I recently had a plumbing company install a tankless water heater in my crawlspace. Everything with the install works and looks great, but we are experiencing a very strong chemical smell (like nail-polish remover or a strong sharpie marker) that comes out of our faucets when we call for hot water. It usually lasts for about 10 seconds then goes away. I notice it whenever we haven't used the hot water for awhile. For example: the smell is very strong first thing in the morning when turning on the shower.

The plumbers used PEX pipes to install a hot water recirculation pipe that is soldered to the original copper pipes. Other PEX pipes were soldered where they needed to connect to some other areas of the copper pipes including a new Pressure Reducing Valve.

Does anyone have any insight or advice on what could be causing this issue?

Thanks, Mike

  • 1
    I suppose this could be from the flux used in soldering. Apr 12, 2018 at 19:49
  • That is what I originally thought too. it's been a week now, and any water soluble flux should have been washed away and gone by now, right?
    – Mike
    Apr 12, 2018 at 19:51
  • One would think so. Apr 12, 2018 at 21:51
  • PEX soldered to copper? Don't you mean crimped to copper that was soldered to other solder? PEX doesn't leech, so I doubt it's the pex, did they use PVC on the supply side somewhere?
    – virtualxtc
    Apr 12, 2018 at 23:01
  • It sounds like the smell you're describing is acetone (often in nail polish remover and sharpies as a solvent). I have no idea why that would be present though.
    – Hari
    Apr 12, 2018 at 23:30

4 Answers 4


Update: We are still not sure what caused the issue, but we had the plumbing company come out and do a vinegar wash of the entire system. This worked and the smell is gone.


I think it is burn-in of the heater from something inside the heat-exchanger since the wet side is a closed system. Believe I'm perplexed by the same problem, so let me expand to establish that we have the same complaint and I believe the same root cause. If those assumptions hold, our system similarities are of more interest and the differences are candidates to be dismissed. I put in a new propane-fired tankless water heater with integral recirculation pump and remote cross-over valve 4 weeks ago on CPVC piping in the garage. After the heater has been idle for a while, the water that has been sitting in it gets a strong chemical smell. You can't smell it at the heater, and it takes a while to pass through the pipes to the fixture (sink/shower) depending on the length of the pipe run. The smell only comes from hot water, so it is not coming from my well water (are you on well or public?). The smell goes away in a few seconds.

To me, there are still logic problems with this answer. If true, it would seem the complaint would be far more widespread and discussed online. Also, I hope to distinguish between water that's been sitting in the heat exchangers leeching the smell from sitting versus some smell coming from initial heating. I plan to do a test allowing water to sit overnight then disable the heater and run the hot water and see if it still stinks.

Note that the burner cannot in my opinion contaminate the water and the smell is IN THE WATER. Pipe dope from supply piping could contaminate the heat exchanger, but I found no residue on the inlet screen whatsoever.

Also note, we are NOT talking about iron or manganese induced rotten egg-smell

  • PTFE is good to +/- ~500 degrees. A cracked heat exchanger, or a (plastic?) holding tank operating above its recommended temperature is all I can think of. Call the manufacturer.
    – Mazura
    Nov 3, 2019 at 19:56

It's the PEX tubing!! I just had some added when I had a whole house filtration system put in. Pex leaches over 150 chemicals. Now I'm back to ground zero and I have to get it taken out and replaced with copper. It does smell worse with hot water running. Mine smells like pure plastic.

The plumber adding the PEX tubing to my line and it defeated my whole purpose of having cleaner water. Now it's going to cost me an arm and a leg to get them to take it out and replace it with copper.

PEX should be banned!

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Jan 15, 2020 at 15:31
  • 1
    A flush of ten minutes is recommended after install Then let water stand in pex for 3 day’s followed by a second flush of System. Not all pex pipe is the same when it comes to chemical smell. A lot of research is still being done on water quality in pex systems. There are millions of homes with pex throughout the home that have zero complaints. Do you know the manufacturer of the pipe used at your home?
    – Kris
    Jan 16, 2020 at 15:08

I can 100% say its not the PEX. I have ran into one of my customers that has this issue and they though flushing the tank would relieve the smell before telling me the real reason why they are flushing the tank. I removed a high amount of sediment from the tankless unit but I personally smelled no rotten egg smell coming from the unit itself as when I flushed it. I had a bucket of the fresh water and a bucket of the chemical water with all the sediment. These rotten egg smells really only comes from the anode rod tank units not the tankless units. Though I can within no doubt say that PVC when glued and primed can give nasty chemical taste and strong odor. Its possible the smell is so faint that cold system doesn't induce a strong smell but the hot side when heated can make the smell much stronger causing you to notice it when the hot system is operating. I have been plumbing for over 10years installed pex systems in 1000s of homes. Only issue I have had with water quality is the taste of the primer in the system in new homes. I always recommend flushing the system for an hour by just opening a hose bib. Theirs a strong possibility that you have pex that someone could of dropped a large amount of primer on or in. Causing the Strong chemical odor. PEX itself is harmless. It would require constant sun contact to emit chemicals as does all plastic.

  • Yeah, sulfur-reducing bacteria are the usual "rotten egg" culprits, and certain anode rod types create an environment more favorable for them to grow. (The usual fix for stubborn cases is a powered anode that protects the tank while not creating an environment for bacterial growth) Oct 20, 2021 at 2:27
  • Am I reading your answer correctly that you've glued PEX fittings? I thought they were crimped/clamped, not glued. Is there a fitting schedule that calls for gluing them?
    – FreeMan
    Oct 20, 2021 at 10:54

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