On Sat, 18 Oct 2014 20:13:17 +0000 (UTC), Jerry Peters
wrote:

ALso plastic doesn't conduct heat all that well, leading to an
inefficient WH. Also a gas HWH has a pretty powerful burner, mine's
rated at 40,000 BTU/hour input.

No, no. You don't want any thermal conduction through the plastic
liner. The whole idea is to keep the heat inside the tank, not
radiate or conduct it to the outside. That's why there's a mess of
fiberglass insulation between the steel water tank and the outside
cosmetic steel cover. Some people add an additional water heater
insulating "jacket" on the outside of the heater. A thermal
insulating plastic layer between the water and the steel tank should
improve efficiency.

An easy way to tell if your water heater is a piece of junk is to
measure the case temperature of the water heater. Use a contact
thermometer (Thermocouple or thermistor, not optical IR). It's
probably the worst at the top of the heater. If the outer case is
warmer than the ambient air, you're wasting energy heating that
atmosphere instead of the water. Same with a refrigerator. If the
case of the fridge is colder than ambient, you're cooling the kitchen.

I suspect you suspicions about the
plastic buckling/warping and separating from the steel tank are
correct.

That was just a guess. I'm not so sure any more. The plastic liner
and steel tank are both fairly flexible, so they can bend and bulge
without breaking anything. You'll never see it because it all happens
inside the tank. If they're glued together (bonded) properly, I don't
think they will come apart. I was wondering why the water tank didn't
have stiffening ribs, which would allow the use of thinner steel. I
guess(tm) stiffening might interfere with the necessary flexing of the
tank with temperature.

It should also be possible to dope the alumina ceramic coating with
something to help it match the coefficient of thermal expansion for
the steel. Even so, stratification, and the difference between
temperatures on both sides of the steel tank, will create enough of a
temperature gradient to possibly microcrack the ceramic.

Incidentally, it doesn't take much movement to wreck a ceramic
coating. I had a nice ceramic coated steel tea kettle that I usually
heat on the kitchen stove top to about 180C. One day, I stupidly put
it directly on top of my wood burner running at about 300C. I
compounded the error by boiling off all the water. My first
indication of a problem was the sound of something like popcorn from
inside the kettle. That was the inside coating flaking off and
bouncing around. As I approached, a large piece of the outside
coating flew off in my general direction. I had to use a broomstick
to remove the kettle. I haven't calculated the differences in
expansion, but for something as small as a kettle, it was much smaller
than my predicted 1mm.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558