Water pipe; will solder flow upwards?

My girlfriend's father helped me install a new water heater, and I watched closely in the interest of eventually being able to do something my darned self someday. Well, it turns out my chance came sooner than I thought. The pipe feeding cold water to the tank has a slow leak at one of the connectors (copper pipe fitted to another copper pipe). Looking at it closely, it doesn't have the silveryness of the other connections, so maybe we forgot to solder it, but it's such a slow leak that this seems unlikely.
Anyway, the leak is from the BOTTOM of the connection, so if I want to fx this without cutting pipe and dismantling everything all over again, the solder would have to suck upwards into the joint. So my questions are:
1) with a cutoff valve being right above where this leak is, can I assume that shutting it off and draining the water heater a little bit will give me a water-free inside of the pipe at that point so I can successfully heat up the pipe enough for solder?
2) Assuming "yes" to the above, will solder flow upwards into this joint, "sucked" up by the heat?
Thanks for any input here, I'm a floundering newbie. I appreciate your time.
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CompleteNewb wrote:

Yes, drain the heater a bit, but do this with a hot tap upstairs open to admit air. Otherwise, you will create a vacuum in the tank and that will suck the solder past where you want it.
As you heat the joint, apply some flux from below. Then melt the solder into the joint. Capillary action will try to flow the solder upward.
Jim
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May work as long as you can get some air into the tank to displace the water.

Yes Clean the joint as best you can. Wipe some flux on it. Put the heat right on the joint where the leak is until it get hot enough to melt solder when you touch it to the joint. Put a quick wipe of solder, then wipe the joint with a rag.
You want the point of the flame above the joint where the tubing slips into the valve, you want to touch the solder right on the joint. The entire procedure will take but a minute or so.
--
Ed
http://pages.cthome.net/edhome/



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dont solder close on the input side of the tank you can melt the dip tube.
you will have to take the joint apart and clean all pieces completely.
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Solder will flow up, but not in this case. The reason you have a leak is that you didn't clean or flux the joint properly. Now that you have heated it, it is even more oxidized that it was before. No chance it will take solder. You have to take it all apart and redo it properly. Sorry, but that's the way it is.
The same thing happened to my water heater a few years back. It was only a drop every 5 minutes or so and I chose to ignore it, as they sometimes clog up by themselves. It never did, but when I put on a pressure reducer, going from 90psi to 55 psi, the leak stopped. If your leak is worse than that, you better fix it; I don't think it can result in failure, but the cost if it does is just too high.
As someone else suggested, you have to be careful using heat near the inlet. Consider using a union there.
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On Tue, 29 May 2007 03:46:10 +0000, Toller wrote:

Whats the purpose of a pressure reducer? Besides reducing pressure of course?? does this add any benefit to the household water system to drop the pressure?
Thanks,
dnoyeB
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If it's excessively high or fluctuates excessively, yes. Otherwise, no need.
90 lbs is too high for household (except perhaps for outdoor spigots). Leads to water hammer, failed toilet/dishwasher/washing machine valves, etc., ...
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Solder does flow up to some degree.
[The following presumes the leak is between the shutoff valve & the water heater] But in this case you might be better off by removing the pipe in question and instead of fixing that particular joint, remove it completely and install a flexible copper pipe. Note that they are not too flexible really, but they do bend a bit. Mostly the advantage here is that they have threaded ends - no need for a plumber or even a torch in the future should you have some need to do anything to the water heater. Solder on a threaded end to the cold water pipe, then some pipe dope or teflon tape for the threads & you're back in business.
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