What purpose does the vertical, capped copper pipe serve on a washing machine water hook up?

Question

Just curious, on a washing maching water hook-up, why is there a tall (~1 ft) pipe going toward the ceiling; it is coming off both of the water lines and has a cap on the end? So, the copper piping bringing in the water comes down in an L shape but right before the k**b to turn the water on/off there is a pipe going up toward the ceiling. I don't see these on the shower lines or any other water line...

Answer 1

they are water hammers to keep pipes from vibrating.  if they do stop working.  you can shut the water off to your house, open a hose bib outside than go inside and turn on your washer hot and cold this will allow the waterlines to drain and allow air to enter into your system.  than close everything and turn your water back on and you now have air back in your arrestors

Answer 2

whoever installed the water lines for the washer hook up was trying to eliminate water hammer. water hammer happens when the water valve in the washing machine shuts off quickly, and causes the pipes to bang throughout the house. those extensions will work as long as they are full of air, if they fill with water, they no longer will do what they were intended to do. you can buy water hammer arrestors that will do the job better, from any place that sells plumbing supplies.