Since October 1980, a small house in Upton, Long Island, New York has been heated and cooled by a liquid-source heat pump using a shallow serpentine earth coil as a heat source/sink. This paper, introduces and describes the system, presents system performance data for the winter of 1981-82 and the summer of 1982, and discusses these results. The experimental test house is a 1120 ft² (104 m²) three-bedroom ranch-style house of energy-saving construction with a heating load of 4.1 x 10³ Btu/°F·day (7.8 x 10⁶ J/°C·day) The heat pump used during most of the period reported on here is a commercially available water-to-air unit sized to just meet the building's design heating load with no auxiliary heat. The earth coil contains 507 ft (155 m) of nominal 1-1/2 in. (4 cm) medium-density polyethylene pipe and is buried 4 ft (1.2 m) deep. An antifreeze solution consisting of approximately 25% ethylene glycol in water is employed to permit subfreezing earth-coil operation. Two independent data-acquisition systems, a datalogger-microcomputer system backed up by a Btu-meter, monitor the space-conditioning system's performance. During the winter of 1981-82, the system, which extracted 14.7 x 10⁶ Btu (15.5 x 10⁹ J) of heat from the ground, operated with a seasonal performance factor (SPF) of 2.46. No resistance heating was used. The earth coil's minimum daily U-value observed during this season was 1.93 Btu/ft·hr·°F (3.34 W/m·°C). During the summer of 1982, a total of 10.0 x 10⁶ Btu (10.6 x 10⁹ J) of heat was rejected to the ground. The system SPF was 1.91, with a minimum daily U-value of 2.04 Btu/ft·hr·°F (3.53 W/m·°C).