In this paper we present measured performance and efficiency metrics of Ground Source Heat Pumps (GSHPs) for space conditioning and forwater heating connected to a horizontal ground heat exchanger (GHX) loop. The units were installed in a 345 m2 (3700 ft²) high-efficiency testhouse built with structural insulated panels (SIPs), operated under simulated occupancy conditions, and located in Oak Ridge, Tennessee (USA)in US Climate Zone 4. The paper describes distinctive features of the building envelope, ground loop, and equipment, and provides detailedmonthly performance of the GSHP system. Space conditioning needs of the house were satisfied by a nominal 2-ton (7.0 kW) water-to-airGSHP (WA-GSHP) unit with almost no auxiliary (resistance) heat usage. Recommendations for further improvement through engineeringdesign changes are identified. The comprehensive set of data and analyses demonstrate the feasibility and practicality of GSHPs in residentialapplications and their potential to help achieve source energy and greenhouse gas emission reduction targets set under the IECC 2012 Standard.