Hi. I just bought a house with two apartments in a 2 story home. Apartment 1 is downstairs and apartment 2 is upstairs. I have separate water meters for each. I have separate water heaters for each. The water heaters and meters are in the basement.
I just found out that somewhere in the house the water systems are connected. Example. Water meter of apt 1 turns when apt 2 has turned on the water, or apt 2 gets hot water even though apt 2 hot water is off.
Anyway, they are mixed up. I have traced all lines in the basement. They are all correct, so the system must be connected on the first or second floor somewhere. Here is my problem. There are 4 copper lines going up from the basement and they come out in the ceiling of the downstairs apt. which is about 10 feet of where you cannot see them. The 4 lines are tightly clumped together. How can I identify which line is which? A friend said something that he heard you could connect a battery and a voltmeter but he did not know any specifics? How do the pros figure this out. I just want to know which line is which.

A friend said something that he heard you could connect a battery and a voltmeter
I would put any faith in that.


they come out in the ceiling of the downstairs apt. which is about 10 feet of where you cannot see them

I am assuming you mean that you can see where the pipes pass to the upper apartment from a distance of about 10'. You said ceiling of first floor apt. Do you mean ceiling of basement?

First thing I would do is try to determine if the cross connection is in the hot or the cold or both. Then I would try to determine if it occurs at all places. Does Apt 1 get water when meter for Apt 1 is turned off? Does Apt 2 get water when meter for Apt 2 is turned off? To determine hot or cold close input valve to water heaters. Does what ever is happening, happen at all faucets or just, say the kitchen or just the bath?

Know these things will help narrow the search for the cross connection.