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<DIV>>This water heater <A
href="http://m.lowes.com/pd/Whirlpool-50-Gallon-240-Volt-6-Year-Regular-Electric-Water-Heater/50397576">http://m.lowes.com/pd/Whirlpool-50-Gallon-240-Volt-6-Year-Regular-Electric-Water-Heater/50397576</A>
has an EF = 0.95 energy factor, ie it uses Qdm Btu/day, about 5% more than
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<DIV>the average daily useful water heating energy
https://en.m.wikipedia.org/wiki/Energy_factor as tested in a 67.5 F room
with 6 64.3 gallon draws of 135 F water heated from 58 F in the first 6 hours of
the day, with an 18 hour rest period after that.</DIV>
<DIV> </DIV>
<DIV>Oops. <A
href="https://en.m.wikipedia.org/wiki/Energy_factor">https://en.m.wikipedia.org/wiki/Energy_factor</A>
says:</DIV>
<DIV> </DIV>
<DIV>"During the test 64.3±1.0 gallons of water are drawn from the water heater
in six equally spaced draws that begin one hour apart."</DIV>
<DIV> </DIV>
<DIV>After I calculated the 88 F water temp after 6 hours and realized that 6x64
gallons is a lot of hot water for a family and reflected on the R1.82 wall
insulation calc, I realized that the <U>total</U> draw is 64.3 gallons, which
makes the useful water heating energy Qu = 64.3galx8.33lb/gal(135-58) = 41243
Btu, approximately. And EF = 0.95 = Qu/Qdm by definition, so Qdm = Qu/0.95 =
43413 Btu, and the standby loss with no water draws is 43413-41243 = 2170
Btu/day, approximately, ie 0.64 kWh/day, or 1628 Btu during the 18-hour rest
period, when a timer might help save energy.</DIV>
<DIV> </DIV>
<DIV>With no timer, 1628 Btu = 18h(135-67.5)G makes the water heater's thermal
conductance G = 1.34 Btu/h-F. With 50 gallons of water, thermal capacitance C =
50x8.33 = 416.5 Btu/F, approximately, so the cooling time constant RC = C/G =
416.5Btu/F/(1.34Btu/h-F) = 311 hours, so with a timer, the water would cool from
135 F to 67.5 + (135-67.5)e^(-18h/311h) = 131.2 F in 18 hours. Reheating it to
135 before the next draw would require 416.5(135-131.2) = 1583 Btu. So the timer
saves 1628-1583 = 45.3 Btu/day, or 1.9 Btu/h or 1.9/3.412 = 0.55 watts, on a
continuous basis.</DIV>
<DIV> </DIV>
<DIV>How long would it take this $21.57 R10 water heater blanket to pay for
itself at 15 cents/kWh? <A
title=http://www.lowes.com/pd_24399-1410-SP57/11C___?Ntt=water+heater+blanket&UserSearch=water+heater+blanket&productId=3133229
href="http://www.lowes.com/pd_24399-1410-SP57/11C___?Ntt=water+heater+blanket&UserSearch=water+heater+blanket&productId=3133229">http://www.lowes.com/pd_24399-1410-SP57/11C___?Ntt=water+heater+blanket&UserSearch=water+heater+blanket&productId=3133229</A></DIV>
<DIV> </DIV>
<DIV>Nick</DIV></DIV></DIV></BODY></HTML>