Dielectric unions: Code vs. actual practice

jesmed1

Question about dielectric unions: when are they required by code for joining dissimilar metal pipe, and when not? I do maintenance in a 100-year-old condo building with a lot of heating plumbing in the basement. Most of the supply and return lines are 2" iron pipe that meet 1-1/4" copper near the boilers. All the iron-to-copper adapters are threaded on with Tru-Blue thread sealant, and they all seem fine, professionally done, no problems. But I have seen crappy homeowner jobs where someone has joined galvanized and copper plumbing with no dielectric union, and the joint is corroding badly. Before I moved here, someone had installed a galvanized nippled in a short run of copper leading to an outside water faucet, and that joint was corroding so badly that it was leaking. I replaced the offending pieces with a correct threaded brass elbow and threaded brass nipple.

So my question is, why do some dissimilar metal pipe joints not corrode and others do, and when does code say a dielectric union is required?

dko

One factor can be water quality. Higher/lower conductivity. No electrolytes no galvanic corrosion.

Cheap dielectric unions are a menace, gaskets will be the first thing to deteriorate and start a drip.

Jamie Hall

I can't answert your question about code -- but there is a world of difference between joining black iron or even plain steel to copper and joining galvanized to almost anything. Galvanized is zinc coated -- and while the zinc protects the steel, after a fashion, it aggravates corrosion when in contact with copper-- with the copper coming off the worst, frequently.

jesmed1

Jamie Hall said:

I can't answert your question about code -- but there is a world of difference between joining black iron or even plain steel to copper and joining galvanized to almost anything. Galvanized is zinc coated -- and while the zinc protects the steel, after a fashion, it aggravates corrosion when in contact with copper-- with the copper coming off the worst, frequently.

Thank you. That explains why the galvanized-to-copper joints I've seen have been the worst.

dko

jesmed1 said:

Thank you. That explains why the galvanized-to-copper joints I've seen have been the worst.

Zinc is much farther away on the galvanic chart than carbon steel is to copper.

jesmed1

dko said:

jesmed1 said:

Thank you. That explains why the galvanized-to-copper joints I've seen have been the worst.

Zinc is much farther away on the galvanic chart than carbon steel is to copper.

OK, but steel and cast iron are still dissimilar enough from copper that there should be some reaction. Is the Tru-Blu just enough to prevent it?

JUGHNE

I believe on hydronic heating piping there is little problem as the same water is circulated thru the piping.

Domestic fresh water puts things in a different perspective.

Most die-electrics on water heaters will leak. But there are plenty of WH tanks with copper male adaptors just screwed into the tank bungs and never a problem.

jesmed1

JUGHNE said:

Most die-electrics on water heaters will leak. But there are plenty of WH tanks with copper male adaptors just screwed into the tank bungs and never a problem.

My belief was that code requires dielectric unions on applications like that. But dko's photo above shows a leaking dielectric union on a water heater, so if that is indeed to code, the crappy fitting made things worse, not better, as you say.

And here's our water heater, with a copper adapter screwed right onto a galvanized nipple, which apparently is a bad dissimilar-metal situation with copper. It's 10 years old, but no corrosion, I assume because the plumber used a massive amount of teflon tape and pipe dope. He probably figured the water heater was only going to last 10-15 years anyway, and the connection to galvanized would be OK during that time.

So maybe this is a case where it's better to ignore code and do what actually works.

Mad Dog_2

In NYS, the gas utility requires Dielectric unions before running 🏃‍♂️ nat gas underground & when popping back up at the pool heater or Gas BBQ, et cetera.  In 40 years, I've installed a good deal and many without them.  I really can't tell you how effective they are.  Most corrosion I've come across is from soldering Flux that was never wiped off the joint or a small weep or drip that turns in to a green Oyster.  

I've seen many copper to black & Galvanized steel, Brass to black & Galvanized steel that look as good as the day they went together.  Mad Dog 🐕 

jesmed1

Mad Dog_2 said:

I've seen many copper to black & Galvanized steel, Brass to black & Galvanized steel that look as good as the day they went together.  Mad Dog 🐕 

That is interesting that you've seen many copper-to-galvanized with no problems. I'm sure you've seen a lot more pipe connections than I have.

The few copper-to-galvanized connections that I have seen, with the exception of my water heater above, were badly corroded, maybe because whoever did them was a homeowner who didn't know to use a lot of teflon tape and/or pipe dope.

hot_rod

I think most water heaters have dielectric nipples now. The code question is more about are unions required. Copper and stainless flex are legal as are some of the copper to pex connectors, as unions.

All dissimilar metals will react in the right, or wrong condition. Conductive water has a lot to do with it, sea water, very conductive ,is used to test dissimilar metals in many reports.

Even old copper and new copper tube can react, according to CDA data

jesmed1

hot_rod said:

I think most water heaters have dielectric nipples now.

Thank you, I didn't know there was such a thing. Then I'm guessing that's what we have in my photo above.

https://www.supplyhouse.com/Bluefin-DEN100-400-1-x-4-Galvanized-Steel-Dielectric-Nipple-w-Pex-Insulator

jumper

Cast iron is supposed to be compatible with anything.

EBEBRATT-Ed

I seldom see corrosion on copper to steel in a heating system as @JUGHNE mentioned it's the same water and the oxygen has been driven out of the water for the most part. Plumbing is a different story.

Personally, i have never met a dielectric union I liked they always seem to leak.

Many have suggested a bronze ball valve between two dissimilar metals.

jesmed1

EBEBRATT-Ed said:

Many have suggested a bronze ball valve between two dissimilar metals.

Interesting. I hadn't heard of that, but it is mentioned as a common practice that seems to work in this tech bulletin from the Milwaukee Valve Company.

https://www.milwaukeevalve.com/wp-content/uploads/technical-service-information/DissimilarMetalsandPiping.pdf

"Furthermore, it is common for bronze valves to be installed with threaded ends
on one side connected to carbon steel piping and sweat ends on the opposite side connected to copper
tubing with no adverse effects."

We have many such connections near our boilers where the newer 1-1/4" copper runs into older 2" steel/cast iron, with a bronze gate valve between the two.

Intplm.

Dielectric unions work wonders but their gaskets!? Not so much.
Dialectric nipples are great with little if any failure.
Unions on water heaters are a recommendation and not a code requirement in my neck of the woods.
However, a dielectric "fitting" is required between dissimilar metals.
Often I have substituted a brass fitting in place of dielectric fittings with better results.