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We bought a house with a Rinnai RUC98 tankless hot water heater already installed. We didn't notice anything about the water at first except only that when we did laundry, the dried clothes smelled fine UNTIL we wore them and sweat a little. The sweat releases this smell from our clean clothes that I don't really know how to describe. A lot of on-line description say it's a "rotten egg" or sulfur smell but I'm not sure if that's correct. It's just a damp, stinky, wet-dog odor. So clothes smell if you sweat even a little bit, towels go smelly in 2-3 showers (we're not a hotel and don't wash our towels after every shower) as they are hung up to dry, bath and kitchen hand towels start to smell in the same way. The only inconsistency was that our teenage daughter's clothes didn't "smell". After much confusion and investigation, I learned that my wife and I were doing our laundry in hot/warm water whereas she was doing her loads in cold water. So the city water coming into our house is fine and the culprit seems to be the hot water heater!

But what's causing the hot water to make our clothes smell, make our hair and skin smell after showers (to the point that we need to rinse ourselves with cold water before exiting the shower)? Washing our laundry only in cold water has solved the smelly clothes issue (before we discovered it was the hot water, we even replaced the very old washer and dryer with brand new ones). But warm showers (can't always rinse in cold water because we live in Minnesota winters half the year) still transfer the smell into our towels. By itself, hot water from the tap put into a glass does not smell (at least to my nose, I can't smell anything at all).

Has anyone had this problem with your tankless water heater? Some research on-line pointed to a lot of information on sulfate-reducing bacteria (SRB) that are common in tank water heaters with anodes but not much on tankless water heaters. Can SRB also live and grow in tankless water heaters without anodes? If so, how can I safely get rid of the bacteria? Should I just flush the system with distilled vinegar/commercial descaling chemical as part of the regular maintenance process and hope the smell goes away? Call a professional plumber? Help. Thank you.

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    On demand water heaters do not make clothes stink but tanked water heaters may if there is air in the tank and not at a high enough temp to kill bacteria. The problem is most likely in the incoming water. The anode is there to stop prevent tank damage from electrolysis.
    – Ed Beal
    Jun 13, 2021 at 1:15

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1: Clothes are inadequately dried.

Sitting in the washer too long can do it. Too much water left in the clothes and inadequate drying can do it. Things grow in the clothes.

  1. Clothes were once inadequately dried and now are colonized.

Especially for towels - I think they get colonized and then the colonies spring to life. Colonies survive subsequent washes. The colony is on some clothes from somewhere and now is on all your clothes and towels.

Your daughter's clothes do not smell because they were not colonized by the smell germs / mold.

3: Solution: bleach. It does not need to be all destroying. 1/8 cup in a full load? Wash the smelly clothes with a little chlorine bleach (normal amount) in wash water as directed on the bleach bottle. Bleach is a phenomenal disinfectant. It will kill odor colonies on clothes and prevent new ones.

I learned this by experimentation and observation. You can do the experiment. Gather a load of towels you know will be smelly after you use them a few times. Wash them on hot with bleach. I predict they will not become smelly. You can then adapt this to purge colonies from the rest of your laundry.

If I am right and all of you now smell good (except your daughter who never smelled bad) report back with that green check.

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  • Not fully dried may be a good reason. My latest front loader is a big disappointment compared to my earlier ones the new one doesn’t clean the clothes at all compared to our previous model.
    – Ed Beal
    Jun 13, 2021 at 1:18
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    Thank you for this suggestion. I know we've tried the bleach to sanitize an empty washer BUT NOT ON ALL OUR CLOTHES. So if what you're suggesting is correct--that these colonies are in our washed clothes and towels and come back to life--it makes sense to me. Thank you.
    – Alex
    Jun 16, 2021 at 17:28
  • From experience, I am sure this the reason.
    – Rohit Gupta
    Jun 5 at 21:27
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Bacteria and mold are not reported as growing in tankless water heaters. AFIK the absence of an anode has nothing to do with mold growth, and the presence of an anode in a hot water tank does not inhibit mold or bacteria.

Off odor might be coming from odor substances in the incoming city water. Alternatively, sometimes clothes washers develop mold, especially front loaders. If your clothes washer has a cleaning cycle, then run one cycle. Some front loaders are known for harboring mold.

Do you have any "dead legs" in your hot water line? Is the hot water line fully swept by water flow?

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  • Jim, thanks for your response. Our new washer does have a cleaning cycle that asks to be run about once a month or around 40 wash cycles. I've done this whenever it's required.
    – Alex
    Jun 12, 2021 at 16:30
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    Years ago in Dallas TX at a certain point in the summer the city water would suddenly develop an off odor and taste. The explanation was that a lake supplying Dallas was "turning over" and sending algae into the outlet stream.
    – Jim Stewart
    Jun 12, 2021 at 16:35
  • Fully agree with this answer I should have read prior to making a comment. And the comment of a new washer that needs a cycle to clean itself sounds like my experience, my wife has to wash my farm clothes 2x (I sweat on the farm not as much on the job)
    – Ed Beal
    Jun 13, 2021 at 1:22
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I will say, the odd smell coming from a tankless is very much a thing. The one in my grandmother's house is doing the exact same thing. Bed sheets have a slight, briny, seawater like smell. Hot showers the same. I'm sure the heater hasn't been de-scaled in years. She's no longer in the home, it's kept for frequent visitors. Next time I visit, I'm going to de-scale it.

Fairly easy DIY provided the heater was installed with the needed valves & hose ports (it's obvious). I use a citric acid cleaner for coffee makers, Dezcal (much better than vinegar, smells alot less, works quickly, food safe, and Rinnai recommened in other parts of the world). Need a small submersible pump, 5 gal. bucket, and a couple of short hoses.(~$80 including a multi-use jar of Dezcal powder) Shut off water & gas at the heater, remove & clean the inlet filter, hook up the hoses (discharge side of pump to the cold water inlet). Circulate Dezcal solution for an hour (minimum). Run some clean water through to flush the heat exhanger & you're done. Plenty of videos out there. Some say to open the temperature & pressure relief valve as part of the process (the one with a little metal lever). I don't touch it, as they often leak slightly when closed again.

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Have you tried to flush the Rinnai? We find we have to do a flush 2 times a year. Call your plumber to get that done.

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  • Your answer would benefit from the reason Rinnai needs to be flushed. I have never heard of such a thing
    – Rohit Gupta
    Jun 5 at 21:26

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