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This paper examines the impact of the secondary fluid temperatures at the evaporator and condenser on the energy use of water-to-water ground-source heat pumps (GSHP). In the first part of the paper, the energy consumption reduction associated with small temperature differences between source and load temperatures is evaluated from a thermodynamic point-of-view by examining the coefficient of performance (COP) of an ideal refrigeration cycle. Then, the performance map of a typical water-to-water heat pump is examined to determine real COPs for a range of source/load temperatures and flow rates. In the second part of the paper, annual simulations are performed on a ground-source heat pump system providing space heating and domestic hot water (DHW) for a well-insulated single-family house. Two different load temperatures and two different source and load flow rates are examined for a total of eight cases. The concept of seasonal performance factors (SPF) is used to account for all the energy flows into the system including pumping energy. Results show that the highest value of SPF4 (2.44) is obtained when the source and load flow rates are 9.0 gpm (0.56 l/s) and 4.5 gpm (0.28 l/s) respectively, and the return load temperature is 40 °C (104 °F). There is a difference of 8% between the lowest and highest values of SPF4 for the eight cases studied here indicating that the choice of the source and load flow rates as well as the load temperature is relatively important to limit the energy use of GSHP.