Dielectric Union

A dielectric union contains a non-conductive sleeve, washer, or spacer between two threaded ports that would otherwise touch. Copper to steel, brass to galvanized, aluminum to copper—any two different metals in contact with water will corrode each other through galvanic action. The insulator breaks that circuit.

In the field, dielectric unions show up at water heater inlets and outlets (especially when code requires them between copper lines and galvanized tanks), at where copper meets steel in mixed-metal systems, and at isolation points in radiant or hydronic loops. They're cheap insurance against pinhole leaks and early failures that come from ignoring this detail.

The catch: the insulating element can degrade or fail over time, especially under high temperature or if water chemistry is corrosive. They're not a permanent solution in hostile conditions, and some codes restrict their use in certain applications. Install them right—hand-tight is usually fine, don't over-torque—and they'll do their job for years.