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What Is a Dielectric Union?

By Paul Scott
Updated May 16, 2024
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A dielectric union is a term used to describe two-part fittings of dissimilar metals which are electrically isolated from each other to prevent galvanic corrosion. The term is most commonly used to describe a family of plumbing pipe fittings. Galvanic corrosion occurs where two different metals are exposed to an acidic solution. This combination effectively forms a battery and causes a flow of electrical current between the two metals. To prevent the corrosion caused by this current flow, the dielectric union uses an insulating insert between the two metals to prevent electrolysis from occurring.

When two different metals are placed in contact with each other in an acidic solution, electrolysis occurs which causes an ion flow between the two metals. This flow of electric current causes molecules from one of the metals to be deposited on the other, thereby resulting in what is known as galvanic corrosion. The metal which donates molecules will slowly be eroded away while the one that receives the material will build up a layer of corrosion byproduct. Most domestic water sources are mildly pH biased; galvanic corrosion is likely to occur wherever dissimilar metals are used in plumbing fittings. One way of preventing this destructive process is to insulate the two parts of the fitting from each other.

The dielectric union achieves this insulation by a plastic insert located between the two parts of the fitting. Bi-metal fittings typically consist of brass and steel halves for connecting similarly differing pipe types. A dielectric union has an additional plastic or non-conductive fiber sleeve which surrounds the brass part of the fitting and a washer to form a flange barrier. The sleeve and washer effectively isolate the two materials from one another and prevent the galvanic battery effect from occurring. One point to keep in mind when considering this phenomenon is that the voltages involved in galvanic electrolysis are very small and pose no danger.

Dielectric union fittings meant for use with copper pipes generally require the brass part of the union to be soldered onto the copper tubing. The plastic sleeve should be removed from the fitting to prevent melting before this is done and only replaced once the brass has cooled down. The washer should also be moved as far down the copper tubing as possible while soldering for the same reason. When the fitting is assembled, care should be taken to avoid tearing or nicking the plastic sleeve. A small area of contact between the two metals will lead to far more aggressive, localized corrosion than full area contact.

Do New Water Heaters Need Dielectric Unions?

Typically, dielectric unions are no longer needed. The secret is out about corrosion. Water heater manufacturers know that corrosion is a real problem in plumbing applications nowadays, so they build new models with countermeasures in place. The installation kits included with new heaters have fittings and nipples now that contain insulation.

Plumbers still sometimes install extra dielectric unions regardless of whether or not the heater manufacturer has included it. This could be for many different reasons, but it is generally accepted that adding an extra dielectric union when it is evident that one already exists adds no additional protection.

Benefits of Extra Dielectric Unions

  • Easier to Access
  • More Visible During Inspection
  • Redundancy
  • Peace of Mind

When To Use Dielectric Unions in Plumbing

Corrosion occurs when these two different metals touch one another in the presence of water. For many years now, plumbers have used the dielectric union for instances where two different metals must be in contact, such as copper pipe joined to galvanized iron.

Dielectric unions are used when laying out the plumbing system of new construction, or when adding fixtures to the plumbing system, such as a new water heater or sink. It's important to plan ahead on a plumbing project and figure out where the unions are going to be located. They are used in sequences of the plumbing where two dissimilar metals are supposed to touch.

Building inspectors are checking for this union in applications where they are required. However, in some situations where dielectric unions used to be needed all the time, they may not be needed anymore. Newer appliances, such as water heaters, come equipped from the manufacturer with dielectric unions. Because of this pre-installation of insulated fittings for newer appliances and because of the widespread use of brass fittings, standalone dielectric fittings are being used less frequently than in decades past.

How To Solder Dielectric Unions for Water Heaters

Building codes require that electric unions be installed in plumbing applications where two dissimilar metals connect. A building inspector will be looking for these before giving a green light. Many plumbers still add dielectric unions when installing water heaters primarily because it's easier for the inspector to see it.

Installation of a dielectric union requires it to be soldered. This isn't the type of soldering found in electric wiring, but a plumber's type of solder. The heat from the solder could ruin the union, so follow the steps and be mindful of the flame.

Soldering a 3/4 inch Dielectric Union

Unscrew the nut holding the dielectric union together

  • This will separate the unit into its component parts. One part is usually galvanized steel with female threads. The other part is sometimes brass. There should be a plastic or rubber washer between them. This piece is the insulator that keeps the corrosion from happening.

Wrap plumbing tape around either the cold water inlet fitting or the hot water outlet fitting, whichever piece is getting the union

  • This preps the threads on the appliance's fitting so the fitting will never leak past the threads

Screw on the galvanized union piece to the taped fitting and tighten it with a wrench

Slide the union nut a few inches down the end of the 3/4 inch copper pipe which is going to be connecting to the heater

  • This is to get the nut in position ahead of time so it is better prepared for the final step. Make sure the nut threads are facing the appliance so that it tightens in that direction

Spread solder flux on the end of the copper pipe and on the inside of the brass or copper half of the union

Slide the brass or copper union piece onto the copper pipe-end

Solder the union piece onto the copper pipe-end using a propane torch and lead-free solder

  • This part might require some experience or at least watch a video of someone doing it. The amount of time it takes to solder the metal pieces together can vary.

After the joint cools, insert the plastic or rubber washer between the galvanized pipe and the copper pipe

  • It is crucial to ensure the insulator is seated properly and isn't going to get damaged.

Slide the nut down from the copper pipe and screw onto the galvanized threads, tightening with a wrench

  • As the nut is tightened, check the insulator again for bending or breaking. The nut should tighten the pipes together, forming the dielectric union.

If necessary, repeat the steps for the other fitting coming out of the water heater

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Discussion Comments
By anon329750 — On Apr 11, 2013

Are dielectric unions needed between galvanized and black pipe connections?

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