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Eric_ht 09-07-2008 10:47 PM

Copper Soldering Problem
I'm just a DIY'er with copper plumbing. I appear to have a PH imbalance in my well water (another story) so my 30+ year old copper plumbing gets a few pin holes and have to be repaired. Over the past few years I've sucessfully repaired 5 or 6 a year. My usual problems with soldering have been due to water in the lines which I usually solve with some loaf bread in the line.

But this past weekend I cut a piece of 1/2 inch copper and was attempting to solder the couplings to it before attaching it to existing line. After 4 attempts I could not get the new piece to accept any solder in the coupling!

I sanded and cleaned both pieces (tubing and coupling), I used sand paper and steel wool and never touched it with my bare hands. I also fluxed both pieces. The solder just rolled off and would not go into the joint. So I ended up at lowes at 10PM buying plastic snap on couplings!

Could the flux have been bad? Can you get the joint too hot? Is steel wool bad for cleaning?

Like I said I'm not a professional plumber but i've never had this happen before!


Termite 09-07-2008 10:57 PM

You more than likely overheated your flux and it liquified and rolled out of the joint. Overheating the joints sometimes lets that flux get out of parts of the joint, which can result in leaks over time due to poor solder coverage of the full depth of the joint.

I hate the grayish-black flux that comes in cans. I'd suggest getting a little pint tub of the amber-colored paste flux. It is nice and thick and seems to be a little more forgiving with heat than the black stuff.

Sounds like you know the bread dough trick, but water or steam can cause the joint to never heat correctly and you'll never get solder in it. Might double check that and wad up some bread in there just for good measure.

Eric_ht 09-07-2008 11:03 PM

Thanks Mite. The flux was an amber/brown looking stuff. I've always wondered about heating it too much. I think I tend to over heat it, thinking the hotter it is the better the solder will melt and go in. I'm probably over looking the importance of the flux. It just seems the flux seems to start dripping out after a few seconds and the joint isn't hot enough? This piece was being soldered while it was off the main line so water was not a problem on this one.

Termite 09-07-2008 11:38 PM

Some flux will always drip out when the copper gets hot. Be sure you're fluxing the pipe as well as the inside of the fitting thoroughly. I like to use a lot, but I can't honestly say whether it helps or not.

My suggestion is to keep touching the solder to the joint until it melts and gets drawn in by the flux. That way you're not heating too long before you try getting the solder in.

It takes practice, but you'll get it.

Nestor_Kelebay 09-08-2008 08:17 PM


Here's the $64,000 question:

Did you provide a route for the hot pressurized air inside that pipe to escape?

You see, it is capillary pressure that sucks the molten solder into the joint. However, as you heat that joint, you heat the air inside the pipe too, and that results in an increase in pressure inside the pipe UNLESS you provide some path for that pressured up hot air to escape OTHER THAN the joint you're trying to solder.

I expect what happened is that when the joint heated up, the air pressure inside the pipe pushing the flux out of the joint was stronger than the capillary pressure wanting to suck molten solder into the joint. The air pressure won, and you lost.

Just remember that there always has to be a way for the air pressure that builds up inside the pipe to be released so that capillary pressure doesn't have to fight an uphill battle to pull the solder into the joint.

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