Need Help Understanding Dielectrics

JHMartin

Hi,
I understand that one is supposed to use a dielectric union between dissimilar metals. On my two boilers it goes straight from iron to copper with no visible corrosion. On my hot water tank, it appears to have a dielectric union and is corroded terribly. Disclaimer: I did not install either (see pics).
I could use a primer on dielectrics. Any help would be appreciated.
Thanks,
Jay

Larry Weingarten

Hi, There is a lot to this, but a simple answer is that oxygen plays a big role in making the corrosion/rusting happen. In the closed boiler system, there is little oxygen. In the open domestic hot water system, there is lots of oxygen. My preferred way to hook up a water heater is to use a plastic lined nipple at the tank and then a corrugated stainless or copper flex connector above that. I only use flexes that have true dielectrics built into the ends. The true dielectrics look like the attached photo, with plastic between the flex line metal and the nut. This approach puts distance between different metals and does not expose any poorly protected steel to the water. I avoid using standard steel dielectric unions as they often cause trouble down the line.

Yours, Larry

SuperTech

I avoid using dielectric unions for the same reason Larry stated above. They tend to be prone to leaking. Most water heaters have dielectric nipples in the tank but I find that different tanks have different quality dielectric nipples. What works for me, I put a 3/4" brass coupling on the hot water nipple and a 3/4" brass tee on the cold water nipple. I connect the type L copper with male adapters. On the bull of the tee I pipe in the expansion tank with brass nipples and fittings. 

If the quality of the dielectric nipples the water heater comes with is questionable then the brass will provide adequate dielectric separation. 

109A_5

Hello @JHMartin,
Either that corroded mess has been leaking for quite some time or maybe the tank has a draft issue and combustion gas spewing out. How does the other pipe look ?

Alan (California Radiant) Forbes

From what I’ve learned, you need at least 4” of brass to stop electrolysis. And I always like unions on my water heater and boiler connections. 

Larry Weingarten

Hi, Here's the galvanic scale and a link to one: https://mavink.com/post/07787690C3B228DB8866EDC13829CA3822AM38F38F/galvanic-corrosion-compatibility-chart


The farther any two metals are from each other on this scale, the more one must corrode to protect the other. There are other factors which affect this. How far the metals are apart physically, and how conductive the electrolyte is both have a huge effect. If you have rain-soft water, the corrosion will be far less than if you have very hard or salty water. That's two cents

Yours, Larry

JHMartin

109A_5 said:

Hello @JHMartin,
Either that corroded mess has been leaking for quite some time or maybe the tank has a draft issue and combustion gas spewing out. How does the other pipe look ?

Hello,
The other pipe looks fine.
Jay

Jamie Hall

" If you have rain-soft water, the corrosion will be far less than if you have very hard or salty water"

True if the water is naturally soft -- if it is ion exchange softened, the corrosion will be worse, not better. And I might add that rain-soft water is also corrosive, just by different mechanisms...

mattmia2

that water heater is doing that because it was leaking, probably since it was installed

RickDelta

....... the cast iron "T" has the blue "gorilla thread tape" on it ......must have been replaced not long ago, hence has not yet rusted?

Atop the tank ...... just looks like a leaking union issue.

Larry Weingarten

Hi @Jamie Hall , agreed! It's hard sometimes to simplify complex things to make them understandable. My experience with softened water is often the "softened" stuff is far more conductive than the hard (or just plain) water being softened. Most people don't live with rain-soft water. San Francisco has some nice water, on average about 60 PPM of TDS. Where I live, it goes from 40 to over 2000. Anyway, water quality varies so much, it's one reason why I like PEX.
To the rusted dielectric, I see the pipe above the union is very dark. Might there have been a leak higher up that ran down to the dielectric?

Yours, Larry

mattmia2

softened water doesn't remove the minerals from the water, it just replaces them with more soluble minerals so it is still more conductive

Intplm.

@JHMartin

Excellent subject question.

Hmm. Those pesky dielectric unions. I haven't had much problem with them leaking.
I think what happens is they are treated and installed like a regular union and tightened way to much like a regular union.

Wrenching too tightly piercing the insulating material in between and thus, defeating there purpose.

That being said. I do like the fact as to how brass can often have a better out come.

I do not fully understand how brass can work so much better.

Larry Weingarten

Hi @Intplm. , I might have an answer about why brass dielectrics do better than the steel ones. All I've ever seen with the steel ones is an electro-galvanized coating to protect the steel from the water. This thin zinc coating deteriorates pretty fast, and then the steel can rust... in potable water. Also, with hot water, I've seen rubber gaskets shrink just a bit. When this happens with a steel dielectric, the whole thing ultimately turns into a flow-restricting ball of rust... or maybe I just have really bad water.

Yours, Larry

Intplm.

Larry Weingarten said:

Hi @Intplm. , I might have an answer about why brass dielectrics do better than the steel ones. All I've ever seen with the steel ones is an electro-galvanized coating to protect the steel from the water. This thin zinc coating deteriorates pretty fast, and then the steel can rust... in potable water. Also, with hot water, I've seen rubber gaskets shrink just a bit. When this happens with a steel dielectric, the whole thing ultimately turns into a flow-restricting ball of rust... or maybe I just have really bad water.

Yours, Larry

Now that makes perfect sense.

Thank you @Larry Weingarten