In the rental house I'm in now, the T&P release valve on the water heater is connected with copper discharge tubing which exits the garage. See this picture copper tubing.

The right-side of the pipe seems to be simply threaded into the T&P discharge valve with some plumber's tape, but the left side is connected to an (otherwise soldered) series of copper pipes by this nut looking thing. I'm curious how this arrangement was likely created, given the fact that these pipes are obviously not flexible connectors. Was it likely that the left side was connected first, and then the water heater was positioned into place exactly so the threading on the right worked? Or was this horizontal copper pipe screwed into the T&P release valve first (allowing the water heater to be positioned semi-independently) and then that nut-like thing somehow helping the copper piping to be connected to the elbow in a way which helped the fact that otherwise the piping would have to have been seemingly cut to the exact perfect length to work. Perhaps the nut allows for some fitting to be placed there and then the horizontal tube was soldered into place while connected to the water heater?

I don't need to re-engineer this or do something similar, but I'm curious how this connection would have likely been made with rigid piping. What is this nut? How would the left-hand side likely been connected?

  • 5
    since there is no water in the pipe, do a bit of investigating and loosen the nut .... you will quickly realize how everything was put together ... btw, it is called a union fitting
    – jsotola
    May 25, 2019 at 23:08
  • That is indeed a union and is often used as a coupler when there is not enough space for fitting up soldered connections, or for repair work, or sometimes when someone just has one handy in their fitting stash. @jsotola you should create an answer from your comment... May 25, 2019 at 23:43
  • 3
    The union (nut thing on the left) was likely added to make it possible to replace the relief valve without sweating pipe. Normally the just sweat (solder) together the pipe, that makes it impossible for someone that can’t sweat pipe tho.
    – Tyson
    May 25, 2019 at 23:45
  • 1
    @gammapoint, if you ever get the chance you should thank the plumber who installed it; it's a good practice in that scenario. May 26, 2019 at 1:35
  • Appreciate all the comments. I wasn't around when this was installed, so not possible to thank the plumber, but I now can appreciate someone putting one of these in.
    – gammapoint
    May 26, 2019 at 19:18

1 Answer 1


That fitting is called a union, it was installed to allow for the T&P and/or the entire water heater to be replaced without needing to cut and resolder the relief valve discharge line.

As for the order of operations during initial installation, there really is no way to tell for sure. Assuming the discharge line exits the utility area where the heater is located then a very likely scenario is that the installer may have "roughed in" the discharge line prior to the closing of the walls with sheetrock and then simply piped to it from the T&P valve once he placed the water heater in its final location. In this case it is likely that they would have begun on the right with the male adapter going into the T&P valve, then measured and cut the pieces and added the fittings, working to the left until they made connection with the previously installed discharge piping. It is also possible that they piped it the opposite direction, from left to right, but personally I think its easier to go right to left in this case. The union isn't required either by code or by the fact that it is rigid copper piping, it is simply there to ease the replacement of the tank itself and the relief valve.

  • What? Way to many words union fitting is for.....
    – user101687
    May 26, 2019 at 1:23
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    @Robert Moody, I respectfully disagree; the original post had many words and questions, specific to this scenario. A general statement of what a "union fitting is for" would not have satisfied the OP. This answers pretty much all of it in as succinct a way possible considering the OP. +1 May 26, 2019 at 1:33
  • I have read your comment 5 times and seem confused. succinct a way OP +1= Always drawn out not every one Harvard Grad, The average Joe cant read .My point. Even stuff i know ill read it and be what he say or is he just trying to talk smart.
    – user101687
    May 26, 2019 at 1:50
  • @Robert Moody, OP= "original post" or "original poster". The +1 was to indicate that I upvoted this answer because I feel it is helpful to the OP. The person whose answer you commented on is a new contributor to this forum; as such we try extra hard to keep things positive and constructive so as not to discourage further participation. If you feel you can answer the OP in a better way you should submit an answer, instead of a comment. Then the community will upvote your answer (if it addresses the question better). May 26, 2019 at 6:17
  • Found that very useful information. I myself are new and getting old ,and type and spell like a third grader .In the short time I have been on my reputation has risen. But have had a few more smart remarks than having positive help . Alot of times from long time users. to the point I was getting discouraged and stop being on this page. I try to put things short and helpful and correct as can. So any one can read and understand from a third grader up..It has take me more than a half hour to type this. So no one really knows how much effort i put into this .
    – user101687
    May 26, 2019 at 12:08

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