Good and bad copper sweat welds question

I've taken apart some of my own test welds, and while most of them look great (solder the length of the joint) every once in a while I do one that I know is bad before it even cools, because the hot solder just keeps rolling off the pipe instead of being sucked into the weld. When I take it apart to confirm, I see that there is no solder in the weld, just a small amount of black residue which I'm guessing is burnt flux.
Is that caused by something specific? Pipe too hot or too cold before welding? Is it something that can be fixed if the weld is in place, or will I have to always separate the joint and re-clean it, re-flux, and re-weld? I'm thinking if the pipe is too hot, I should be able to let it cool and then just try to scrape in some flux (it would get sucked in as the pipes re-heat and it melts) and re-solder.
I'm thinking especially if this happens on a joint where I _can't_ tell it is bad until it is too late (hard to reach valve installations, etc) that it would be nice to be able to just fix it in place- I'm working on a really old house and trying to put valves in-line in a couple of places I can barely reach, and I'm not sure I'll be able to see well enough to be certain that it is ok, and getting a valve back out to clean it will mean taking apart 2 joints, doubling my chances of any errors when putting it back (versus just fixing one weld).
Any advice welcome (other than "hire a pro", I wish I could afford ti but I can't). Thanks! Keith
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hire A Pro Mate!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
if you get both side clean and flux your joints will flow right. are you sanding the pipe and brushing the inside of the fitting to get it bright copper,if not it wont flow,bad flow is also caused by not getting both sides hot enough
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Just for the technical correctness, they are solder or sweat joints, not welds. Welding is a different process.
and while most of them look

Sounds like a dirty joint. Are you cleaning the joints first with steel wood, emory, wire brush, etc.,? Then using hte proper flux?

As mentioned, not clean enough, or, yo are overheating, burning up the flux and then adding some carbon to the mix.

Take it apart and start over. It will never get better unless you clean it.

No, there will be contaminants the the flux will not get rid of.

I'm thinking it would be nice too, but it just does not work that way.

Make another cut and put in a coupling to put it back together. That way, only one joint is taken apart, but still two to put it back. One being easier though.

Practice. Do the easy joints first and get the timing right. By the time you get to the difficult joints, you will have more experience. Double check your technique of where to heat the joint, how hot to heat it, how much solder to use. Rule of thumb is the length of solder is about equal to the diameter of the joint.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

99% of the time this is just a problem of not having the pipe clean enough. Steel wool with some flux in it does a good job.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Plenty of good advice on the joints. When there is any doubt about leaks, I generally pressurize the pipe with air, spray on soapy water and check for bubbles. Once you get water back in it makes it pretty hard to unsolder sometimes. Blowing the water out of the pipes with air also helps when soldering or unsoldering. Don Young

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.