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Mildew smell from hot water

hamiltonworks | Posted in General Questions on

Hello folks,

CZ 5b
Recently I returned home after being gone for couple of weeks. My wife and I have noticed a pungent mildewy type of smell that seems to be coming from the hot water only side. (40 gal, natural gas heater, on city water). The smell is more musty/mildewy than like sulphur. I did some minimal research and didn’t find a lot of info, with the exception that maybe some bacteria has formed in the tank…
Possible remedies are:
1. introducing a couple of liters of hydrogen peroxide into the tank and letting sit for about 4 hours, then flushing.
2. Replace anode rod with a powered anode rod.

Anyone out there have any insight on this?? Would be much appreciated! Thanks so much!

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  1. Expert Member
    BILL WICHERS | | #1

    The usual chemical to use to sterilize water systems is bleach (the chlorine in it, specifically). Get some plain bleach with no additives, add a little and circulate it throughout the system. You can get more information as to how to do this by searching for "how to shock a well", since it's a common maintenance procedure used with water wells. That will clear up anything living in there making stink.

    The other thing to do is to replace your anode rod with the "other" kind, which usually means installing an aluminum anode rod instead of a magnesium one. If your water heater doesn't have a lot of clearance around it, you get get a segmented "flexible" anode rod that will be easier to install in a tight space.


    1. hamiltonworks | | #5

      Got it, thanks Bill!

  2. Expert Member

    How hot is the water?

    1. Expert Member
      Akos | | #3


      I've seen this happen with tanks that are kept too cold. You want your hot water tank at 140F to pasteurize any bacteria. This tends to be too hot for faucet and shower, so most codes nowadays require a thermostatic mixing valve on the tank to bring it down to 120F for the hot water supply.

      The reason this didn't happen before is that city water has chlorine in it and even small amounts of it can kill bacteria at high temperature. When you went away and there was no water use, there was no fresh chlorine brought in so the bacteria eventually started multiplying.

      1. hamiltonworks | | #4

        Ah makes sense, thank you. Water temp is definitely below 140, it's set on "hot" which I think is 120. And there's no thermostatic mixing valve.
        So, the order to proceed would be: do as Bill describes above- anode rod and bleach, and then install a mixing valve and keep thermostat on higher temp...

        1. Expert Member
          MALCOLM TAYLOR | | #6


          Unless it's due for a replacement, I'd leave the anode rod alone. Shocking the system will clear up the problem. Even a slightly lower temp on the tank is fine as long as it isn't left stagnant while you are away for long periods.

          1. hamiltonworks | | #7

            Cool thanks Malcom. Tank is 12 years old, to my knowledge the anode rod has never been replaced. Maybe it's a good time?
            I was thinking I was saving some carbon by keeping it on a lower setting... But as you stated, once cleared should be ok on lower setting unless it's left stagnant for long periods. Thanks for the heads up!

          2. Expert Member
            MALCOLM TAYLOR | | #8


            I've never had much luck getting anodes out of heaters after that long. Maybe plan of replacing it, but don't sweat it if you can't.

          3. Expert Member
            BILL WICHERS | | #17

            An impact wrench is really helpful breaking loose old anode rods, and it's much less likely to damage things in the process. Next best is a breaker bar and a socket, but then you have a lot more issue with the possibility of the water heater twisting on you and causing problems. I would not try to use a pipe wrench -- you really need a socket that goes all the way around the nut.

            In the automotive world, "neverseize", anti-seize compound, would be put on those threads, but I'm not sure that stuff is approved for use on water systems. Pipe tape should perform a similar function. Car parts are famous for seizing up. Just this past Monday I had to use a carbide tooth reciprocating saw blade to cut out the lower bolt on a rear shock absorber on my wife's car after I popped off the head of the bolt (on a hardened bolt!) because it was so seized.

            12 years old is certaintly past time for an anode rod replacement. The purpose of those anode rods is to rot away to protect the tanke -- the rod rots instead of the tank. If you keep the rod in good shape, the tank will last a very long time. The only real problem is they tend to be hard to remove since the nut tends to seize in place. If you can get it out, replace it, otherwise it's not the end of the world. Try draining down the tank from the lower drain valve and see if any sediment comes out, which might also be contributing to your smell problem. I agree with the others that if you have the water heater set to too low a temperature, you're more prone to have issues with things growing in the water.


  3. hamiltonworks | | #9

    Sounds good, I'd probably open a COW (can of worms). Thanks Malcom!

    1. Expert Member
      Akos | | #10

      If you don't mind opening a can of worms, replacing an anode before completely worn can significantly extend the life of a tank. I have a used resistance tank I picked up many moons ago to use as an indirect storage tank. The tank is now 20+ years old and on its 3nd anode. The first anode replacement was a pain, luckily I did it before installing the tank. Took a lot of heat and a big impact gun to get it out.

      1. Expert Member
        MALCOLM TAYLOR | | #11


        Rant: Why do they send them out like that? The top encased on foam, and tightened until you are worried about snapping off the threaded connection?

        1. Expert Member
          Akos | | #12

          If it would be too easy to replace, people would not be buying new tanks every 10/15 years. This way only curmudgeon like us go through the trouble.

          1. hamiltonworks | | #13

            Haha exactly. I suppose a contributing factor will be the quality of my water to begin with...

      2. DC_Contrarian | | #14

        I have a 36" breaker bar I bought for replacing anodes, I find I can usually get it if I use a 6' section of pipe as a cheater. Usually it takes an extension to get down on the nut, and if I don't use an impact-rated extension they've been known to snap off.

        One tip is to remove the anode when the tank is brand new, then put it back with both tape and dope and only tighten as tight as necessary.

        1. hamiltonworks | | #15

          DCContrarian, that's some serious leverage! My anode is attached to the nipple of the hot water line. I suppose I could put a piece of pipe over the wrench handle for a cheater. To top it off, the threads are nearly buried under the top of the tank, so I have minimal room to even get a pipe wrench on. (Malcom basically eluded to this above) I will destroy the threads if I attempt to remove it, and thus once I try, it must come out, or I'm in for a new tank... However, If I do get it out, I can put a new anode with a longer nipple so I actually have some purchase, and will use tape and dope as described. Thanks!

          1. Expert Member
            MALCOLM TAYLOR | | #16


            I've used a system similar to what DC describes. With that much torque the problem then becomes keeping the tank from rotating. I had no luck with two helpers holding it. If you need that much force an impact gun is a better bet.

          2. DC_Contrarian | | #18

            If the tank has nipples on the top, and they're steel dielectric like they should be, you can put another 6' piece of pipe between them. It helps if the tank is full of water. You want to press firmly but slowly and stop if anything doesn't feel right.

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