There is a leak in the copper piping, and the solder won't stick. How do I fix this?
all the answers are correct here. your biggest problem will be draining ALL the water out of the system before replacing the problem fitting where the leak is occurring. i suggest using unions in case you have water problems.
with a torch, clean the pipe first; sand it with very fine emery cloth, then apply flux to the area where you want to put the solder. also, you should drain the pipe so it's empty of water, that way the pipe will get hot enough for the solder to stick, and penetrate the joint. good luck.
Shut off the water and drain the pipe. If there is water at the joint you cannot create a solder joint. The solder won't flow if the copper is not clean before you apply the flux. Clean the pipe with emery cloth or sand paper. Apply the acid flux, heat the joint and apply a non-lead solder. The solder will flow to the hot spot. So place the flame on the fitting rather than the pipe so the solder is drawn into the joint.
first, the copper has to be very clean. sand it down or use some steel wool. all the water must be out of the pipe or it will never get up to temp. use soldering flux (acid free) on the clean copper, apply liberally. then apply your heat. you need enough heat to get it up to temp relatively quickly, before the flux all burns off and the copper starts to oxidize. apply solder, let the copper pipe melt the solder, not the torch. it should flow nicely, and not be clumpy. if it's clumpy, it's not hot enough.
First you have to drain the water from the line or you won't be able to get the pipe hot enough to solder. Next you have to clean the surface of the pipe with sand paper... steel wool... or emery paper. Next you apply flux paste this is the thing that forms the bond between the solder and the copper pipe. Heat the pipe with a torch and then touch the solder to the pipe. When heated enough the solder will be sucked into the joint between the two pieces of copper. Let cool and then clean the surface if you want to. Turn the water back on and you are done.
