Navien NR210NG temperature sensor?

[RobFII]

Hello all. Apologies if this has been covered before. I have a Navien NR 210 NG tankless and recently it has started to do the hot/cold sandwich. It has slowly gotten worse where the water will get very hot and then cold. I can feel and see the water pressure suddenly changing (bypass valve cycling?) then it goes cold and then warms up again and repeats. I have done yearly flushing of the system. At first I thought the problem was the flow sensor. When I pulled out the old, it did not spin freely. That didn't fix the issue. Can someone point out to me how the unit determines the water temp and where this sensor is located? I have combed over the parts breakdown and can't see anything obvious. The unit is over 10 years old but I would have expected to get a longer lifespan with regular maint. Thank you all for any help!

 

[breplum]

They’re indeed are thermistors, but they’re not listed in the parts breakdown. As per the sheet I’m attaching the diagnostic is to check the voltage and resistance.

 

[breplum]

Start planning for an updated WH. The only model we recommend is the NPE A2 series.
NR was a first round version and I’m not fond of them at all.

 

[breplum]

Also check cross connections at your house. Turn the cold water to the water heater off and open a hot tap. Water flow should stop completely. If it doesn’t then you have a cross connection. It’s very common for old price Fister single handle, shower valves or Kohler, single handle, shower valves and some KWC as well

 

[GReynolds929]

Is it an "A" unit, do you have a recirculation system?

 

[RobFII]

Thank you so much for the valuable info!!! I couldn't quite wrap my head around how it measured water temp with no thermistors!! I was afraid someone would say to start planning for a new WH. We have really enjoyed this unit up until a few months ago so I guess I shouldn't complain. I will post with my findings.

Also, how are the thermistors mounted? Looking at the images, they don't appear to have threads. Are they easy to replace?

They’re indeed are thermistors, but they’re not listed in the parts breakdown. As per the sheet I’m attaching the diagnostic is to check the voltage and resistance.
View attachment 46381

 

[RobFII]

Is it an "A" unit, do you have a recirculation system?

It is not an A, no recirc pump.

 

[GReynolds929]

The thermistors are usually secured by one or two screws and sealed by an oring.

Outlet thermistor PN: 30002655A

It could also be the bypass mixing valve PN: 30004837B