Dielectric union fitting on supply pipe. Plumbing Components:

Using A Dielectric Union To Join Galvanized And Copper
Water Supply Pipes

In This Article:

A discussion of the dielectric union and it's installation.

Related Articles:
Skill Level: 2-3 (Basic to Moderate)  

By , Editor


This is a dielectric union. This device electrically separates the water heater from the household piping.

It also provides a convenient way to disconnect the piping for removal or servicing of an appliance, such as a water heater or water softener.


Components Of A Dielectric Union: 

The big nut clamps the brass part to the steel threaded part.

The black washer and purple insulator keep the brass and steel parts from touching.

The brass part gets soldered to a copper pipe. The steel part is threaded onto a piece of steel pipe.


Why is all this complicated plumbing necessary?

If you recall from high school chemistry, two dissimilar (different) metals, when placed in an acidic solution, will create a battery. And one metal will usually erode away as the chemical reaction progresses. The other metal may have a buildup of new material, which may be a chemical combination of the eroded metal and the acid. Since virtually all domestic water is slightly acidic or slightly basic, this electro-galvanic action can occur in any metal plumbing system.

When copper and steel pipes are connected together directly, the "battery" has a path for electrical current to flow. (This current is tiny, and the voltage is not a safety hazard.) 

If the current cannot flow, because there is no electrical connection (interrupted by the plastic insulators) then the "battery" never discharges. In theory there will always be a small voltage between the different metals. The metals do not erode.


Installing The Dielectric Union:

I used a pipe wrench to install the threaded end of the dielectric union.


Then I took the union apart and put a piece of copper pipe in the brass part, and soldered the joint.

I did the soldering on a workbench, using a piece of galvanized sheet metal (a scrap from an old section of heating duct) as a heat shield.


After the joint cooled down, I put the plastic insulator on the brass part, followed by the steel nut.

The dielectric union was then ready to use.


I soldered the next fitting (in this case an adapter fitting for connecting to a threaded pipe) onto the other end of the pipe. 


The union was assembled, using a big 15" adjustable wrench. A large pair of "Channel Lock" pliers also works for tightening the nut.



Tools Used:

  • Pipe Cutter
  • Pipe Cleaning Brush
  • Propane Torch
  • Pipe Wrenches
  • 15" Adjustable Wrench

Materials Used:

  • Dielectric Union
  • Copper Pipe
  • Solder


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Copyright © 2000, 2005 HammerZone.com

Written January 28, 2000
Revised January 12, 2005