How to Install a Dielectric Union on an Electric Water Heater

Copper and galvanized steel plumbing pipes don't mix. When you join them in such a way that they are touching each other, they develop a small electrical charge that corrodes the joint, and the result is a leaking water line. In order to prevent this, plumbers join copper pipes to galvanized steel ones with dielectric unions, which are fittings with nonmetallic washers that keep the pipes separated. You need to use dielectric unions to connect an electric water heater to a system of copper pipes if the heater came with preinstalled galvanized nipples.


Separate a 3/4-inch dielectric union into its component halves by unscrewing the central nut. One half is made of galvanized steel and has female threads; the other half is brass or copper and has a slip joint. Between the halves is a plastic or rubber washer.


Wrap plumbing tape around the threads of the cold-water inlet or the hot-water outlet nipple attached to the top of your electric water heater. The cold-water inlet is usually identified by a blue sticker on the top of the heater; the hot outlet by a red sticker. Screw on the galvanized half of the dielectric union and tighten it, using a wrench.


Slide the union nut onto the 3/4-inch copper pipe that you are going to connect to the heater with its threads facing the end of the pipe. Spread soldering flux on the outside of the pipe and on the inside of the brass half of the union, using the brush that is supplied with the flux. Slip the union onto the pipe. Solder the union onto the copper pipe, using lead-free solder and a propane torch.


Bring the copper and galvanized pipes together after the fitting has cooled. Insert the washer between them and screw the union nut to the threads on the galvanized half of the fitting. Tighten the nut with a pipe wrench or a large pair of adjustable pliers.


Connect the other water heater port to the water pipes in the same way.

Things Needed

  • Plumbing tape
  • Soldering flux
  • Lead-free solder
  • Propane torch
  • Pipe wrench
  • Adjustable pliers


  • A union is a type of compression fitting and it will leak if the pipes aren't straight with respect to one another. You can't stop this leaking by wrapping plumbing tape around the threads of the central union nut.


  • If the water pipes approach the heater at an angle, install whatever fittings necessary to make them exactly perpendicular to the top of the heater by the time they reach the nipples.


Photo Credits

  • Comstock/Comstock/Getty Images

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