Navien NPE-210A: How did this get into the flow sensor??

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TL;DR: How did a rather large fragment of copper tubing make it past the inlet filter screen and end up obstructing the flow sensor? Is there any way this fragment is actually from an internal component of the heater? Bonus Question: Is it normal to see very fine metal shavings in the condensate clean-out, perhaps from the brass service valves?

Hi Everyone,

I've been a lurker on Terry Love for 15+ years. Although I've never really contributed by posting, I have definitely gained some wisdom in these forums. I'm hoping someone can help me out with my current issue today.

I have a Navien NPE-210A that was installed in 2014. I've been meticulous about the annual flushing and cleaning of the filters (air, inlet, and recirc). I hadn't had any issues until this past month. The day before Thanksgiving (of course!), the wife alerted me that she could not get any hot water. I looked at the unit and could see that it was not firing up, even though it was powered up and the hot water was running. It was also not displaying any error codes. I opened the hot faucet at the utility sink in the garage, which is where the unit is installed, and I was able to get it to fire up for a few seconds, but then the heat turned back off (unit stopped heating but remained powered up).

This forum and other resources led me to the conclusion that the problem was likely either the check valve or the flow sensor. Since it was the day before Thanksgiving (and the check valve was only about $4) I decided to grab both parts from the local supply house and replaced both. The unit worked perfectly afterwards until yesterday, when I started having the exact same symptoms of no hot water (not firing up), but no error codes. When I tapped on the information button on the panel, it was not showing any water flow, even though multiple faucets were fully opened. Since I had just replaced the check valve and flow sensor, I couldn't imagine it was either of those parts, and I started to suspect my initial problem last month had actually been something else. I wondered if it was the PCB, so made an initial post on here to see if someone with more knowledge might be able to help me out.

While waiting for that post to be approved by the moderators, I decided to go ahead and "go through the motions" of inspecting the flow sensor and check valve, even though they were brand new. Low and behold, I discovered a large fragment of metal had lodged inside the flow sensor and completely stopped it! After fishing the fragment out and reinstalling the flow sensor, the unit is now working as expected.

That's all good news, but now I need to know how this fragment of metal got inside the flow sensor. It looks to me like it is a tarnished/corroded piece of copper tubing, but it is way too big to have made it through the screen of the cold water inlet filter. I inspected the filter to make sure it wasn't damaged or anything like that.

I wondered if it could be a part from the heat exchanger, that perhaps had traveled backwards and into the flow sensor, so I phoned a friend who works in the plumbing business, and he assured me that everything inside the heat exchanger is stainless steel. He also agreed that the fragment looked to be a piece of copper tubing.

I'm attaching a picture of the fragment and looking for ideas from the experts on what I should be inspecting.

Bonus question: Although it's not mentioned in the routine maintenance section of the official manual, I learned about the condensate line clean-out yesterday and decided to go ahead and clean that out. There was a bunch of dark-colored fine sediment in there, but when you swirl it around you can also see a bunch of really fine gold-colored flake/shavings. Is this to be expected in the condensate clean-out? Maybe it's from the brass service valves? The flake/shavings looked like it was small enough to pass through the screen on the inlet filter.

Thanks in advance!


  • Metal piece extracted from flow sensor.jpg
    Metal piece extracted from flow sensor.jpg
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