Short definition
A dielectric union is a fitting that electrically isolates two dissimilar metals at a connection — typically copper to galvanized steel. Without isolation, a galvanic cell forms at the joint and the steel side corrodes preferentially, pinholing through within a few years. Most commonly required at the cold inlet and hot outlet of a residential water heater where copper supply lines meet the heater’s steel nipples.
What it is
The fitting looks like a standard threaded union — brass body, central threaded ring — but with a critical addition: a non-conductive plastic or nylon liner and a rubber gasket separating the two sides. When tightened, the metal halves never touch each other; current can’t flow across the joint, so the galvanic cell that drives differential corrosion can’t form.
Modern alternatives include dielectric flex connectors — brass-bodied flexible water-heater supplies with the dielectric isolator built in. These are now standard on most residential water-heater installations in Washington and elsewhere.
Why it matters to a homeowner
If you’ve never heard of a dielectric union and your water heater connections show greenish or rust-colored corrosion at the nipples, this is probably what’s missing. The classic failure pattern in older WA homes is a 1970s install where copper went straight onto a galvanized water-heater nipple — by 2000, the nipple is pinhole-leaking from the inside out, slow drip becoming a wet floor.
When a quote talks about “dielectric upgrade” or “code compliance at heater connections,” they mean adding (or replacing) this fitting. It’s a standard part of every modern water-heater install in Washington.
Common variants and what a dielectric union is not
- Dielectric union vs. brass nipple. A brass nipple separates copper from steel but does not electrically isolate them — brass conducts. A true dielectric has a plastic or nylon liner that breaks the circuit. Some older codes accept brass nipples as a buffer; current best practice is a real dielectric.
- Dielectric flex connector — brass-bodied flexible water-heater supply with dielectric isolation built in. Standard on modern installs.
Common failure modes
- Mineral buildup bridging the gap — over years, hard-water deposits in the gasket region can re-create the galvanic cell.
- Plastic liner cracked by heat during sweating of adjacent copper — installer error.
- Wrong orientation — some dielectric unions have a flow direction marked.
- Most common: never installed at all — direct copper-to-galvanized contact corrodes within 3 to 8 years.