A home is expected to provide comfortable living quarters for its occupants even when external conditions are uncomfortable. A comfortable thermal environment depends upon many factors. [1,2] Clothing, furniture, building insulation, metabolic activity levels, adaptation via human thermoregulation, and psychological perception are but a few examples. Today, significant energy resources are used in many parts of the world to maintain a comfortable thermal environment for human habitation.
Key end-use categories where energy is consumed in the residential sector include space heating, water heating, air-conditioning, refrigeration, lighting and other appliances. The first four categories directly control thermal comfort while the heat generated by lighting, other appliances, and human activity indirectly influence the home thermal environment. Regional climate variations, fuel availability, socioeconomics, human behavior, and other factors also set residential end-use energy demand and thereby act to regulate the home energy budget. [3-6]
Thermal comfort in many buildings is primarily controlled by heating, ventilation and air-conditioning (HVAC) systems. A thermostat dial is a common household example of such systems. HVAC equipment is typically used to heat the ambient air in the household and includes central warm-air furnaces, boilers, heating stoves, fireplaces, heat pumps, solar thermal collectors, and other space heaters. For many HVAC systems in use today, the major fuels are natural gas, fuel oil, liquid petroleum gas, and electricity.  Other important fuel sources, which once were, are and may again become primary energy sources for home heating in many parts of the world, include wood, coal and solar energy. [7,8]
Each system and fuel source is used in residential dwellings to supply, store, distribute and control the flow of energy in the home to help maintain thermal comfort for its occupants. Energy is therefore required to shelter humans from the extremes of weather and climate.  Improvements in HVAC energy efficiency and building design have helped to preserve these energy resources without sacrificing thermal comfort. [9,10] In many parts of the world, however, space heating still accounts for the largest fraction of home energy use and represents a substantial fraction of total primary energy consumption.
Total U.S. primary energy consumption in 2005 was about 1.06 × 1020 J. Consumption of fossil fuels, including coal, natural gas and petroleum, accounted for about 9.05 × 1019 or 85% of total U.S. primary energy consumption. Natural gas and petroleum accounted for nearly 63% of consumption in 2005 with about 6.64 × 1019 J. Biomass, which includes wood energy, was about 3.29 × 1018 J or about 3.1% of total energy consumption. [11,12]
The approximately 111.1 million households in the U.S. consumed a total of about 1.16 × 1019 J or 11% total primary energy consumption in 2005. If electrical transmission losses are included in addition to the metered electric power delivered by a central utility to a residence via power lines, then U.S. households consumed about 2.04 × 1019 J or about 19% of U.S. primary energy consumption. Note that energy used to transport non-electric fuels to U.S. households is not included in these estimates. Other sources of indirect energy, such as food, humans and consumer goods, that enter a residence are also not included. From this, about 11% of U.S. primary energy was consumed in homes in 2005. 
The energy consumed in homes is directed to several end-use functions, including space heating, air-conditioning, water heating, refrigerators and appliances. Space heating using electricity, natural gas, fuel oil, kerosene, or liquid petroleum gases accounted for about 4.54 × 1018 J in 2005. This figure does not include wood fuel used for space heating, which itself amounted to 4.54 × 1017 J. Then, if all wood fuel was used for space heating, then about 4.99 × 1018 J were used for space heating. This estimate does not include energy derived directly from solar space heating, coal, nuclear or other sources except that which is included in the metered electricity delivered to U.S. households. Then about 43% of home energy consumption and 4.7% total U.S. primary energy consumption was due to space heating in 2005.  For comparison, energy consumed for residential space heating is then of the same order of magnitude as the energy consumed for plastics, which was previously estimated to be between 2.5% and 4.0% of total U.S. primary energy consumption. 
© Curtis W. Hamman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. McDowall, Fundamentals of HVAC Systems, (Elsevier,2007).
 R. De Dear, "Thermal Comfort in Practice," Indoor Air 14, 32 (2004).
 J.P. Nelson, "The Demand for Space Heating Energy," The Review of Economics and Statistics 57:4, 508 (1975).
 J.D. Sheaffer and E.R. Reiter, "Urban Climate Effects of Energy Demand for Space Heating," Meteorology and Atmospheric Physics 38:4, 202 (1988).
 K. Rehdanz, "Determinants of Residential Space Heating Expenditures in Germany," Energy Economics 29:2, 167 (2007).
 R. Haas et al., "The Impact of Consumer Behaviour on Residential Energy Demand for Space Heating," Energy and Buildings 27:2, 195 (1998).
 G.V. Sanders., "Good-By to Coal Shovel Drudgery," Popular Science 145:5, 194 (1944).
 A. Lovins and H.A. Bethe, "Energy Strategy," Foreign Affairs 55:3, 636 (1977).
 E. Hirst and J.C. Moyers, "Efficiency of Energy Use in the United States: Transportation, Space Heating and Air Conditioning Provide Opportunities for Large Energy Savings," Science 179:4080, 1299 (1973).
 S.R. Hastings, "Breaking the 'Heating Barrier': Learning from the First Houses without Conventional Heating," Energy and Buildings 36:4, 373 (2004).
 "2005 Annual Energy Review", U.S. Energy Information Administration.
 "BP Statistical Review of World Energy 2010," British Petroleum.
 "2005 Residential Energy Consumption Survey", U.S. Energy Information Administration.
 C.W. Hamman, "Energy for Plastics", Physics 240 Stanford University (2010).