If you've ever had a water heater connection fail prematurely or seen a greenish-white crust forming where copper meets steel, you've seen galvanic corrosion in action. Dielectric fittings exist to stop that from happening — and in many jurisdictions, they're required by code.
What Is Galvanic Corrosion?
When two dissimilar metals (like copper and steel) are in direct contact in the presence of water, an electrochemical reaction occurs. The less noble metal corrodes faster than it would on its own. In plumbing, this is most common at water heater connections, where copper supply lines meet steel tank fittings.
What Does a Dielectric Fitting Do?
A dielectric fitting creates an electrical barrier between the two metals, interrupting the galvanic circuit. It uses a plastic or rubber sleeve and washer to prevent direct metal-to-metal contact while still making a watertight connection.
When Are They Required?
Most plumbing codes — including the Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) — require dielectric separation wherever copper connects to galvanized steel or black iron pipe. The most common applications are:
- Water heater inlet and outlet connections
- Boiler connections with mixed-metal piping
- Any copper-to-steel transition in a potable water system
Always check your local code, but if you're connecting copper to steel, use a dielectric fitting.
Dielectric Union vs. Dielectric Nipple — Which Do You Need?
Dielectric unions are used where you need to be able to disconnect the joint later (water heaters, boilers). They create the separation and allow future serviceability.
Dielectric nipples have a plastic liner inside a steel shell — they're used in tight spaces where a union won't fit, or where the connection doesn't need to be taken apart.
For water heater work, unions are the standard choice. For boiler supply/return connections in confined spaces, a nipple may be more practical.
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