A vapor compression system includes a compressor, a gas cooler, an expansion device, and an evaporator. Refrigerant is circulated through the system. The high side pressure of the vapor compression system is selected to optimize the heating capacity. In one example, the optimal high side pressure is obtained by determining the high side pressure that correlates to the maximum current required to operate to the water pump. In another example, the actual temperature of the water entering the gas cooler, the water exiting the gas cooler, and the ambient air temperature are measured and compared to a predetermined value to determine the optimal high side pressure.