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Daily Hot Water Use

The Home Energy Saver web site uses the following equation[1]
to estimate average daily hot water use in gallons per day (Lutz, et al. 1996).
This equation was modified and improved from Lutz et al's version by
subtracting out the constant assumed hot water use of clothes washers and
dishwashers (the variables *cloth* and *dish* in Equation 4), and adding two
variables (*cwGals* and *dwGals*) that allow users to individually
specify their clothes washer and dishwasher hot water use (e.g. specifying
loads washed at certain temperatures). These two variables (*cwGals* and *dwGals*) are used to allocate the water heater energy consumption
into three portions; clothes washer, dishwasher and other. The energy consumed
by the water heater for the clothes washer and dishwasher portions is reported
with the Major Appliance energy in the final report. The calculation of hot
water use by the clothes washer is described here and the calculation of hot water use by the dishwasher is described here.

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where

Use_{wh}
= hot water consumption (gallons/day)

occupants =
number of persons in household (sum age1-4)

age1 = number of
people aged 0-5 yrs

age2 = number of
people aged 6-13 yrs

age3 = number of
people aged 14-64 yrs

age4 = number of
people aged 65- yrs

T_{tank}=
water heater thermostat setpoint (°F)

tank_size = rated
volume of water heater (gallons)

T_{in} =
inlet water temperature (°F)

average_temp =
average annual outdoor air temperature (°F)

adult_at_home = 1
if TRUE, 0 if FALSE, adult at home during day

*dish* = dishwasher
hot water use embedded in original Lutz et al. equation (Lutz, et al. 1996,
Equation 12)

*cloth* = clothes
washer hot water use embedded in original Lutz et al. equation (Lutz, et al.
1996, Equation 8)

*cwGals* = calculated gallons of hot water used by
clothes washer based on user inputs, see Clothes Washer Energy Consumption [replaces more generic
estimation method (cloth)] (gallons/day)

*dwGals* = calculated gallons of hot water used by
dishwasher based on user inputs, see Dishwasher Energy Consumption [replaces more generic
estimation method (dish)] (gallons/day)

The original model set *pay* to 1.3625 if residents do not pay
for energy to
make hot water (to reflect less water-conserving behavior), but since
HES does not have an option for multi-family or solar, pay is currently
fixed at 1.

*senior* = 0.379 if only seniors live in household and
it is a multifamily residence, otherwise senior = 1

[1]
The original development of the water heating
analytical method was sponsored by the U.S. Department of Energy, Office of
Building Technology, State, and Community Programs as part of their appliance
standards analysis program (US DOE 2000c).