Plumbing Soldering brass fittings to copper

Stephen Tashi

Science Advisor
When a heavy brass fitting is soldered to copper pipe, how can I be sure the brass is hot enough? Does the visual appearance of flux indicate when a joint is hot enough to apply solder?
When soldering a brass fitting (e.g. ) to copper (e.g. a 3/4 inch copper coupling) I think there is a danger that copper gets hot enough to melt the solder, but the brass fitting (being more massive) isn't hot enough. Can I use the visual appearance of flux (e.g. Oatey H-20@5) on the brass as a clue? Should the flux liquify or bubble? Should I apply the torch mainly to the brass instead of at the middle of the joint?

Another question, on the above example is whether the raised lip on the brass fitting should be set snug against the end of the coupling - or should I leave a small gap for the solder to flow into.

I ask the general question, because I soldered the particular example and even though the solder flowed around the joint, the joint leaked.


Science Advisor
Gold Member
I have soldered a lot of copper pipe and a fair amount of copper pipe to brass valves. If you had a leak the reason could be poor application (incomplete) of flux or you did not have the solder flowing how you thought you did. You may also have poor luck with a dented fitting. I have had that happen. It has become so second nature to me that it is hard for me to imagine screwing one up. What I have found is that once the solder starts to flow easily into the joint, it is hot enough. As long as you have adequate flux it should hold.
I have soldered some seemingly impossible to get at joints tucked up in between ceiling joists/ductwork/etc. and have had good luck as long as the work was clean, well coated with flux and enough heat to easily flow the solder. It is easy to assume the work needs to be heated all the way around but this is not necessarily the case. I always solder brass to copper on a bench if possible and then solder the assembly into place just to be safe. A heavy brass fitting definitely takes more heat and I prefer doing that work where I can see all the way around the fitting.
As to where to heat the joint, heat the outer part. I assume the fitting you linked to fits around the copper pipe and not inside a copper fitting.


Science Advisor
What type and grade of solder are you using, silver solder?
If you are soldering that pex fitting to Cu pipe, then if I was doing it i'd run the torch over both, that brass fitting isn't that big, so should heat fairly quickly, if the brass fitting was larger eg a valve or something (with the valve taken out), then I almost eyeball the differences in mass and apply heat proportionally more to the larger mass.

I always clean both surfaces with plumbers sand paper (even if new fittings), apply flux to both, bottom the pipe out in the fitting heat with torch, you'll see the flux start to boil off, I then repeatedly touch the fitting and/or pipe with the solder wire (hold the flame away while doing this) to check if the temp is there, once it is it will wick in there nicely. Just be careful not to let any solder run onto the pex side of things or that will never seal!

Regarding leaks, I had one annoying one which looked like it was soldered properly, but after about and hr under pressure the thing sprung a pin hole leak, seems the flux somehow bubbled or something in the solder, allowing water out. Pain because the pipe was full of water and closed off so figuring out how to get the water out again and reflow it was the challenge. But once that was figured out, I just re applied flux, reflowed the solder and voila, leak gone.

Stephen Tashi

Science Advisor
I always solder brass to copper on a bench if possible and then solder the assembly into place just to be safe.
That's what I did on my second try. Working at a bench, I soldered the brass adapater from 3/4 inch copper to 1 inch PEX onto a copper couping. Then I soldered the coupling on the copper line, working in a trench. So far, no leaks.
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Science Advisor
Gold Member
Flux should bubble, but not turn black, burn and dry out. Use a hotter torch to reach temperature more quickly if that’s happening. Always heat the fitting. Bottom pipes fully into fittings.

One more thing that helps—clean the soldered joint with a small wire brush dipped in water and wipe with a rag after the joint cools. That will prevent ugly green corrosion later.

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