Yes I agree guys...to a point. Gas fired water heaters generally have a life expectancy of about 11 - 13 years (data from the AGA @
http://www.oldhouseweb.com/stories/D...d/10382.shtml).
In the past I have found rust flakes built up as bad or worse than Jerry's pics in heaters 8 to 10 years old with no apparent water leaks and, knowing the unit is most likely nearing the end of its expected life anyway (according to charts at least), have recommended replacement rather than service based on that. After all the price tag to replace a gas fired, natural draft, water heater in my area can run $800 - $1200 including installation. A tidy sum.
As with any metal subject to the harsh environment like that of a water heater tank and its chimney, I imagine many factors come into play when it comes to corrosion. Amount of moisture in the local gas supply, humidity in the tanks operating environment, how hot the burner is set to, how long the burner runs (ie. a family of 6 taking showers twice a day will certainly cause more heat stress to the tank by virtue of the amount of time the burner is lit, even if the control temp is set to medium, vs. a family of 2), and so on. None-the-less, if we find the amount of flakes in the chamber as seen in my photo in a unit only 5 years old but find the same amount of flakes in a different unit that is 9 years old, what could be inferred from that if anything do you suppose?
There does not seem to be any way of knowing if a large portion of the rust came from only one area of the chimney either. In our case all the rust is directly on top of the burner head and so
had to have fallen down the tanks center chimney/heat tube so we know the tube is the source at least. The situation could thereby be creating a weak spot in the tube area that may leak in the not too distant future? We cannot even know if the tube is corroding more or less evenly. Being a suspicious HI wouldn't we have to consider the worst case, that the tank now has a major weak spot in it because it has not deteriorated evenly?
At any rate, to satisfy my own curiosity, no matter the age of a tank the amount of rust flakes in the burner chamber do not seem to be a measure of the amount of life left in the tank metal before it fails, or does it?
Just some musings.
My plan at this point is to continue recommending replacement if the tank is 10 or more years old but I may recommend service for the purpose of possible warranty repair or replacement if the age is less than that. I believe it may be wise to also recommend the client obtain a copy of the licensed plumbers repair ticket to be sure the homeowner didn't just shop vac up the rust and call it good.