Dielectric Unions

When your hot water heater hooks up to your water pipes, it actually creates a problem. When two different metals are connected together in the presence of an electrolyte (in this case tap water) you get a reaction called galvanic corrosion. This means that atoms from one metal break off and cling to the other metal.

To avoid this plumbers use a special coupling called a dielectric union. It’s a coupling that acts as an electrical barrier between the two metals. One side is made of copper; the other, steel. Between the two sides, there is a non-conducting washer, usually made of rubber, which prevents the metals from interacting.

Dialectric unions are required by code in most states. If you install a new water heater, and the water pipes in your home or building are copper, you have to use them.

DEU’s will sometimes corrode after 10+ years on the job. If your water is hard it is an exceptionally strong electrolyte. It can overcome the barrier inside the DEU and cause galvanic corrosion. However, if the DEU corroded it means your tank didn’t.

In some states it is acceptable under code to use a brass connection between the steel of the water heater and copper water pipes. A brass fitting is marginally less reactive with steel than copper, and can be used in some areas. Most still require dielectric unions.

Since dialectric unions have become code, some water heater manufacturers have actually been building dialectric connections into their heaters. In these dialectric nipples, the steel pipe is lined with the dialectric barrier.

Photo by: Richard McGarry of
www.McgarryAndMadsen.com

Photo by: Fred Sweezer Sr. of
www.thelongbeachhomeinspector.com

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