Energy and cost analysis of residential water heaters

A detailed computer model is developed to calculate energy flows for residential electric and gas water heaters. Model equations are derived from applications of the first law of thermodynamics, analysis of manufacturers' literature, and related studies. The model is used to evaluate the energy (and associated initial cost) impacts of alternative designs to reduce water heater fuel use. Model results show that the largest heat loss in an electric water heater is conduction through jacket walls (14% of energy input). An additional 5% is lost through the distribution pipe for a 7.6 m (25 ft) long pipe. The remaining 81% of energy input is used to heat water. In a gas water heater, conduction losses through jacket and distribution pipe are 12% and 3%, respectively; and 33% is lost up the flue (due to main burner and pilot light operation). Only 52% of energy input to a gas water heater is used to heat water. Several energy-saving design changes are examined using the energy model. Changes for both electric and gas water heaters are: increase jacket insulation thickness, reduce jacket insulation thermal conductivity, reduce thermostat setting, and add insulation to the distribution line. Application of all these changes to an electric water heater would reduce electricity use 17% and increase initial cost 27%. Additional changes examined for gas water heaters are: reduce pilot rate, eliminate pilot and add electric ignitor and flue closure, and reduce excess air for combustion by increasing flue baffling. Implementing all these changes to a gas water heater (except addition of electric ignitor) would reduce gas use 27% and increase initial cost 26%. These results show that there are large opportunities for reducing water heater energy use with only small initial cost increases.