18 May 2017
When the ASHRAE headquarters building in Atlanta, Georgia was renovated in 2008, two separate heat pump systems were installed and instrumented: – a variable refrigerant flow (VRF) air-source heat pump serving the first floor and a ground source heat pump (GSHP) system, primarily serving the second floor. Two previous articles by the authors have covered energy consumption of the two heat pump systems, determination of the heating and cooling provided by both systems, and system coefficients of performance (COP) for both heating and cooling on a monthly and annual basis for both systems. This paper focuses firstly on analysis of both systems with the mean system COPs determined as a function of outdoor air temperature. Over a significant part of the temperature range, system cooling COPs increase with increasing outdoor air temperature, contrary to expectations for the heat pumps. Several reasons are identified for this trend – standby power consumption of the heat pumps, heat pump fan power consumption for ventilation, decreased efficiency of the circulation pump at low speeds, and heat pump startup losses. Significant changes in the GSHP system COPs due to a change in the differential pressure setpoint used to control the circulation pump speed will also be examined.