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."