With increasing fuel prices, various energy-saving techniques have been considered for residential appliances. Space heating accounts for the largest energy use in residential applications. One method proposed by manufacturers to keep operating cost low is to combine residential water-heating and space-heating functions into a single system. This paper presents the field test results of the performance of a combined water-heating/space-heating system under actual usage conditions. The system was installed in an 1187 ft2 (110 m2) home (Green Home) located in the Washington DC area. Data were collected for a one-year period. The water-heating performance was established by evaluating the hot water usage data during the test period. Water-heating/space-heating performance was established by using hot water data plus the amount of hot water circulated for space heating during the winter months. Analysis of the data indicated that the system produces hot water at an efficiency averaging 48% for water heating in the summer months and produces hot water at an efficiency ranging between 63.5–69.4% for combined water-heating/space-heating during the winter months. The relative humidity was maintained in the home at substantially lower level than that of the outside air, and the inside temperature was kept at the desired setting.