Backflow Preventer

Hi, At my church, the water company wants us to have a Double Check Backflow Preventer Valve installed. I think I can put it in myself. I know it needs to be inspected yearly but I think I can install it. Question is, Water line enters the basement and goes into a 90 directly vertical into the meter and then to the ceiling where it goes horizontal. Can I install it on the vertical pipe line or does it have to sit the other way? does it matter?

Thanks,

Joe

Backflow Preventer