Is it possible to solder up a leaking copper union?

[rwh442]

I have a leaking copper union around my DHW mixing valve. There is a slight blemish on the seal and I CANNOT get the thing to stop leaking. I tried valve lapping compound to try and "smooth" it out. No good. Pipe dope sealants last for about a month or two then start a slow leak that gradually ends up leaking faster. Just thought about opening it up, adding flux and soldering it together. Anyone ever try this with success? If not I'll have to do a lot of plumbing redos. I have grown to hate copper unions.

 

[woodsman23]

Unions must be dry before soldering. Why not just cut it stick in some balled up bread to catch any excess water close it up and solder it back up, bread will break down and just drain out of the line.

 

[rwh442]

I'm just trying to save some time by soldering the union up (if possible) at the threaded connection. If I have to repipe and solder there will no longer be another union - guaranteed.

 

[Eric Johnson]

The bread is an old plumber's trick that works pretty well. As woodsman noted, the parts being soldered must be dry. A quick & dirty alternative would be to add a bottle of boiler Stop-Leak (about $10) to your system. Heating system pros scorn this approach, but it works. If you solder it, make sure there's a vent open somewhere to relieve the pressure. You don't want to try to be pouring solder into a pressurized leak. Dry and pressure-free are the critical elements to success.

 

[mole]

You might want to try a little Permatex #2 (the non-hardening type) on both mating surfaces. I think it works better than pipe dope.

 

[pybyr]

If the other suggested fixes above don't work, and before taking it out or trying to solder it (you'd probably need a bigger-than-homeowner torch to solder the actual mating surfaces of the union if it's a union of any size), what about getting a piece of silicone rubber gasket sheet (or other appropriate material) from some place like McMaster-Carr and cutting a homemade gasket to go in-between the mating surfaces?

 

[djblech]

I was told by a plumber to smear a little silicone calk on each surface before you tighten. Thats easy to try
Doug

 

[rwh442]

I think I have some Permatex #2 and definately silicone caulk around. I'll try those first.

We can probably start another thread about the quality of copper unions. Probably 75% good?

 

[Tony H]

If you do end up soldering it you will want to have an oxygen - acetylene torch so you can get enough heat in the nut to make a good connection. Propane will not be hot enough and probably Mapp gas won't be either. Advise from a plumber I know when I had one leaking but it turned out to be fixed with a little permetex and tightening with a very large wrench.

 

[drizler]

If you want to stop that leak forever yet not mess with solder try putting some automotive windwhield urethane sealer on it. That stuff hardens like a rubber rock but you will probably have to cut it off if you ever need it again. It will seal up about any leak you can find though. Personally I was never one for sealing with silicone as it doesn't stick to a lot of things too well. A similar product to urethane is 3M 5200 Marine sealant. That is used to seal fittings drilled in fiberglass boat hulls underwater so it will do the same permanent job. Either one of those will definitely do the deed.

 

[in hot water]

cut the union out and solder a repair coupling, or two in place. Those unions are nothing but trouble. If you end up soldering it, it's really not a union anymore anyway. Soldering the large thread will not solve the problem, you would need to solder the faces together.

More often then not they end up with solder on the faces when yon install them and they never come apart again.

If water is a problem you can buy compression repair couplings, (open center) it still serves as a union and doesn't need a torch.

The only good copper or brass unions are the ones with an o-ring grooved into them.

hr