ENERGY ADVISOR
About the Water Heating Module
To prepare the estimates, we use a detailed method developed here at Lawrence Berkeley National Laboratory. 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.
This module calculates energy consumption for heating water in three steps. The first step is to estimate average daily hot water use. This calculation is based on number and ages of people living in the house, presence or absence of a dishwasher and a clothes washer, the water heater temperature setting and tank size, and the local climate. [1]
Once the average daily hot water use has been estimated, a simple calculation is done to determine the daily energy use by the water heater. The calculation uses the energy consumption characteristics of the water heater as determined by the DOE Energy Factor test, air and water temperatures, and how much hot water is used on an average day. The last step is to convert the daily energy use into annual consumption of specific fuels, (e.g. electricity and gas).
Once energy use is calculated, we multiply by the energy price to arrive at the final energy cost. Prices for each fuel are defaulted to the average for the state in which the building is located (using the 1994 annual averages reported by the Energy Information Administration), or they can be overridden with custom inputs by the user.
You'll recognize some of the equation variables as things you are asked to provide on the web page. To simplify the process, the rest has been defaulted for you.
Jim Lutz,
Staff Research Associate
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[1] 1996 Lutz, J. D., Liu, X., McMahon, J. E., Dunham, C., & McGrue, Q. T. "Modeling Residential Hot Water Use Patterns" No. LBL-37805. Lawrence Berkeley National Laboratory. November 1996
Daily Hot Water Use
Use the following equation to estimate average daily hot water use in gallons per day.
| vol = | [ | -1.78 + 0.9744*per + 6.3933*age1 + 10.5178*age2 + 15.3052*(age3+age4) -0.1277*thermostat + 0.1437*tanksz - 0.1794*wtmp + 0.5155*atmp + 10.2191*athome - (1-dw)*dw_use - (1-cw)*cw_use | ] | * IF(senior_only,0.379,1) * IF(NOT(pay),1.3625,1) |
| variable | units (or equation) | description |
|---|---|---|
| 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 | |
| per | = sum(age1,age2,age3,age4) | total number of people in household |
| tstat | °F | water heater thermostat setpoint |
| tanksz | gals | rated volume of water heater |
| wtmp | °F | inlet water temperature |
| atmp | °F | average annual outdoor air temperature |
| athome | = 1 if TRUE, 0 if FALSE | adult at home during day |
| pay | = 1 if TRUE, 0 if FALSE | residents pay for energy to make hot water |
| senior_only | = 1 if TRUE, 0 if FALSE | only seniors live in household and it is a multifamily residence< |
| dw | = 1 if TRUE, 0 if FALSE | dishwasher present in home |
| cw | = 1 if TRUE, 0 if FALSE | clothes washer present in home |
| dw_use | =0.692^per+1.335^per gal/day | hot water use by household dishwasher |
| cw_use | =1.1688^per+4.7737^per gal/day | hot water use by household clothes washer (if use hot or warm water for clothes washing) |
| variable | units (or constants) | description |
|---|---|---|
| vol | gals/day | average daily hot water use. |
| RE | recovery efficiency of water heater | |
| UA | BTU/hr-°F | standy heat loss coefficient of water heater (if not available see section below) |
| Pon | BTU/hr | rated input power of water heater |
| Ttank | °F | tank thermostat set point |
| Tinlet | °F | inlet water temperature |
| Tamb | °F | air temperature around water heater |
| dens | 8.293752 lb/gal | density of water |
| Cp | 1.000743 BTU/lb-°F | specific heat of water |
| fuel type | equation (with units) | description |
|---|---|---|
| electricity | = 365 * Qin / 3412.76 kWh/yr | annual water heater electricity use |
| natural gas | = 365 * Qin / 100,000 therm/yr | annual water heater natural gas use |
| propane | = 365 * Qin / 91,500 gal/yr | annual water heater propane use |
| fuel oil | = 365 * Qin / 138,690 gal/yr | annual water heater fuel oil use |
| variable | units | description |
| Qin | BTU/day | daily water heater energy use |
| variable | units | description |
|---|---|---|
| EF = | Energy Factor | |
| RE = | Recovery Efficiency | |
| Pon = | BTU/hr | rated input power |
| Qout = | 41093.7 BTU/day | Energy content of water drawn from water heater during 24 hour test. |