Cold water mixing with hot water - Navien tankless water heater, NPE210A

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Vista, CA
I have a Navien tankless water heater – NPE210A. Two weeks ago, the hot water started turning colder after one of two minutes in the shower.

Navien’s support had me troubleshoot the issue as follows:

1. Turn the unit off.
2. Close the isolation valves to the cold-supply and recirculation lines.
3. Open the pressure relief valve.

In my case, water was coming out from the pressure relief pipe. This indicated that the issue was not with the unit but with the house’s plumbing. According to Navien, had the issue been with the unit, water would not come out from pressure relief pipe.

4. With the pressure relief valve open, I check every one-handle fixture in the house.

In one bathroom, I can clearly hear water moving through the pipes. It turned out that the shower cartridge was bad. I replaced it and solved the problem.

Hopefully, my experience will help someone.
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