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re: electric hot water heater--reducing costs
4 jul 1999
elaine gallant wrote:
>hot water heaters are not constructed to be turned off and on daily. i have
>heard stories of the switch breaking competely off in a couple of months of
>this off label use...
most water heaters don't even have a switch, just the circuit breaker in the
fusebox. it seems to me that turning the heater on and off daily won't hurt
it, but it won't save much energy either. we've already been through this...
if it's 2' diameter and 5' tall, with 2" (us r8) of insulation, with
thermal conductance u = 38ft^2/r8 = 4.7 btu/h-f, keeping it 120 f in a
70 f room takes 24h(120f-70f)4.7 = 5655 btu or 1.65 kwh/day, about
17 cents per day or $60/year at 10 cents/kwh, with no hot water usage.
rc = 40galx8btu-f/gal/4.7btu/h-f = 68 hours, so switching it off for
22 hours per day allows it to cool to 70+(120-70)exp(-22/68) = 106 f, for
an average temperature of 113 f over the 22 hours, and an approximate
heat loss of 22h(113-70)4.7 = 4446 btu over the 22 hours. keeping the
water at 120 f for another 2 hours requires another 470 btu, for a total
of 4916 btu or 1.44 kwh/day, and an energy savings of 739 btu or 0.22 kwh
or 2 cents per day or $8/year, with no hot water usage.
one might save more energy by heating up the water, then turning off the
water heater, then using all 40 gallons for showers, etc, and then leaving
the tank full of cold water until the next usage, 24 hours later. that
might use close to 40x8(120-70) = 16k btu or 4.7 kwh/day of energy at a
cost of about $170/year, vs $170+60-8 = $222 in the last scenario.
one might save more money by getting the utility to install a timer that
shuts off the heater at peak electrical usage times, and heats the water
at some lower off-peak rate, say 6 cents/kwh...
nick
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