The plate-type heat exchanger is found to be well suited for the application of a transcritical $CO_2$ heat pump water heater. A $CO_2$ heat pump water heater test facility was used to evaluate the performance of a high-pressure plate-type heat exchanger. The performance characteristics of heat exchanger effectiveness, capacity, and pressure drop were evaluated over a range of $CO_2$ gas cooler inlet pressures and temperatures. For the ambient temperature range of $10^{circ}C$ to $30^{circ}C$, the maximum observed capacities ranged from 4.46kW to 5.34 kW, with the capacity increasing approximately linearly with increasing ambient temperature. Over this maximum capacity range, the effectiveness of the gas cooler ranged from 98.7% to 99.4%, with the effectiveness increasing with decreasing ambient temperature. The largest gas cooler pressure drop was approximately 130 kPa at a volumetric flow rate of 128 $cm^3/s$, which is the flow rate resulting from the optimum system operating conditions at an ambient temperature of $30^{circ}C$.