off the sink and then the shower cools down again.

Other sinks don't fix the problem for this shower, but if another shower is having this issue, then the sink in that bathroom can make the shower hot (even though previously that sink didn't help the master bathroom shower).

I first didn't believe this... perhaps my skin was just getting used to the temperature or something? So, I bought a testo 115i pipe clamp thermometer with bluetooth and recorded the whole series of events. It made a pretty plot of the temps and then I annotated the figure with the steps of turning the sink on and off (see below).

We have a Navien NPE-240A tankless hot water heater. It is 10 years old. Our system has a re-circulation line and the Navien unit will run the recirc pump intermittently to keep hot water in the line (and this works fine). When someone else was showering, I went to the Navien unit and watched the outgoing temp on the display and it was unchanged at 125 the whole time (with about 0.8 gallons per minute of flow).

I have noted a few others with a similar (unsolved) problem: Why does my shower only get hot when the sink faucet is on? and Hot water in showers gets very hot when other hot water is turned on

I had a repair person come visit, they looked at the unit and called Navien technical support. There were no error codes and everything seemed to be set properly in the unit. I have never cleaned/descaled it (I use Seattle public water), and I plan to clean it soon to see if that helps. I also have one other idea that I will try and then post as the answer (unless someone can figure it out first).

Edit:

Here is a plot showing temperature at the HWH recirculation pump inlet. The first 40 minutes of the plot show the temperature as the recirc inlet gets hot when the recirc pump is on, and then cools as the recirc pump is off. Near the end of the plot is a large drop of 30-40F (to reach almost the temperature of the cold water from the city) which happens when I turn on a sink. Once I turn off the sink, then the recirc pump turns itself back on and the temperature rises to about 120F and then slows cools after the pump is off and no fixtures are on.

Answer 1

Problem solved

I deduced that the recirculation pump backflow valve inside the HWH must be broken. The water at the shower cools because it is pulling water from the HWH "hot" side and also pulling water the wrong way through the recirculation line (which normally only flows INTO the HWH). This backflow water gets very cold since it is coming from the cold line of the city water.

Then, if I turn on a particular sink (one which is between the shower and the end of the recirculation line), that sink will get all of the cold water from the backflow through the HWH recirc line and thus the shower's hot water line will be 100% from the "hot" side of the HWH.

If I try to turn on a different sink to fix the shower (one which is between the shower and the HWH), then it will make it colder in the shower since then that sink will get the hot water and cause the shower to get more backflow recirculation line water.

This plot shows the hypothesis for what is going on:

So, I found the correct $6 part (NAVIEN 30008360A CHECK VALVE) and replaced it (after draining the system following some nice youtube videos). Here is a picture of the broken part next to my good one:

And, finally, here is an annotated temperature diagram of the system behaving properly. In this you can see that the temperature at the HWH recirc line inlet goes up and down as the recirc pump turns on and off. There are no changes to the inlet temperature when I use the fixtures: