Navien NPE-240A Water Heater “Surges” or “Pulses”

[twagoner]

Our Navien NPE-240A Tankless Water Heater “surges” or “pulses” when demand is more than a couple of gpm. Yesterday we had both showers going simultaneously and they both started “pulsing” hot-no hot-hot-no hot at about 1 second intervals until one was shut off. We were able to repeat this behavior using other taps in the house. There were no error codes. Today the problem is gone, but what could have caused this? I’m guessing bad/dirty flow sensor or bad/dirty check valve.

We moved to this house two years ago and the water heater was in place then. The mfg date is August 2014. A few weeks ago I felt that it was time to perform the recommended maintenance – cleaning the filters and flushing the system. It turns out that is was installed improperly. There was no way to flush it without cutting pipes. There we no shut-off valves, unions, or flushing ports and the expansion tank was installed the hot side. So I assumed it hadn’t been flushed in ten years. I took care of the piping issues and flushed the system and cleaned the filters and trap. There did not seem to be much junk in the system. There is a recirculating system consisting of about 50 feet of ½” Pex tapped into the 3/4” supply near the guest bathroom at the far end of the house.

I’m guessing that some of the junk I loosened in the flushing made the recirculating check valve stick temporarily. Would that do it or is some other part failing? Should I replace the check valve or the flow sensor? They both look easy and cheap to replace.

Any thoughts would be appreciated.

 

[Breplum]

1. I'd start just replacing the internal check valve inside the unit.
2. fyi: 1/2" PEX is not the right size for recirculation line. It is supposed to be 3/4" for PEX...as it will add long term strain on the pump...but your situation shows there is leeway in factory specs.

 

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[Fitter30]

Flushing or pumping vinegar/ commercial cleaner? Minerals dissolve in solution for the most part. Unit that hasn't been chemically cleaned the minerals create hot spots that puts stress on the heat exchanger could create leaks. Leak small leak will show up in the condensate or flash into steam cools the flame or puts some of it out.

 

[twagoner]

1. I'd start just replacing the internal check valve inside the unit.
2. fyi: 1/2" PEX is not the right size for recirculation line. It is supposed to be 3/4" for PEX...as it will add long term strain on the pump...but your situation shows there is leeway in factory specs.

Thanks for the quick reply.
1. I will replace the check valve - looks pretty easy.
2. The installation manual does recommend 3/4" for the recirculation line, but goes on to say you could use up to 100' of 1/2" copper. I don't know how that would relate to the 50' of 1/2" PEX that I have, but it seems it might be ok. (After all, it's been there 10 years.)

Also, there is no shutoff near the water heater on the recirculation line. Would you suggest installing one?

Tom

 

[twagoner]

Flushing or pumping vinegar/ commercial cleaner? Minerals dissolve in solution for the most part. Unit that hasn't been chemically cleaned the minerals create hot spots that puts stress on the heat exchanger could create leaks. Leak small leak will show up in the condensate or flash into steam cools the flame or puts some of it out.

I used a commercial cleaner. I have not seen any evidence of leaks yet. I have not yet looked inside the unit for leaks. I will do so this week.
Thanks

 

[Breplum]

Post says "flushed". Please clarify that you actually descaled as per factory recommendation and then did it for far longer than normal to account for massive mineral scale that you likely have/had.

 

[twagoner]

Post says "flushed". Please clarify that you actually descaled as per factory recommendation and then did it for far longer than normal to account for massive mineral scale that you likely have/had.

Yes. I flushed it following the instructions in the owner's manual with a commercial descaler for about an hour. There didn't appear to be an abundance of debris produced. I plan to do it again next week and I will do it longer as you have suggested. I will also clean the intake and recirculation line filters again.

 

[Breplum]

The internal check valve on all Navien A series has been a sticking part from the beginning. As much as I like Navien, they are terrible about never fixing the sticking o-ring on the check valve, even the newer A2 series. It is a under $5. part if you get at a local Navien supplier.

 

[twagoner]

The internal check valve on all Navien A series has been a sticking part from the beginning. As much as I like Navien, they are terrible about never fixing the sticking o-ring on the check valve, even the newer A2 series. It is a under $5. part if you get at a local Navien supplier.

Thanks - I have ordered a check valve and will install it when it arrives.

 

[twagoner]

Here's a followup. I replaced the check valve and the flow sensor after flushing the water heater. It was working great. Then a couple of weeks ago - no hot water. I opened the cover of the water heater and water was sprayiing out near the top. I considederd that I was looking at a leak in the heat exchanger and the thing was toast. Probably the ten years of scale I removed was holding it together. My warranty was void because it was installed improperly. So I thought I would call a plumber and get it replaced. He came out the same day and gave me a quote for over $13,000.00! He would not break down the price for me. So I looked up the unit he was selling me and I could get it for $1,800.00. I asked if he could quote me just the labor cost. Never heard from him again. It's not like it was a hard job. The plumbing and venting were all in place and in good shape. I figured two guys could do it in a day.
I went to home depot, spent a little over $600 for a Rheem 50 gal and put it in myself. I am a retired home inspector and I have replaced at least ten water heaters in buildings I've owned. I sweat copper as good as a plumber and I have all the tools, gas sniffer included. It was up and running the next day. I could buy about 20 water heaters for the quote he gave me. I cannot figure out why it would be $13,000.00 to replace. If that's the going rate, I should have been a plumber.
Any thoughts?

 

[Breplum]

That $13k quote was total nutso. I swapped out an old Noritz tankless for a new Navien the other week for a neighbor. Six hours including running 2" PVC up the old double wall vent cap. Of course I have a ProPress rig so about three minutes re-arranging piping and adding water heater flex lines.
The neighbor helped hoist up onto the wall, and the rest was was light duty plumbing.
What was improper about your original install?

 

[twagoner]

No way to flush without cutting pipes. Expansion tank on the hot. Here's a photo of the bad install and the recent estimate