Smell in water

I recently ran some new copper water lines for a friend and they are complaining of a smell coming out of the water since the water lines were ran. The smell is worst first thing in the morning. It almost smells like gas. The local utility has tested the water and can find nothing by their tests but they smell it too. Before that the building was empty for a year and a half. I've talked to severaL friends in the plumbing business and they have no good answer. The only thing I can come up with is the city turned on the gas to the place and the water heater was calling for heat but wasn't full of water and possibly cooked the tank. The city had the water on at the street but not inside the building. Anyone have any ideas?? I used 1/2 L copper and fittings, solder and flux. Nothing that I can see that would cause a smell.

O.K., does the smell come from the cold side, hot side or both?

Were they living in the house B4 you changed the lines?

If there is a meter pit in the yard I would take some from there and see if the smell is B4 the meter or after.