Ground source heat pump (GSHP) systems are significantly more energy efficient than conventional heating and cooling systems, but they have suffered from low market penetration. This study analyzes the implications of and barriers to nation-wide GSHP retrofits in U.S. single-family houses based on national databases of housing units and home energy use. Our model estimates maximum annual savings of 1.26 quads (1.33 EJ) of energy, $7.1 billion in energy costs, and abatement of 64.8 million tons of CO(2)eq. Economics is the major barrier as typical GSHPs cost $164 less to operate annually but cost $8990 more to install than the conventional alternative HVAC systems. Spatial and economic constraints exclude 7.7% and 89% of homes respectively, leaving only 10% of homes suitable for retrofit. Applying these two constraints, savings reduce to 0.15 quads (0.16 EJ), $3.0 billion in energy costs, and abatement of 12.1 million tons of CO2eq. A 30% federal tax credit helps increase the percentage of GSHP-suitable homes from 10% to 30% while reducing the average payback period from 9.1 to 4.8 years among those homes. More effective policies to lower high cost premiums would be needed to promote large-scale GSHP implementations. (C) 2016 Elsevier B.V. All rights reserved.