What are dielectric unions, and do I need them?

I’ve had some talks with other plumbers about dielectric unions, it seems that nobody agrees whether or not they are necessary.  We use them on our water heater installations because they only cost 8 bucks for the pair, and if they don’t help they certainly don’t hurt.

I have searched a bunch of web sites and spoken to  to find information on whether or not Dielectric Unions are really necessary when two different metal pipes meet, and found many stories about them,  but  the problem is it’s conflicting.

What are dielectric unions?

dielectric unions

Description:

Dielectric Unions are used in commercial and residential applications to prevent accelerated corrosion and deterioration in the piping system due to galvanic and stray current. It is installed between pipes made from dissimilar metals, and features a female iron pipe thread to solder connection. It consists of a union nut, two tailpieces, and a gasket that separates the tailpieces to prevent an electric current from occurring. Series 3001A is ideal in applications such as hot water storage tanks, water and air conditioners, processing tanks, steam, gas, and water piping. Their  maximum operating pressure is 250psi

“Dielectric Unions are used in commercial and residential applications to prevent accelerated corrosion and deterioration in the piping system due to galvanic and stray current”.   But what is that you ask?

If you recall from chemistry class, two dissimilar metals, when placed in an acidic solution, will create a battery.

And one of  the two metal supply pipes on your water heater will usually erode away as the chemical reaction progresses.  The other metal will usually have a buildup of new material, which is a chemical combination of the eroded metal and the acid. This build up reduces the ability of the water to flow threw the tank properly, and will break off and enter the water heater.

Since virtually all domestic water is slightly acidic or slightly basic, this electro-galvanic action can occur in any metal plumbing system, but water heaters with well water systems especially need to be protected from  this corrosion that will from on the top of the water heater feed and return lines.

When copper/brass/ galvanized steel pipes are connected together directly, the “battery like charge” has a path for electrical current to flow. (This current is tiny, and the voltage is not safety hazard in any way.)

If the current cannot flow, because there is no electrical connection (interrupted by the plastic insulators in the dielectric unions) then the “battery” never discharges thus the metals do not erode.

This electro-galvanic action is called electrolysis and in time it will eat pin holes in copper pipe. Dielectric Unions are insurance and well worth the money.  When properly installed there is no reason to worry about leaks, and they make future water heater replacements much easier.

Most new water heaters will have dielectric inserts on the feed and return line, but dielectric unions are another good line of  defensive against corrosion and permanently  needing a water heater replacement.

As you can see here in this picture below we have installed a pair of
dielectric unions on a new 50 gallon electric Rheem water heater.

dielectric unions on a water heater

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