wrote in message news:d1f57637-b812-4e5e-a7c6-

stuff snipped

Interesting theory, but I suspect it's not true. I don't accept the
idea that thermal shock is the dominant failure mechanism.

Community Association Underwriters of America, Inc. (CAU), one of the
largest insurance providers in the United States for community associations,
residential and office condominiums, cooperative apartments and homeowners
associations says on their site:

How do Water Heaters Fail?

A water heater holds and transfers water continuously - from installation to
replacement or failure. Over time, deposits will accumulate on the bottom of
the tank. These deposits corrode the tank liner and heater elements. Water
quality, particularly water hardness, directly influences the amount of
sediment deposited.

Moving water also causes wear on the tank and piping. The (!!) hotter the
water, the greater the fatigue on the parts it touches. (!!) The constant
heating of cold water also subjects the unit to extreme temperature swings.
No household appliance works under tougher conditions than the storage water
heater.

In most cases, water heaters fail gradually, but not always. Some of the
telltale signs of imminent failure include water accumulation beneath the
heater, a hissing or whistling sound characteristic of a worn valve, and
chronic hot water shortages during periods of normal demand. Prompt
corrective action is required once the signs of failure appear.

When the corroded bottom of a tank fails without warning, the water already
in the tank and the continuously fed cold-water supply create a deluge. If
not stopped, this water will continue to flow. In these cases, it's crucial
to stop the flow of water by turning off the cold-water supply valve at the
water heater or at the water main shut-off.

Community Association Underwriters of America, Inc.

http://www.cauinsure.com/

http://www.cauinsure.com/Include/Doc...520Heaters.pdf

I'd say they give fairly serious weight to the thermal shock a unit endures
as a cause of failure. Certainly not 100% but I'd say a fair number of
cracked tanks die from thermal fatigue.

With my limited experience with a water heater "plant" at a photofinishing
plant I QC'ed at, water heaters are indeed sized professionally to never
drop below half. If you run out of hot water, you are undersized in most
cases. I would bet that running low with large amount of winter-cold street
water has a negative effect on longevity very similar to what Nestork
proposes. And it means you have a sizing problem - the tank(s) is not big
enough.

As aside and a thread shift - the tank water heater deluge they mention
could be worse. I think I would rather have a water heater tank crack than
a washing machine hose split and turn into a fu&ing sprinkler. BTDT

Two major upgrades were monitoring and double-hulling the hoses. Now even
the stainless steel braided super high quality replacement hoses are backed
by a Floodstop and by covering them with smurf tube over their length so any
spraying occurs in a sheath and runs down to the floor instead of watering
the ceiling. And the workbench. And the washer. And the dryer. And the
pet food shelves. )-:

--

Bobby G.