Servicing Navien NPE-240A for Error Code E438

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Burlingame, CA
I have a Navien NPE-240A in a closet, which also contains our home networking equipment. It is about 3 years old in a new construction house. Lately, it had been tripping our AFCI circuit breaker, which I learned are prone to nuisance tripping. Each time the circuit breaker was reset, the Navien flashed error E438.

I replaced two AFCI breakers that were constantly tripping, which did mitigate the tripping, but it happened again, which not only takes down our hot water, but also our home network.

I'm generally a DIY and after some research, bought several parts that could be the root cause for both the E438 and possibly the AFCI tripping.

I have the following replacement parts:
  • Flow Sensor
  • Circulation Pump
  • Check Valve
  • Inlet Filter
The issue I'm having is that I cannot drain the heater to begin servicing it. I took the following steps:
  1. Unplugged unit
  2. Closed gas line
  3. Closed cold inlet and hot outlet valves
  4. Connected hose to cold water drain and opened valve
In all the videos I watched, the water drains to a trickle very quickly. In my system, the water just flows continuously. I tried it a second time and this time opened the pressure release valve. Not only did the water flow continuously, but it also overflowed the condensate drain and made a huge mess. I couldn't get it to stop until I turned off the main and only later realized that closing the pressure valve stopped the water.

In some videos, I've seen a valve on the circulation pipe, too, which my installation doesn't have. Is this my problem? I assume water is flowing back into the system and out the cold water drain, even though the cold water inlet is closed. Is there an internal valve I can close to isolate the Navien for servicing?

Attached is a picture of my installation.

Hey, wait a minute.

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