Navien Combi Boiler relief valve actuating frequently

[mech-e]

I have a navien ncb 240-130 installed in January. It worked fine for the first few months, then the relief valve started venting occasionally, a few times per day, with a sudden burst of chatter for a few seconds then it stops. I'm not sure if this started when the heat was on. The heat has not been on for weeks and the frequency has been gradually increasing to many times per day, including when no water (heat, DHW, or cold water) is being used at all. We do not have DHW recirculation and the leaking happens even when the burner is not on. The pressure reading on the display jumps suddenly from 15 to about 34 while the relief valve actuates. There are no error codes. If seen discussions concluding that this may be an internal leak between DHW and heating sides of the heat exchanger.

2 questions:

1) Could it be something else besides a leaking heat exchanger?
2) What can I do to diagnose the issue without replacing parts to see what happens?

I use multi meters, pressure gages, etc. all the time, so I'll take suggestions and post the results.

 

[Twowaxhack]

@breplum

He’s the guy who can help you.

 

[arctic bill]

my first guess would be the expansion tank . and then the PRV for heating like the

 

[breplum]

I am NOT trained on combi units (because I didn't get into hydronic heating, just forced air). I've installed them and repaired other brands as well, so, I have basic understanding, but, frankly, when it is beyond my skill set, we call Navien (or brand's) tech support.
Unfortunately, they typically do not want to talk with anyone other than an experienced field plumber, just because there was too much time wasted with homeowners.
Sounds like possible problem with the pressure regulator on the make-up water. Is there a regulator and a backflow preventer? Odd but I'd guess a bad regulator, but hard to get a gauge anywhere to verify.