The reason that it has an arrow is, there should be a cap on the side. When you go to drain the lines, and the arrow is in the right direction, you can drain it by taking the cap off, and it will drain what ever is in the line from the valve on. If the arrow is reversed, an you try taking the cap off, the thing will SHOOT off with pressure because its in the wrong direction and pressure is not taken off by shutting off the valve...
FYI...
Jesse
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