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Old 07-17-13, 09:12 PM   #12
ham789
Helper EcoRenovator
 
Join Date: Dec 2011
Location: tigard, oregon
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OK, but with most things in life, the devil is in the details.
What's the temperature of the water when you turn off
the heater?
What's the temperature of the water just before you turn
it back on.
What's the capacity of the heater.
From those three numbers, you know how many BTU's
it's gonna take to get the water hot again. Time is not a
factor, it's all about the BTU's.

That's also the number of BTU's of waste heat that your
air conditioner will have to get rid of.

If the water temp changes a LOT, you need more insulation
on the water heater.
If it's not a lot, you can make a linear estimate of the
heat lost at maximum temperature vs the heat lost
at lower temperatures as the water cools...that we already
calculated.

My guess is that, for a well insulated water heater,
the difference is in the noise level.

The bottom line in all this is that it takes about the same
amount of energy to reheat the water as you lost by turning
it off. All you're saving is the difference between the
total heat lost at max temp vs the total heat lost
at an average temperature about half way between
the two temps we measured above. That's the extra
heat your air conditioner has to take out.

And that having microsecond resolution in when you
turn it on or off is inconsequential.
Hence, the question about thermal time constant.
You may discover that turning it off at all is mostly
wishful thinking.

But, it's all about the exact details of your particular
situation.
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