Basically the two that are running perpendicular to the UV are to completely shut off flow to the UV other than that they are always wide open. The one that is in the middle of the two pipes running perpendicular(on the pipe parallel to the UV is to control how much flow is actually allowed to run to the UV. This I keep about 1/4 closed so that only a fraction of my 900 gallons per hour gets sent to my UV. In the pic you cant really see the other pipe running down on the left to the UV because it is behind my drain line, but it is the same as the right and the ball valve in the middle is behind the skimmer.