> they are all rated at # of warrant able years...

Warranties are a marketing device and have little to do with a product's 
character or quality.

 > I suspect maybe that
> lifespan has not so much to do with the energy source as with the
> construction of the tanks and corrosion from minerals in the water.

Back in the 1970s I had a water heater that gave up the ghost, and a new 
friend who operated a dairy farm just down the road. My friend was 
getting a bigger hot water heater for his milk house, and he gave me the 
old one. It was one his dad bought second hand back in the 1950s. It was 
just like an ordinary domestic hot water heater, except the tank was 
make of monel metal. So I installed it, and it served us well for 30 
years. We sold that house back in '95, and last summer I stopped by to 
visit with the current owners. I asked about the hot water heater and he 
said it was still going strong, no problems. So, there is a hot water 
heater that has lasted half a century. If you want a long lasting hot 
water heater I suggest looking for one from the dairy suppliers. But, in 
this day and age it could be better to just go with the flow and buy 
whatever you can get at your local hardware store. Row, row, row your 
boat, gently down the stream.

And whatever you do, don't water proof your screens.

John

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