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 Posted: Sat Jul 24th, 2010 02:04 am
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suki
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 I happened to check the electrical resistance of the dielectric union on the hot water tank I am replacing and found it was a continuous circuit from the copper tubing to the steel tank nipple. Does this mean the dielectric  union failed? Is there any special care to be taken when installing dielectric unions?

I am also confused regarding the electrical state of a gas water heater. Should the tank itself be isolated from any electrical ground? Besides addressing problems with dissimilar metals, isn't that one of the purposes  behind a dielectric union? Many homes, especially with basements, use the copper water supply line as the electrical ground because it is at least 6 feet underground and therefore is considered an excellent ground source. The only recommendation that I am aware of is to use a heavy copper jumper wire between the cold water supply line and the hot water line exiting the hot water tank. Your thoughts please.

 

 

 

 

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 Posted: Sat Jul 24th, 2010 09:11 pm
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sky_tech
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The short answer is that the typical dielectric nipple connection (as opposed to an insulating union) DOES have electrical conductivity between the pipes.  The connection, though exists only along the exterior (the dry path), where the pipe surface is out of the water.

Where the dielectric part comes into play is in the wetted path. A dielectric nipple connected to another pipe (or flexible copper line) prevents contact between the steel of the nipple and the copper line connected to it IN THE WET INTERIOR. Galvanic corrosion occurs at a wet junction between dissimilar metals, thus that is where you want the insulation.  A direct electrical connection in the dry areas is fine, and is desirable for other purposes such as grounding and prevention of stray electrical currents that can cause corrosion.  I believe this is also covered elsewhere in the site.  Here is a link to a manufacturers spec page.

http://perfectioncorp.com/library/menu/documents/ClearFlowSheet.pdf



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 Posted: Sat Jul 24th, 2010 09:21 pm
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Jeeter
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sky_tech, thank you for the nice clear explanation and the link to a drawing. -- John

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 Posted: Sun Jul 25th, 2010 06:51 pm
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suki
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I know that a plastic lined steel nipple has conductivity but a dielectric union ,as designed, should electrically insulate the copper supply line from the tank. Since I last posted, I checked new dielectric unions and found that the flange on the brass insert can easily touch the inside of the steel housing of the union if not perfectly centered when installed. It seems that ginding a millimeter off the diameter of the brass flange would make it impossible for it to touch the steel and yet still insure a good seal with the plastic washer inside the union.

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 Posted: Mon Jul 26th, 2010 01:53 am
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elenano
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My experience in the field with dielectric unions was mostly negative. They were hard to unscrew. It was really hard to get the top out from between them on a commercial water heater I wanted to service. And like as not, they didn't work. I can't count the number of times I found dielectrics with water and rust dribbling down from them.

Still, if that's all you've got.....

Randy Schuyler

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 Posted: Mon Jul 26th, 2010 06:07 am
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eleent
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Hello:  I'd chuck the steel dielectrics and use lined steel nipples with flex connectors that have true dielectrics in their ends.  A true dielectric has both the rubber washer inside and the plastic ring under the brass nut to prevent contacting the copper tube.  The benefit of this way is you get the protection from galvanic corrosion and put physical distance between different metals.  Also, the problems Randy mentions go away.  It just works better. 

The other concern was with grounding/bonding.  You do want the tank to be protected in such a way so that it is never in the electrical current path and also so that any current that does get there is bled off to ground easily.  This is done by jumpering between hot and cold above the dielectrics and connecting that to a wire leading to the ground on your main breaker panel (which also goes to a grounding rod).  All wire is usually six gauge copper.  This should be done with both gas and electric heaters, but I see where it can make electrics safer if their elements leak current to ground.  Better electricity leak through the copper wire than through the human :shock:

Yours,  Larry

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 Posted: Fri Jul 30th, 2010 02:46 pm
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suki
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Thanks, Larry. You were right, but only the corrugated copper flex lines are true dielectrics. The stainless steel flex lines only have a sealing washer and I don't know how much galvanic activity there is between the stainless steel and copper or brass. Also, if the copper cracks and leaks there is a good opportunity to fix it while the stainless steel would be a tough mending job.

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