How Compression Fittings Work

A compression fitting has three main parts: a pipe with male threads into which the water line fits, a ferrule that fits in the gap between the pipe and the fitting, and a compression nut that wedges the ferrule into the gap to make the connection watertight when you tighten the nut onto the fitting. The nut compresses the ferrule into the gap, which is why it's called a compression fitting.

When you're installing a compression fitting, it's important to align the pipe with the fitting. If the pipe enters at an angle, the gap between it and the male threaded fitting won't be uniform; the gap will be uneven, and the ferrule will make a bad seal. Misalignment is the most common reason for compression fitting failure and leaking.