A dielectric union is a two-piece plumbing fitting with a nonconductive barrier that separates dissimilar metals to curb galvanic corrosion at the joint.
Dielectric union meaning in plumbing
It’s a union with two different ends, each matched to a material, and a middle that stops electricity. One end threads or sweats to copper or brass. The other mates to steel or galvanized. Between them sits a plastic sleeve and a gasketed seat that keep metal faces apart. The nut backs off for service without cutting pipe, which is where it beats hard couplings.
| Connection | Risk without isolation | Typical fix |
|---|---|---|
| Copper to galvanized steel main | Pitting at the first steel thread and brown water | Dielectric union or brass converter fitting |
| Copper to carbon steel on hydronic coil | Localized rust near the interface | Dielectric union sized to flow |
| Water heater copper to steel tank outlet | Thread loss and stuck nipples | Dielectric nipples or flex connectors with isolators |
| Stainless to carbon steel in domestic water | Accelerated attack on carbon steel | Dielectric union or plastic-lined connector |
Many water heater kits ship with flexible stainless connectors that include plastic washers. That washer breaks the metal path and gives you the same benefit as a dedicated dielectric union. Several manufacturers point this out in their install tips, like the A. O. Smith guide that notes flexible connectors often include built-in dielectric fittings.
How a dielectric union works
Two shells meet at a seat that holds a nonmetal gasket. A sleeve slides over one shell so the two metals never touch. Tightening the union nut compresses the gasket and locks the assembly. Because the barrier stops current at the joint, the galvanic circuit loses a path. Without that circuit, the driving force for attack falls off.
The design also brings service advantages. Unions give you a break point for quick removal. On a heater changeout or valve swap, the nut backs off, the joint separates, and you keep adjacent work clean. For old houses with patchwork piping, that service break often saves hours.
When you need one (and when you don’t)
Use one any time dissimilar metals meet in potable water or open hydronic loops. Copper to galvanized steel is the classic case. Stainless to carbon steel is another. If you’re joining copper to brass, you can usually skip it because the metals sit close together on the galvanic chart. Many local rules spell this out. Illinois, for instance, states that copper to dissimilar metals must use a brass converter or a dielectric type union; see the Illinois union rule.
Water heaters get special attention. Tank tappings are steel on most glass-lined units. That means the first inch of thread is a hotspot. You can break the circuit with dielectric nipples, plastic-lined flex, or a true union. Major brands mention this in plain terms; the A. O. Smith install steps show options for solder, compression, and flex with isolation.
Mixed-metal repairs
On a patch where you switch from steel to copper, a dielectric union is fast and tidy. Sweat or press a copper stub to the union on the bench, thread the steel side in the field, then make your final joint cool. That staging keeps heat away from gaskets and saves the seat from damage.
Existing unions and flex connectors
If a heater already has plastic-lined flex, adding a second dielectric break rarely buys you anything. One break in the circuit does the job. Doubling up can add turbulence pockets and another set of gaskets to maintain.
Dielectric union vs dielectric nipple
Both break the galvanic path. A nipple is a one-piece fitting with a plastic liner that runs through the bore. It threads into the tank and accepts your pipe or connector. A union gives you that plus a handy service break. On tight closets or drop-in replacements, the union’s break point is worth the small cost bump.
There’s also the brass option. A short brass coupling or valve between copper and steel offers a middle ground. Some codes accept brass as a converter because it sits between the two on the galvanic chart. Where brass is allowed, it’s a rugged choice with fewer gaskets. Check your local rules before you pick a path.
What can go wrong
Leaks show up when seats get nicked or overtightened. Cross-threading the union nut can pinch the gasket and create a weep that only appears under high demand. Hard water can leave scale at the seat and hold the faces apart. Water with grit can scratch the gasket during assembly and cause a slow drip that’s tough to chase later.
Another pitfall is using the wrong size. Undersized unions add restriction and noise. Oversized parts can be bulky and hard to wrench in a chase. Match the union to the run, not the fixture rating. Flow charts from the valve or meter will guide your choice.
Close variation: dielectric union for water heaters
If your heater connects to copper, you’ll want an isolator at each hot and cold outlet. Choices include union-style dielectric fittings, dielectric nipples, and flex connectors with plastic washers. The goal is the same in each case: break the metal path and stop the joint from acting like a battery. Most kits sold at retailers ship with the flex route because it’s quick and serviceable.
Bonding and continuity
Because a dielectric break stops current, it can interrupt electrical bonding through the piping. If your jurisdiction uses the metal water piping as part of the bonding path, electricians may add a jumper around the meter or heater to keep continuity. That work belongs on the electrical permit. Don’t rely on the water alone to bond a system that needs a rated conductor.
Sizing, materials, and ratings
Domestic runs usually use 1/2-inch or 3/4-inch unions; larger sizes show up on boilers and multi-family risers. Shells are often steel on one side and brass on the other. Some brands use all-brass shells with inserts. Gaskets range from EPDM to nitrile. Look for NSF/ANSI 61 and 372 marks if the joint touches potable water. For pressure and temperature, choose parts that match the rest of the run so the union isn’t the weak link.
| Spec | Typical range | Where to check |
|---|---|---|
| Nominal size | 1/2 in. to 2 in. | Product sheet |
| Pressure rating | Commonly 150–300 psi | Stamping or cut sheet |
| Approvals | NSF/ANSI 61, 372 as labeled | Listing mark |
Installation steps that save callbacks
Dry fit first. Hand-start the nut to make sure threads seat clean. Back the nut off, then prep the ends. On the steel side, use a quality thread sealant rated for potable water. On the copper side, finish any soldering before the gasket goes near heat. If you press copper, wipe oil and grit from the sleeve so the gasket seats flat.
Snug the nut with a smooth jaw wrench. Stop once the leak stops; overtightening can deform the seat. Open a valve to purge air, then run hot and cold through the joint. Test under both low and high flow so you catch micro weeps that only show when the piping moves.
Placement tips
Keep unions out of walls and ceilings where you can’t see them. Place them near valves or equipment that will need service. Aim for straight runs on both sides so the gasket isn’t twisted as the nut tightens. Where space is tight, a short nipple and a street elbow can give you the swing room to pull the nut later.
Care and service
Once a year on a heater call, scan union joints for white crust, rust stains, or damp insulation. If you see deposits at the seat, relieve pressure and crack the nut to check the faces. Replace gaskets that look flat, torn, or hard. When a union has been in service for a long time, it’s smart to keep a spare on the truck so you don’t chase threads on a corroded shell.
Why galvanic corrosion happens
Put two different metals in contact with each other and a conductive liquid, and electrons start to move. The metal that gives up electrons turns into ions and loses mass at the joint. Water supplies vary, so the pace of that reaction varies too. Salts and minerals change conductivity. Temperature swings change gas levels in the water and shift the reaction again. Isolation gives you control when the water chemistry isn’t in your hands.
Trade groups have written about this for years. The Copper Development Association explains how mixed-metal joints in water systems can attack weaker partners and why nonconductive breaks help keep that reaction in check. For more background, see their article on mixed metal systems.
Choosing between union, nipple, and flex
If you want a clean service break, pick the union. If you want a low-profile thread-in part at the tank, pick a dielectric nipple. If you want speed with a bit of vibration relief, pick a stainless flex with plastic washers. All three isolate. Your site layout, access, and tool set decide the winner.
Think about future tasks too. A union near a shutoff saves steps during a cartridge change or a heater swap. A nipple can be great where space is tight and you just need a thread adaptor with a liner. A flex helps when alignment is tricky or when you don’t want to heat near a tank port.
Code, listings, and best practice
Most jurisdictions want an isolator when dissimilar metals meet on potable water. Some accept brass as the converter. Others call out dielectric unions, nipples, or flex with isolators. When in doubt, read the local book and the equipment manual. Water heater brands, such as A. O. Smith, show several compliant paths and remind you not to solder near nonmetallic parts.
For potable water, look for NSF/ANSI 61 and 372 marks on the fitting or box. Those listings tell you the wet parts are fit for contact and low in lead. Stick with listings that match your system type so inspectors don’t kick work back over a label.
Pro tips from the field
Stage hot work away from gaskets. Build hot joints on the bench. Let them cool fully before you assemble the union. Heat is the enemy of elastomers.
Use the right sealant. On the steel side, a paste or tape approved for potable water works well. Don’t go heavy with tape near the seat. Strands can foul the gasket.
Keep faces clean. Wipe the seat and sleeve with a lint-free cloth. A single grain of grit can spoil a seal that should last for years.
Support the run. Unions don’t like bending loads. Strap the line so the seat isn’t fighting pipe weight or pump thrust.
Reliable guidance you can trust
Trade groups and manufacturers have published clear notes on mixed-metal joints and isolation. The Copper Development Association covers the science and the field pitfalls in detail; see their technical article. State rules often echo the same message; the Illinois union rule is one example that names dielectric unions or a brass converter for copper joined to dissimilar metals. Water heater makers back this up in their install pages, like this A. O. Smith walkthrough for flexible connectors and soldered runs.
Pick the method that fits the site, size it to the run, and treat the seat like the sealing surface it is. You’ll tame corrosion where metals meet and make the next service visit a whole lot cleaner.