Dielectric Couplings - Worse than nothing?

Metal Content: Pipes and pipe threads.
Thirteen months ago I replaced our home's electric hot water heater (it
died from a rusted out tank) for the second time in seventeen years. Six
to ten years is about par for heaters where we live, almost no one gets
full warranty life from them. Makes me wonder what's in our water...
The new heater's installation booklet recommended installing dielectric
couplings on the supply and outlet piping to reduce galvanic corrosion
of the steel tank. I'd heard about them, but don't think I ever saw a
hot water heater with a pair of them stuck in the pipes on top of it.
Anyway, I decided to go first class and install them, because two unions
plus two 3 inch galvanized pipe nipples only cost about $6.00, and I was
they might gain me a couple of more years of life out of the new
heater's tank.
For those who don't know what I'm referring to, the dielectric couplings
I used look like regular unions, but the "piping side" half is brass,
with a female sweat fitting, and the "tank side" half (and the nut) are
zinc plated steel, female pipe threaded. Plastic rings inside provide
both liquid sealing and insulation, so there's no electrical
conductivity through the fitting.
I dutifully installed the nipples on the tank, put the unions on top of
them, and sweated the brass ends of the unions to the original copper
supply and outlet pipes.
After I got it all done, I began wondering just what would be gained by
putting an insulating break in the pipes when the break was bypassed
with a low resistance path because the heater's tank is grounded through
the ground wire in its power feed and all the copper plumbing in the
house is also grounded, both per code.
I couldn't Google up an answer, and my posted question on
pdaxs.services.plumbing (a newsgroup whose member's language and decorum
are highly reminicent of those last encountered in my junior high school
lunchroom.) provided a lot of opinions, none of them indicating any
technical understanding of the subject. Lurking on that newsgroup a bit
led me to believe that all you have to know to earn a living as a
plumber is that you can charge double after 5 PM and that shit won't
flow uphill. All that makes me not want to think about the many
unpleasant consequences to health which can result from improperly
installed and maintained plumbing.
I forgot about it....
Cut to yesterday afternoon....I was putting something away in "my"
storage closet, which is home to the hot water heater, and noticed some
water weeping from the pipe threaded joint between one of those unions
and the galvanized nipple connecting it to the tank. I figured that a
fresh wrap of teflon tape and a bit of wrenching would stop that drip
real quick.
When I got things opened up I found that the whole length of the nipple
was about three quarters full of rust alread, and the leak wasn't coming
from the made up threaded joint, but through a pinhole corroded through
the nipple, at the start of the "exposed" portion of the male threads.
I didn't have another nipple the right size handy, and SWMBO was already
grumbling, so I rolled my own out of two straight male copper sweat
fittings and a few inches of copper pipe and got the hot water back on
again.
Now I'm wondering what I'll find on the inside of the other galvanized
nipple tonight, and whether I just wasted my time (and $6) installing
those dielectric couplings last year. I can understand the need for
insulating pipe couplings in other applications, such as preventing a
home's grounded gas appliances from shorting out the old "impressed
current" galvanic protection systems used on buried steel gas mains.
But, I'm really beginning to believe that their recommended use on
electric hot water heaters may be just one more urban legend of the
plumbing industry that no one's bothered to question or test.
Comments guys?
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)
"If you can keep smiling when things go wrong, you've thought of someone
to place the blame on."
Reply to
Jeff Wisnia
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the dielectric couplings are to stop the galvanic erosion of the metal.. the two different metals will start to erode because of chemical reactions... just like batteries do as they are suppose to.. if you put up alum. gutters on your house and use steel screws to put it up then you run a chance that when the water comes you will have the two disimilar metals that are wet and they will start with a chemical reaction on each other... it has nothing to do with the electric wires in th house our a ground wire in a house.....
Reply to
jim
If the tank is earthed via the heater and the pipes are earthed as well then the dielectric coupling might as well not be there. Does the tank have a sacrificial zinc electrode? If not, make and fit one.
Why does your part of the world use steel tanks (even if galvanized)? I assume that all your piping is copper. If this is the case, you should fit a copper hot water tank next time and your problems will stop.
Regards Mark Rand
Reply to
Mark Rand
Yeah, the dielectric unions don't do a blessed thing to reduce galvanic corrosion if the tank and pipes are both grounded. They only work if one or the other *isn't* grounded.
Gary
Reply to
Gary Coffman

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