I recently inspected a Navien hot water heater. I believe this unit is 6-7 years old. When I opened up the panel there was a brown oily fluid on the bottom of the heat exchanger and on the intake. These are both made of some kind of black plastic. It appears to be condensate but I am not sure why it would be oily. From what I saw online, Navien states that plastic does not contain any oil that would cause this. I can’t find any info on this at this point. Has anyone seen this and know what the cause is? Is this a major defect that could cause harm? Thanks.
Welcome to our forum, David!..Enjoy and participate. 
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A picture is worth 1000 words.
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There is a big issue with Navien units leaking exhaust back into the unit that kills it in about 5-6 years – they are piece of [$$$] junk. Write it up hard to get it fully evaluated by a qualified plumber/HVAC tech experienced with Navien units prior closing. They are not cheap to replace and if the new unit is not identical, the piping/manifold needs to be repiped $$$
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Martin,
Thanks for the reply.
Below are some pictures. Previously I said intake, I meant the exhaust and heat exchanger. You can see the brown oily fluid I am taking about in 3rd picture.
Thanks Simon.
Do you know if the exhaust is causing this brown oily fluid? I have added some photos requested by Mr. Willes.
Thanks again for the speedy reply. I will be going back to this house shortly to do some more looking.
I think excessive circulation of air is pulling oils from the plastic components of the water heater. The heavy condensation inside the cabinet tells me there is excessive moisture buildup on the interior. This is an issue with this particular manufacture and they should be contacted for repairs. David call it out for sure and good catch.
David, it’s not normal for the unit to have condensation and brown spots inside of it. Also check the main control board, it will often get discolored. It should be nice and green, not brown/blackish. The exhaust leaking back into the cabinet causes all these symptoms. The (hot, humid, & corrosive) exhaust in these units leaks from several places. Let a competent tech figure this out. I would simply state in my report that these symptoms are indicative of exhaust leaking into the cabinet of the unit… premature failure may result, high levels of CO can enter the dwelling, etc… recommend HVAC/plumber with a brain to eval before closing…
PS: Martin, you installed 1000s of these, right 
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Simon I’ve never installed this brand or would recommend it 
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Thanks! I am at the location now finishing up. I now see the exhaust termination and intake on the outside are only 22" apart and the intake is actually about 7" higher. I will recommend adding a stack on the elbow to get it below the exhaust (2 on the left). Maybe this is adding to the issue. Again, thank you for your help. I can see this forum is going to be very helpful in the future.
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Two on the left are for the HVAC, and the one on the right is a Concentric vent which may not be installed properly, and too close to the furnace exhaust which may be sukking in some of the exhaust.
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Hey, Jeff. Thanks for the reply. The 2 on the left are for the water heater and the concentric is for the furnace. The basement is unfinished so I can follow the vents from the appliance to the penetration.
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One more picture to the mix. If you look at the exhaust you can see corrosion on the left. There is actually about an 1/8" gap (at the most) between the cabinet and the black plastic which has the brown condensation on it. Does this make the system more dangerous? I am not sure how much exhaust can get through right there, but judging from the corrosion I would bet quite a bit.
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Yep, I think that is your culprit. Exhaust is forced so it is pushed out the gap which causes condensation. As far as the brown oily substance, possible exhaust soot or manufacturing oils or just corrosion.
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Where did you hear that? Your comment is a contradiction to what I have heard and read about Navien units. I frequently see them.
It’s what’s happening here on Long Island, but don’t take my word for it, do your own research:
One can start here:
Then try this:
Navien Exhaust Leak
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Looks like other people have experienced the same problem. Scroll down the pictures on the link below and you will see the “brown” leak at the same location as David’s picture.
https://forum.heatinghelp.com/discussion/160416/navien-npe-240a-leaking-brown-substance
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While that is good info on the recall, it is old. I have never encountered one of the recalled units, but not many of them were sold. I see about a half dozen or so, Naviens a year. Maybe they are more popular in your area. IMO, one recall from 2008 for 3400 units doesn’t condemn the brand for all time to the crap heap. My Pathfinder just got a recall and overall it’s a pretty good vehicle.
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Are you telling us there is no issue with Navien tankless units?
I posted 2 different recalls (one from 2018) to show you their units are poorly engineered & tested, requiring recalls. There is still an ongoing issue with them that Navien is refusing to address (units that are not part of the recall). A class action lawsuit is being brought. People keep discovering that their Navien units are dying because of the same issue: exhaust leak. For example, Navien will replace the unit but won’t pay for the labor. Replaced units are also prorated… if the warranty is 8 years and they replace a unit on 6th year, after 2 years there is no more warranty. The replacement unit has the same problem – it will start leaking exhaust after few years of service. These units are not cheap to install, service, or replace.
I stand by what I said. I was addressing a specific issue, read what I wrote. Every single unit that leaks exhaust back into the unit made by Navien is piece of junk. If the OP’s pics came from your house, you would probably have a different opinion.
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I apologize. I misread (and missed) the 2018 recall.