Two weeks ago I ran new 1/2" copper type L water lines to the kitchen sink, 30' away from the bathroom and the rest of the plumbing. Now in the morning (after the water has been standing overnight), I get a slug of bad smelling water from the cold water line. It starts after running the water for maybe a minute and lasts about a minute. The smell reminds me of burnt plumbing flux or "burnt copper".
What could be the cause of this? What comes to mind is that I overheated one of the joints on the cold water line and some residue is slowly leaching into the water. If so, will this go away in the near term (a month or two), or do I need to locate the bad section and replace it? If replacement is required, any suggestions on locating the bad section? I could time the flow to see exactly how long until the bad smell starts, measure the flow rate, and determine a length away from the faucet based on the cross-sectional area of the pipe, but I'm not sure how accurate that would be.
BTW, there's no problem from the hot water line. In case it matters, I used hard pipe for the runs parrallel to the joists, and I pulled soft copper 5/8" tube through 1" holes in the joists for the perpendicular runs.