Compression fittings use a nut slid over a length of tubing followed by a ferrule or an O-ring. This nut is tightened against another line or fitting, which in turn compresses the ferrule or O-ring, thereby creating a seal. The compression fittings are most commonly found in plumbing applications where a fast, easy seal is required. The fittings are typically a single-use product, but many types of compression fittings can be re-used by replacing the ferrule or O-ring and re-tightening the nut.
Found under many household and commercial properties' sinks, compression fittings provide watertight seals in drains as well as water lines and fixtures or faucet fittings. The fittings rarely require more than hand tightening to achieve a seal and can even be used when mixing plumbing types such as stainless steel with copper or plastic. Compression fittings rarely require pipe sealant or Teflon thread tape to achieve a watertight seal. In rare occasions, a fitting will require a slight turn with a wrench to complete the seal.
Often used in tight spaces that would make soldering a fire hazard, compression fittings are easily installed. These fittings are also used in high-risk applications such as gas and oil lines where application of flame or heat would be too dangerous. The most common cause for a compression fitting to fail is from over-tightening it at the time of installation. By over-tightening, the ferrule becomes out of round and fails to give a complete seal. When the fitting is sealed a leak test is performed. If any leak occurs at the fitting, the nut is slowly tightened until the leak disappears.
While extremely useful in sealing plumbing lines, compression fittings are not as strong or as visually impressive as soldered and sweated connections. In applications which are prone to over tightening or rigorous use such as an outdoor water faucet, a soldered connection may be a better option than compression fittings. Also, in areas and applications that are prone to water line chattering and shaking, the soldered connection may prove to be more durable.
In applications that mandate occasional removal or disassembly for maintenance or meter changing, compression fittings are preferred for the ease of re-assembly. Requiring little or no tools to complete the assembly, the compression fitting is easy to install in cases of emergency and are often installed as a temporary fix to a problem that will eventually be solved with the application of a soldered connection.