Navien Tankless Heater - DIY Fail

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TxInjun

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Hello fellow helpers,

My house has a Navien NPE-240A that came installed new (in the basement) when we bought this renovated house just over 5 years ago. The unit has external recirculation enabled to the kitchen faucet on the 1st floor. Bedrooms and baths are on the 2nd floor, upstairs. We live in a very hard-water area: I descaled the unit for the first time about 9 months ago (April 2019), and intend to do so once a year going forward.

The issue: Recently we haven't been getting consistently hot water in the upstairs showers (mostly lukewarm) but we do get piping hot water in the kitchen and guest bathroom on the 1st floor. I assumed we may have had blockage in the shower lines, so I opened up the shower valve and replaced the pressure balancer / mixer. That didn't fix anything. Next clue was when my wife did laundry: when the washing machine was running on the 1st floor, I was getting hot water upstairs in all outlets. Some browsing suggested that the flow sensor may be sticky and not fully active, which made sense to me that the washing machine load was big enough to trigger the flow sensor so the heater was turning on. The idea was to clean and reinstall the flow sensor.

With that plan, I turned the unit off, shut down the water, gas and power lines, and took out the flow sensor (in the front, middle right of the unit when open). There was an unexpected amount of water that came out (should have fully drained the unit I suppose) but that eventually drained out. I soaked the flow sensor in vinegar, rinsed it, applied some plumbers silicone to the o-ring and the flange connector and put the sensor back in (and reattached the clamp in the picture). With the electricity and gas still offline, I opened up the water line and was VERY SURPRISED to see the full water flow come out of the condensate line - it was overwhelming the condensate pump.

So I turned all the water off again, and removed the flow sensor, checked the o-ring was in place and reconnected. Again, full flow into the condensate line. Now I'm truly puzzled, as I think of the condensate system as completely separate from the water flow - the condensate is just what "condenses" from the hot gases in the combustion side, right? So how is the water getting from the cold water inlet to the condensate? Only thing I can think of, is somehow the pre-heater is cracked / leaking into the condensate sectoin.

Let me know your ideas and thoughts! I have an appointment scheduled with a pro, but I thought someone here may already know what I'm doing wrong. Thanks in advance!

TxInjun
 

TxInjun

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Thought I'd post the fix to my issue, in case others run into this issue.

So ha ha, turns out I did not install an o-ring properly when I put the flow sensor back. There are 2 ends to the flow sensor, and I put the o-ring on the wrong side (the clamp side). In my defense, the "right" side looked like it was a friction-fit seal, and the installation guide that came with the unit showed what I thought was an o-ring on the "wrong" side but it was just the fixed washer there.

Somehow the unit senses the lack of seal and diverts the water out the condensate exhaust, probably a safety feature so that the unit interior doesn't flood from a leak.

Service guy came in (Navien authorized local service rep), he hadn't seen this issue before either. He got on a call with Navien support and the first suggestion was to replace the o-ring and everything is back to working order. It's a $170 slip, however on the positive side we have hot water everywhere (cleaning the sensor did seem to help). For future reference, Navien service is reachable at 1-800-519-8714.

Cheers

TxIn
 
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