Solder corrosion and leaks on copper hot water lines

I'm a residential contractor. I have a house we restored/renovated for 7-8 years. The renovation started in 2006. It is a three story 7,500 sq ft house built in the 1830's. The solder joints at the elbows on the third floor hot water lines are corroding and starting to leak. There is a circulating pump on the system and the hot water lines also run through a geothermal AC/heat unit that has a heat exchanger that warms the water in the cooling mode. The owners moved in the house about 4-5 years after the start of the renovation and 95% of the copper water lines were replaced during the renovation but the pipes basically sat for 3-4 years before they were used on a constant basis. The corrosion is green and white on the outside of the pipe but the copper has turned black also at the corrosion areas. Some of the copper has corroded away on the outside and the copper has turned a red color in those areas. Not every elbow has corroded it appears to be random. The cold water does not appear to be affected from what we can see. It does not happen at couplings where two lengths of pipe are joined together. The copper water system everywhere else seems to be a normal copper patina where the copper has just darkened to a light brownish color. The heat exchanger in the geothermal system stopped working a year or 2 ago and was replaced. Not exactly sure of the date. The geothermal contractor says this is not a possible cause of the corrosion but I don't know enough about them to know. The hot water lines all have insulation installed on them. We have also had to replace the circulating pump twice now since they moved in. The hot water heater is also turned up to almost the highest setting. The third floor hot water system is separate from the other 2 floors. Any thoughts or Ideas that could be causing this? We had the water tested and it came back fine except for a slightly high PH of 8.2-8.5. I sent some of the elbows to a metallurgist and she says its galvanic corrosion occurring from the inside out but is not sure what is causing the corrosion. Any ideas?

contact the department of health, they will give you sterile jars to collect specimen. then have their lab analyse the water, i think its 5 bucks for the lab work

We had it tested by an independent lab and it came back fine with a slightly high Ph of 8.2-8.5

well i can tgell you that those solder joints will leak because the were not soldered properly. also it is good practice to use long sweep 90's on a recirc system to help keep friction domn or use 45's. also circ system should be on a timer if it runs non stop it will eat up the plumbing. it also looks like the flux was not wiped off.

How can you tell they were not soldered properly?

for one all the slag left over at the joints and the fittings were over heated. you can tell by the discoloration. A lot of heat actually changes the chemistry of the copper. Always sand cloth one direction, apply heat to pipe first with back and forth motion then slowly move to fitting. The flux causes capillary action to much heat will cause it to boil. also never cool joints with water let them dry naturally. Most people do not know proper solder technics. Copper that is installed correctly should last 30,40,50 years. Also did you ream pipe back to original pipe diameter? the ridge in the copper after it is cut causes the water to spin which will eat the pipe away and cause turbulance.

Would an overheated joint not leak for 3-7 years?

I was told all the discoloration was from the water sitting in the insulation holding it against the pipe and it was copper oxide. The discoloration is 8-12" away from the joint in some places. Also the pipe did not look like this after it was soldered. It looked like a regular soldered joint. Does this info change your mind at all? Or you think it was still overheated? I've worked with this plumber for 20 years and have never seen his work do this before. He plumbed my own personal house 13 years ago.

Thanks for any help or opinions. I appreciate as much information as I can get.

To clarify, there is a separate water heater for the third floor, and the thermostat is turned up to a rather high setting?

The reason I ask, is that hot water is more corrosive than cold water. Combine that with the PH being high, and that might be the source of the problem.

Yes, third floor has its own water heater and is separate from the other 2 floors. It is set almost to the max for temperature. And yes I agree and have read that higher velocity and higher temperatures increase rates of corrosion.

Is the inside of the pipe pitted if so it was not reamed? Is pipe type m or l? i agree with phish but i dont think the ph is the issue. if so i would think it would affect everything. not familiar with the geotherm is there a chemical in that system

It's type L pipe. No pitting on inside of pipe or leaks through copper. Just at solder joint.

see so you answered your question bad solder joints