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Pipe Soldering

  1. Oct 29, 2009 #1
    I have been told that to solder properly the source of heat is to be directed at the bottom of object to be soldered (6 o'clock), Particularly, During a soldering operation to join two copper pipes.When I asked why that is, I was told it was due to convection distributing the heat around the pipe or object. I don't understand the need for convective heat distribution in this application given the more than adequate thermal conductive properties of copper.

    Does it really make a difference where the heat is directed? the convective heat transfer (and I assume by convective they are referring to heat transfer in the gas phase on the surface of the pipe) should be negligible given coppers ability to transfer heat. What do you think about this?

    Thanks so much.
  2. jcsd
  3. Oct 29, 2009 #2


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    Science Advisor

    I agree that it is splitting a hair that doesn't need to be split. I side with you that the majority of the heat transfer is going to be through conduction. I also can not say I have seen a theoretical dissertation of any kind on the topic. I'll have to see what I can dig up. You have piqued my interest. It honestly sounds like a myth that has been passed along for too long.

    In my experience (I am not a master plumber by any means) I have put the torch in a lot of different positions on the soldered part and never had an issue with solder flow. The only rules I follow are
    - the solder follows the heat
    - don't burn the house down
    - I also try to put the solder at the 12 o'clock position just to let gravity help
  4. Oct 30, 2009 #3
    I AM a plumber, and no, it doesn't matter what physical 'clock' location you put the torch. If so, it would be really difficult to solder a vertical joint! The correct steps are:
    1. Make sure both the fitting and the pipe are really clean. Both joining surfaces should be bright copper, with no discoloration at all. Use emery sandpaper or a specially sized wire brush that can do both the inside and outside surfaces.
    2. Use the correct paste or liquid flux for copper pipe. Coat the inside and outside surfaces liberally. Assemble the joint completely, making sure that the pipe bottoms out in the fitting.
    3. Apply heat from a properly sized torch. A propane or mapp gas torch will do up to 1" copper; anything bigger should be done with acetylene. Do not overheat the joint, or the solder will crystallize and leak for sure. Also, there can be no water inside the pipe. Steam will not allow the solder to melt.
    4. At first, heat the joint equally, moving the flame from fitting to pipe and all around for 5-10 seconds.
    5. Place the flame on the fitting, moving it slightly, and apply solder to the opposite side, wherever that happens to be. Same applies to an upside down joint. Keep testing the temp by touching the solder to the joint area; when it starts to melt, back off a bit on the heat and run the solder around the joint.
    6. Capillary action will 'wick' the solder into the joint and fill it, even if it's down-facing. The heat draws the solder toward itself, up into the fitting and toward the torch location. If it starts to drip out of a down-facing joint, it's too hot. The correct temp should turn the copper different colors, from green to blue to gray, but not flaky black. That's too hot.
    7. Once the solder has flowed around the joint, immediately remove the heat. The stick of solder can be used to fling off any drips. Ideally, there should be no solder running down the pipe or fitting; just a nice clean silver ring visible completely around the joint. If it doesn't go all the way around, apply more flux with a brush, more heat, and more solder.
    8. Obviously, just like the flux, make sure you use non-core solder made for copper pipe--absolutely not acid core. Rosin core electronic solder can be used in a pinch, but avoid it if possible.
    9. Allow the solder to cool and set on its own for a bit--throwing on a cold wet rag while the solder is still molten will shock the joint and probably crack and crystallize the solder.

    A little practice, and every joint will look like it was professionally done. Just start out keeping the heat on the opposite side from the solder, on the fitting and not the pipe, and it should wick up cleanly and completely. Hope that helps!!
    Last edited: Oct 30, 2009
  5. Oct 31, 2009 #4


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    That was a great post, NeuronsAtWork; very informative. I'm pretty good at electrical soldering, but this was a whole new realm for me.
    Perhaps the mods would see fit to copy your lesson into a 'sticky' or FAQ or similar. I'm sure that others in my situation will benefit from it. I know that I'll be referring to it during future building projects.
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