The Performance of an Auxiliary Heat Pump Water Heater Installed in a Dual-Tank System in a Net-Zero Energy Residence
Tania Ullah, William M. Healy
In the effort to achieve low-energy operation of residential buildings, advanced water heating technologies are vitally important. This paper explores the year-long performance of a 189 L (50 gal) heat pump water heater (HPWH) serving as an auxiliary unit to an active, indirect solar thermal water heater with a 303 L (80 gal) storage tank in a net-zero energy test home located in Gaithersburg, MD, USA. The systems were subjected to a representative water use schedule for a virtual family of four between July 2013 and June 2014. We investigate the effect of inlet water temperature on the overall system Coefficient of Performance (COPsys) of the HPWH and the unit's space conditioning impact, as these factors can vary substantially depending on the extent to which hot water demand is met by the solar thermal water heater. Field testing showed that the installed HPWH used 1104 kWh in the year and had a COPsys of 1.41, not reaching the manufacturer's reported Energy Factor (EF) of 2.33 over the course of the 12-month testing period. The difference was largely due to the fact that the hot water load delivered by the unit was much less than if it were the sole water heater. The study of a HPWH in this unique configuration is valuable considering regulatory trends away from electric resistance storage water heaters, such as current standards in the United States that require EFs greater than 1.9 for electric water heaters with storage volumes greater than 208 L (55 gal).