A plumber is testing the potable water piping for a new residential installation. The system must be tested with water at a minimum pressure of 100 psi for at least 15 minutes. What is the exception to using water for this test?

• Air may be used, except for copper piping.
• Air may be used, except for plastic piping.
• Air may be used, provided the pressure is doubled.
• Water must always be used; there is no exception.