Compression joints are used in plumbing in particular situations. New materials and processes are reducing the amount of compression fittings being used today. Still, you will find these connections on refrigerator water lines, under the sink water filtration systems, instant hot water heaters, evaporative cooler water supplies, and supply lines to faucets and toilets. The compression joint depends upon three parts to make it work, namely, the ferrule, compression nut, and an adaptor or valve body with a fitting hub. The nut compresses the ferrule against the hub and makes the connection water tight.
Turn off the water to the water line serving the compression joint.
Unscrew the compression nut from the valve body or fitting by securing the body or fitting between the jaws of one wrench and turning the nut with the other wrench.
Slide the compression nut away from the ferrule.
Cut the pipe just behind the ferrule using the pipe or tubing cutter. The back of the ferrule is the part that is farthest from the pipe or tubing end.
Slide the new ferrule on to the pipe or tubing end.
Brush pipe joint compound on to the ferrule, and pipe or tubing, using the brush attached to the cap of the pipe joint compound can.
Slide the end of the pipe or tubing into the adaptor or valve body hub. Push the pipe or tubing all the way into the hub until it stops.
Slide the compression nut against the back of the ferrule and continue sliding the two along the pipe or tubing until they come to rest against the valve body hub or fitting hub.
Hand thread the nut on to the hub threads.
Tighten the compression nut with one adjustable wrench, while holding the valve or fitting hub between the jaws of the other wrench.
Turn on the water. If there is a leak, continue tightening the compression nut until the leak stops.
Use caution when tightening because even though compression fittings can take a lot of torque it is possible to tighten them to the point where the tubing or pipe is twisted and broken.