[68sting]

I have a 3/4" pex solder/barb fitting that I sweated in a gate valve. I missed a small spot and it leaks. I have tried to reheat the joint and add some solder but I can't get the joint hot enough now to melt the old solder. Any ideas on how to handle this?

[KISS]

Well, if you can't get the joint hot enough, then the pipes have water in them.

You must drain the water and have some taps open upstream.

Usually when the joints are fresh it's usually possible to repair in this way, otherwise... You have to take it apart and do it over.

[ballengerb1]

You may also need to disassemble the valve so you don't melt and of the plastic or rubber parts. You can't sold a pipe with any water in it so undo the PEX pipe and then crack that valve open.

[68sting]

Do I just dab some flux around the hole and add some solder?

[KISS]

Yep. It will either work or not work. No guarantees.

[ballengerb1]

Once you are sure all water is gone and the unit is disassembled to the point where the heat won't damage any washers you should wipe it down, reflux and reheat. You may or may not need solder, I don't know how much you applied the first time. If the two pieces were cleaned properly to start with this should do it. If it doesn't work I would assume a spot of grease or crud is present so you must totally clean the soldered joint after disassembly. This means trying to remove the solder too.

[Milo Dolezal]

You have to have NO water in pipe. Period. No compromise here. What happens is that as you heat up the pipe, water starts vaporizing, vapor expends in volume, creates pressure and blows hot solder out of the pipe. If you have lots of water in pipe, than you may not be even able to heat it up to the melting point at all.

Best and fastest way is to cut the pipe, drain water, install coupling and re-solder - including your original joint.

[iamgrowler]

Do I just dab some flux around the hole and add some solder?

If you do so, you're merely tinning the joint, which may or may not work.

Personally, so I could sleep at night, I'd cut the valve out and start all over from scratch.