|Hybrid Ground Source Heat Pump Systems (HGSHPSs) which include cooling towers are widely used so as to improve Ground Source Heat Pump Systems (GSHPSs) efficiency in cooling dominated applications. A Greek office building with total cooled area 1000 m2 is examined. The whole system is modelled using TRNSYS 17. Systemâs operation is optimized using TRNOPT 17 so as to meet the maximum cooling load during the net cooling period, when no heating loads occur, by minimizing Ground Heat Exchangers (GHEs) depth. Three control strategies, based on continuous observation of critical temperatures, are applied to the optimized system. Each strategy attempts to achieve a further optimization of HGSHPSâs operation by minimizing the electric power consumption. In the first one, the cooling tower is turned on when the difference between the fluid temperature exiting heat pumps and ambient air wet bulb temperature exceeds 10Â°C. In the second one, the cooling tower is on when the fluid temperature exiting GHEs is greater than 28Â°C. In the third one, the cooling tower starts to operate when the fluid temperature exiting heat pumps is greater than 32Â°C. Each of these control points is normalized by the fluid temperature exiting the hot side of Heat Exchanger which comes in between the ground loop and the Closed Circuit Cooling Tower loop. The new set points define three new control strategies which are examined so as to achieve a further improvement to HGSHPSâs operation. [Sagia Z, Rakopoulos C (2012) New Control Strategy for a Hybrid Ground Source Heat Pump System coupled to a Closed Circuit Cooling Tower].