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Stuck with Electric Hot Water Tank - Page 2

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Posted by vaughn on August 21, 2009, 12:29 pm

   I have no special knowledge to argue with, but that seems
counterintuitive and certainly different than my personal experience.  The
dishwashers that I have experience with take lots of hot water and energy
and produce 30% dirty dishes unless you pre-wash them.   If you need to
pre-wash, may as well just wash them and get it over with.



Posted by harry on August 21, 2009, 6:44 pm


Quite right, I wouldn't have one in the house.   If you want cheap hot
water get yourself a solar panel. I made my own & fitted it.  About 75
%of my hot water is now free.

Posted by BobG on August 21, 2009, 7:44 pm

This aint right. The thermostat has several degrees of dead band
between turn on and turn off, so the amt of cold water entering to
rinse dishes cant change the tank avg enough to turn on the element.
This is an irrelevant problem anyway, because it takes a BTU to heat a
pound of water a degree F, and while applying more BTUs will heat the
water faster, this doesnt happen in the hot water heater. Its a bang-
bang system, and once the water is heated, the element shuts off and
it doesnt cost anything. Hook an ammeter to it and watch it. for about
10 hours without running the hot water. It cools slowly for about
2hrs, then runs for about 5min to heat back up. Thats about a 4% duty,
so 96% of the energy is going to heat the water, and 4% to maintain
it. Not much waste there to be salvaged.

Posted by daestrom on August 21, 2009, 8:21 pm

BobG wrote:

All that is true...

But that is not.  The leap from a 4% duty cycle to 96% efficiency isn't

Consider this, if you go on vacation for a week and leave the heater on,
it cycles just as you describe with a 4% duty all week long.  But *all*
the energy it uses for the entire week is wasted. The water temperature
started the week and ended the week within the hysteresis of the thermostat.

The 4% duty cycle simply means that it looses heat a lot slower than the
heating element adds it in so the water takes just a few minutes to heat
up to the shut-off point, while it takes a long time to cool down to the
turn-on point. (assuming no flow through the tank)

When there is flow through the tank, the temperature cools much faster,
turning on the heating unit within a few minutes.  As long as you have
the tap open, the heating unit will most likely stay on and remain on
for a few minutes after you shut off the tap.  Only when the temperature
is all the way back up to the shut-off point will it shut off the
heating unit.


Posted by Gordon on August 22, 2009, 1:02 am


First off; your dishwasher will clean and sanitize your dishes.
No need to put bleach in the water.  You don't need to wash
your dishes before loading the dishwasher.  But soaking them
will help loosen the crud.  A quick swipe will get most of the
crud off.  But they don't have to be rinsed clean. Let the
dishwasher do it's job.

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