Compression Fitting

A compression fitting works by applying axial force through a threaded nut to squeeze a ferrule—a small cylindrical ring of copper, brass, or nylon—between the nut and fitting body. As the nut tightens, the ferrule deforms slightly and creates a metal-to-metal or metal-to-tubing seal that stops leaks.

In the field, compression fittings are popular for copper tubing, PEX, and poly because they don't require soldering, crimping, or special tools beyond a wrench. The tubing simply slides into the fitting, the ferrule seats over it, and you tighten the nut hand-tight plus one-half to three-quarters turn. Overtightening splits ferrules and damages tubing; undertightening causes weeps.

Compression fittings excel for shutoffs, water heaters, and accessible repairs where disassembly matters, but they're slower than soldering for large runs and more prone to leaks under vibration or thermal cycling. Most codes accept them in walls and buried, but job specs and jurisdiction vary. Always use the correct ferrule size and material for your tubing—a bad ferrule match is the fastest path to a callback.