Here's one to stump your best minds... We have a smell of dried urine in the water that's coming from a shower (tub/shower combo), especially when it's first turned on. It's the only appliance in the home where this happens, and even the water supplying the sink and toilet in this same bathroom does not have a smell. The specs and details of this problem are as follows: 1) It does not matter whether the water is hot or cold. 2) There's only one water source into the home, which supplies all the appliances. 3) Yes, I have run the water with the shower head removed, thus eliminating this as a possibility, and the water still smells. 4) To eliminate the possibility of it being gas or a smell coming back from the drain/sewer line, I've also run the water with the drain closed and the overflow blocked off. It is the supply line, not the drain line. 5) We've looked under the tub and in the plumbing wall behind the plumbing for possible leaks, thinking it might be activating mold spores or something when the water gets turned on. It's dry! 6) All supply lines coming out of the slab are copper. I've spoken to plumbers, engineers and architects, and they all agree that because it's a closed system with no way that anything foreign could enter the water lines, that water coming from all outlets would smell, not just one. I too am a pretty bright guy and agree with them, but obviously we are all missing something here... HELP!

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I've spoken to plumbers, engineers and architects, and they all agree that because it's a closed system with no way that anything foreign could enter the water lines,

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And nobody mentioned that you might have bacteria growing in the shower raiser? Amazing!
I've reread your post looking for any sign that the tub spout smells so I guess it doiesn't. Correct?
You can trey this. Remove the shower head and chrome shower arm. B The shower arm threads into a drop eared ell, (see image).
Tonight before bedtime pull up the shower diverter on the spout so nothing drains out and take a funnel placed on the drop eared ell and pour bleach down until it runs out of the ell. Let it set overnight. And because I'm a cautious man take the shower head and shower arm and place them in a pan full of bleach and soak them also. Next morning push the diverter down, drain out the bleach, replace the shower head and arm and turn it on. Did that help? Let me know and good luck, Tom

Tom - This seems to have worked like a charm, and I took your advice and soaked the shower head. I actually found it easier to leave the shower arm on, turn it upside down and pour the bleach right into the shower arm, and because the opening is then higher than the ell, it allows the arm to get bleached at the same time! Because my ell was about 4" in the wall, this also saved the hassle of trying to bend and/or adapt a funnel to be able to pour the bleach directly into the ell. Another thing I learned - The chrome colored coating on a plastic shower head can't handle the full strength bleach for a long period, as it started eating the chrome around the edges. By doing some further research, I found that bleach kills bacteria upon contact, so no real need to soak anything for a long period. Keeping the bleach in the riser as well as using as a soak for an hour is more than adequate. Thanks and I'll report back if the problem arises again, but after two days, no more smell.

Good for you! Glad to hear the bleach worked. The only thing I still wonder about this.

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I've spoken to plumbers, engineers and architects, and they all agree that because it's a closed system with no way that anything foreign could enter the water lines, that water coming from all outlets would smell, not just one.

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All this high powered help couldn't diagnoses a simple bacteria problem. Unbelievable! Good luck Tom