Master Plumber Mark.....    

   WOW...48 years old and still working great ........

Why do some heaters  last only a few years and others  last forever,   what can I do to extend  the life of my  heater? 

1.  High water pressure above 80psi can dramatically shorten the life span of a water heater.   Many homes on  well water  have only around  50 psi,  and the heater seems to last forever.   Installing a pressure reducing valve and thermal expansion tank  to kick down your city water pressure to about 60psi can MORE THAN DOUBLE  the life of your heater and save stress to all of your plumbing fixtures. 

  http://www.wattscanada.ca/pro/divisions/watersafetyflowcontrol/learnabout/learnabout_wprv.asp

2.   Hard water,   iron and minerals in your  water can destroy the heater.  A  properly installed softener will keep minerals from attacking your heater and lengthen its life.

3.  Changeing the Anode rod can help to lenghten its life.

4.  Flushing out your heater from the bottom every 3  months helps keep the sediment from building up in the heater like below..

5 .    Poor Grounding in your electrical system  and  "phantom"  currents in your home...  http://411plumb.com/danger-lurks-on-the-water-service-main-for-the-plumber

A  loose or broken  ground rod at your Breaker Box can cause trouble in  your plumbing .    With a bad  ground at your breaker box,    your copper plumbing system usually becomes the  "back up"  ground for your home .

If you have a plastic in-comming water service, you  really have no back up ground in your home and the power will find another way to ground itself usually through the copper plumbing and  the water heater..

In time this seems to thin down your  copper pipes and destroy water heaters.

Also,  their are  phone and cable grounds tied directly to your plumbing system that  will attack the plumbing if you have a poor ground in your home. 

this home had gone through its second set of water heaters in  3 years...

 the homeowner  grounded everything to the garage floor and jumped the wires on the top of the heater  all down into the garage floor.   total cost   $50 .

Attempting this experiment is  much cheaper than doing nothing.

For more info...

 read this link about "phantom" currents and other grounding problems in your home........

http://web.archive.org/web/20070717185515/http://ecmweb.com/grounding/electric_shocking_truth_grounding/