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I may have forgotten to use soldering flux paste when I replaced a water valve on 1/2" copper tubing. It was in a tight place and sweating pipe is not something I do often. Some of the joints I attached on the ground but others had to be soldered in place, near the ceiling.

In any case, after I put the new tubing and valve in place and soldered it, I looked up and could not recall putting flux on one of the joints.

If I soldered the joint without flux and it does not leak should I leave it alone or redo it?

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    If there really was no flux then you have a "cold" solder joint. The solder may stick in spots but it will not have "wicked" into the "sweat" joint properly and will not completely seal the joint. Generally you should be able to tell this because the solder will not smoothly cover the joint but will be all lumpy and irregular (and the crack between the two pieces will be visible). Unfortunately, just the fact that the joint does not leak is no assurance -- an unfluxed joint is subject to corrosive damage over time, plus heat/cool cycles will stress it, and it could fail at any time.
    – Hot Licks
    May 30, 2016 at 20:06

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There are several possibilities to consider.

  1. If you had really cleaned the joint beforehand with emery cloth or steel wool (both parts of the mated joint) there there is a good chance of a decent joint. The cleaning action helps clear out any oxidation on the parts that prevents the solder from adhering to the copper. (Keep in mind though that the simple act of applying heat to the joint also brings on oxidation.

  2. If you had happened to be using a rosin core silver solder that flux built into the solder wire may have been more than enough to clean the joint for proper soldering.

  3. An improperly cleaned joint may appear to not leak because it is tight or a small amount of solder may have flowed into the joint but it is no substitute for a properly made joint. One way to understand the sweat process success is to carefully observe how the solder sucks into the joint just as the silver solder wire is touched to the joint and melts.

So if your situation is like #3 (and possibly #1) you will really probably want to take things back apart and re-do this work. Last thing you want is for the joint to come apart under mechanical stress on the pipe or start leaking after a year of so of thermal cycling of the joint.

It can be really be worth while to get a handfull of copper fittings and a length of copper tubing and practice a dozen or more test joints to get the techniques down. (Note that standing water in or on a pipe near where you are soldering can be a huge detriment to getting good sweat joints.

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    Yep, if you take the joint apart you need to get the pipe and fittings completely dry inside and out before you attempt to reassemble it. Even a tiny bit of water in the pipe can frustrate attempts to get a decent solder joint.
    – Hot Licks
    May 30, 2016 at 20:07
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    @Yehuda_NYC - And if you try to take apart and re-use fittings that were already previously soldered it is essential to remove all excess solder before re-use. Otherwise they will not fit together properly before re-soldering. Best success comes from discarding used components and starting with clean new ones. Make sure to always use silver solder (lead free) and rosin flux (non-acid) when working with copper water pipes.
    – Michael Karas ♦
    May 31, 2016 at 4:00

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