A control strategy was developed to predict optimal ground source heat pump water flow rates under part load operation. Using this strategy, optimal flow rates are calculated from available design data and minimal on-line measurements. The optimal control strategy was evaluated with validated single speed and tandem speed ground source heat pump simulation models in both heating and cooling mode. A range of building load profiles were implemented when analysing heat pump system performance. The optimal strategy was shown to result in an improvement in system performance when compared to nominal flow rate operation, particularly for low load conditions. Due to the prevalence of low load operation over a heating or cooling season, the seasonal system performance (SPF3) was shown to increase by between 20% and 40% when using the optimal strategy. (C) 2015 Elsevier Ltd. All rights reserved.