Alternative to dielectric union

I've heard that a brass nipple can be used to join copper to galvanized pipe instead of a dielectric union. Is this true? How long does the nipple need to be? (.5" and .75" pipe are at issue.)
Thanks
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On 11/8/03 Bennett Price snipped-for-privacy@itsa.ucsf.edu writes in part:

While it will NOT meet UPC code requirements, in my experience a brass nipple does a pretty good job of electrically isolating the black tank of a water heater from copper tubing.
I suppose it would work just as well for a galvanized to copper connections as the material in galvanized or black you're trying to isolate the copper from is iron.
Doug
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Perhaps I should have added that the question came to mind since I'm planning a changeout from galvanized to copper in the basement/crawl space of my house but intend to leave the galvanized in the walls.
I'm having trouble getting homeowner's insurance (in California) because of the old galvanized plumbing and am told that a 'substantial' replumbing to copper will satisfy the insurance companies. Water heater is in basement so it will go pure copper; house is 1 story high and the galvanized vertical sections left will be about 4 to 5 feet long.
And as I asked initially, 'how long does the nipple need to be?'
Bennett Price wrote:

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usually when doing a basement changover and leaving the galv. risers I'd either use a diletric union, a brass ips valve or a min 3" brass nipple. I haven't had any problems over the last 30 years. A simple brass break between the copper and galv. slows down the reaction enough that will out live the existing galv. pipe that is left.
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It can depend on the jurisdiction, In one city I work in they want 6" Brass nipps, DEU not allowed. In another, ether is ok, yet another DEU only. So you should check with your local inspecting agency. Dale

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Brass
So
Hey guys, the one common thing to all water heaters regardless of the type of plumbing and anode rod materials, or the manufacturers, is the water quality. I've only seen it mentioned what, twice in the whole thread? All water is wet and most of it is clear but that's where the similarities between one location and another cease. Unseen differences in water quality ranges all over the board and you'll find that dictates how well the plumbing/appliance materials it contacts do in regards to their service lives.
Since the EPA changed the acceptable pH range of potable water in 1991 from 6.9-8.5 to 6.5-8.5, most 'city' water has been more acidic. A water with pH 6.0 is 10 times more acidic than a water of 7.0 (neutral) pH. That's not the only thing that will shorten heater and plumbing material life. Things like bacteria, DO, CO2, chlorides, sulfates and TDS content plus chlorine etc. all have a negative impact on material life. Also, electrical grounding and grounds do too.
Take a look here for more info on water caused corrosion: http://www.corrosion-doctors.org/NaturalWaters/Frames.htm
Gary Quality Water Associates
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Like the first gentleman so eloquently stated: Brass is an excellent conductor. So wouldn't it stand to reason that a nonconductive material would break the electrolytic circuit?? how 'bout that PEX! The current will still travel through the water but the intensity of the current is dependant upon the density of the disolved solids in the water (mainly minerals), but its definately better than a metal connector. Make sense?
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If I understand all of this correctly, the issue is not how well a pipe conducts but how electrochemically reactive it is when placed in contact with a pipe of different composition.
I found the following URL's helpful - though they don't directly address the question I posed initially.
http://www.mtec.or.th/th/research/famd/corro%5Chowmetals.htm http://www.corrprev.org.au/Galvanic.htm
Pipeline9000 wrote:

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Dielectric union
My water heater contains galvanized steel inlet water pipes. The house water pipes are copper.
For a test, I did this: - Turned off the gas input valve. - Temporarily disconnected the gas line from the water heater. Then I measured the open circuit voltage and short circuit current across the dielectric union.
Copper Iron ======||||====== <--- Water heater pipes + - Measured across the dielectric union: Open: 5.5 mV Using a voltmeter. Shorted: 33. uA Using an ammeter.
The copper and iron pipes, with the mineral-filled water, create a chemical battery. Connecting the copper and iron with an electric conductor allows current to flow. Then ions migrate from the iron to the copper, slowly corroding the iron. Eventually the iron pipe deteriorates enough that it leaks water. That conductor makes the voltage between the copper and iron metals zero. A battery "short circuit".
If something anywhere electrically connects the gas and water lines, then that invalidates the dielectric union. That "electrically" connects across it. Here is the "test": Failure to sense any open-circuit voltage across the union indicates that condition.
To reduce the current flow to zero, I inserted an "electric insulator pipe" into the heater's input gas line. Then the voltage between the copper and iron metals is no longer zero. No "short", so no current flows. Then the dielectric union actually works. The water heater's inlet iron water pipe lasts much longer.
If all the house water pipes are plastic, then the dielectric union is not needed. A plastic pipe / connector will serve the same function as a dielectric union.
Knowing the shorted current, you can calculate how much iron is lost per year. It's chemistry.
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On Sunday, January 20, 2013 at 4:35:04 PM UTC-8, snipped-for-privacy@att.net wrote:

ross the dielectric union.

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What a great post this takes away all the hand waving, and gives a real exp lanation why dielectrics don't work in the real world. Virtually all gas l ines are grounded; therefore virtually all dielectrics are short circuited.
So the other way to break the galvanic cell (stop the corrosion) is to inhi bit the conduction through the electrolyte (water), which is done in practi ce by having a several inch plastic lining inside the pipe known as a diele ctric nipple. This effectively makes electrolyte (water) a much poorer con ductor. Hence most water heater manufactures build these in or supply thes e.
Now the other unanswered question is why DOES brass seem to work when put b etween Copper and steel? Theoretically brass should corrode iron almost as fast as copper, but plumber after plumber says putting in brass nipples wo rk, to the point that it has become code in some areas.
Perhaps this just moves the corrosion to just inside the thick large anode protected tank. Since there is no "nipple" failure it appears there is no acute problem. Under this explanation a dielectric nipple would still be preferable and would help the water heater itself last longer.
Does anybody have experience with putting brass between a copper pipe and g alvanized pipe, then opening up some time later? In this scenario the galv anized pipe should still readily corrode.
A more out there idea is perhaps brass forms some natural surface oxidation or other surface layer that acts as an inhibition to the electrolyte simil ar to a plastic liner?
Any other ideas?
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