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So the bathroom sink has three lines underneath, all with shutoffs. Hot, cold, and another cold line. The second cold line is unused and older (it's copper, appears to be 3/8). Edit for clarification: yes, all three lines have shutoff valves installed.

If this was the kitchen, I would be thinking refrigerator or filter line. But this is underneath the bathroom sink. Why would there by a third line in the bathroom?

I want to cap it off, stick it back into the wall, and patch over the hole. Any reason I shouldn't?

  • A photo would help. If it's not capped currently, does this mean you can't turn on the water to the house or the sink? How is it attached? – BMitch Apr 29 '15 at 19:52
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    Can you get any further upstream to cap it? Personally, I'd rather cap something in an accessible space, lest Mr Murphy and his law enforcement team come to visit. – Aloysius Defenestrate Apr 29 '15 at 20:39
  • @BMitch - I think it has a shutoff valve, but is not currently capped: all with shutoffs – Johnny Apr 29 '15 at 22:06
  • Thanks @Johnny. You are correct; I've edited the OP to make it more clear that the extra line has a shutoff. – nbm May 5 '15 at 19:40
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If there's a valve, then turn it off and remove the pipe. If you're concerned the valve will leak, then shutoff the water to that line, remove the valve and cap the pipe there (or replace any T's with a straight fitting).

Leaving the pipe and capping the thin refrigerator line will be difficult and error prone, and you're still left with a valve and two places that may leak.

  • Thanks very much for your help BMitch. I would vote up your answer if I was able. The connection goes extra water supply -> shutoff -> T, so I can't shut off the water to that line, but it would be no problem shutting off the house main for the short time it will take to effect your suggested solution. – nbm May 5 '15 at 19:47
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Capping that third copper line with a soldered copper cap in an unaccessible space should not be a problem at all if you do not need it. But I would not trust a push cap, shark bite type fitting.

  • Thanks very much for the advice Damon. I would vote up your answer if I were able. I probably would have tried to do it the "easy way" that doesn't involve soldering, but you make a very good point about not trusting a push cap (the H2O pressure is pretty high in our house) and there is enough space to work in there that putting in a nice, dependable copper cap will not be too difficult. – nbm May 5 '15 at 19:51
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Did or does your house have a swamp cooler? I just found an extra valve under my guest bathroom sink. My guest bathroom is located right by the return air duct so maybe crawl in the attic and see if you have a copper pipe running to the roof from where your sink is. If so, it’s more than likely just for MAU/Swamp coolers. If you don’t have one simply go with your original plan if you haven’t already.

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