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DECARBONIZING SPACE HEATING WITH AIR SOURCE HEAT PUMPS APPENDIX: MODEL KEY ASSUMPTIONS Our goal for the simple model described in Section 5 was to understand whether heat pumps can be competitive under current and potential future market conditions in various regions of the United States. We did this by estimating and comparing the costs of the following alternatives: (1) natural gas furnaces with central air conditioning; and (2) ASHPs, with backup where needed, in three different locations: San Diego, California; Atlanta, Georgia; and Fargo, North Dakota. To estimate the costs of a natural gas furnace, we considered housing size, average insulation values, furnace capacity and efficiency values, a reference indoor comfort temperature set equal to 65°F, and historical weather data as reported in the following formula: The house square footage considered across all three geographies is 1,670 square feet, equal to the median US housing size according to the Electric Power Research Institute.62 The furnace capacity used in this formula is 0.5, based on the assumption that the gas furnace is sized so that it runs 50 percent of the time during the coldest hour.63 For shell heating efficiency, based on data from EPRI, we assumed 0.20 Btu/sq. ft. for San Diego and Atlanta. Shell heating efficiency was assumed to be 25 percent lower for colder climates, thus we used 0.16 Btu/sq. ft. degree hour for Fargo.64 Analogously, for shell cooling efficiency, which was used in the model to size the air conditioner and ASHP (in regions where cooling demand is higher than heating demand only), we assumed 0.29 Btu/sq. ft. for San Diego and Atlanta and 0.23 Btu/sq. ft. for Fargo.65 We estimated heating design temperature for each region by calculating the outdoor temperature that each location stayed above for 99 percent of all the hours in the year (i.e., the first percentile), based on 30 years of hourly data. Similarly, we estimated cooling design temperature for each region, which is used to dimension the air conditioners and ASHPs based on cooling requirements, by calculating the outdoor temperature that each stayed above for only 1 percent of the hours in a year (i.e., the 99th percentile), based on 30 years of data. The weather data was sourced from the National Climatic Data Center66 database, hourly information, from January 1, 1987, to December 31, 2016, for each of the three cities. To estimate the total yearly heating and cooling needs, heating hours and cooling hours were Size of gas furnace needed [Btu] = house square footage x shell heating efficiency ÷ furnace capacity x (65°F - region minimum temperature) 34 | CENTER ON GLOBAL ENERGY POLICY | COLUMBIA SIPAPDF Image | DECARBONIZING SPACE HEATING WITH HEAT PUMPS
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