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A Side-by-Side Energy Use Comparison of a Variable-Speed Small Duct High Velocity and Two-Stage Conventional Ducted Heat Pump in a Net-Zero Energy Home



William V. Payne, Brian P. Dougherty


Two air-source, split system heat pumps were installed in a residential, net-zero energy home that was constructed as a laboratory on the campus of the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland USA. The first heat pump was a two-stage, 7 kW (2 ton), 15.8 SEER, 9.05 HSPF conventionally ducted system, and the second heat pump was a variable-speed, 10.6 kW (3 ton), 14 SEER, 8.35 HSPF, high velocity ducted system. These two systems operated side-by-side, using separate supply ducts and a common return duct, on a weekly alternating schedule to condition the home that was operated with very consistent, simulated thermal loads. We wanted to know if the high velocity system could provide comparable energy use efficiency to the conventional system. The results of this study showed that it did meet the required loads while doing so with slightly greater efficiency; the average cooling and heating COPs of the small duct, high velocity system were 0.396±0.113 higher and statistically equal, respectively. A new firmware was provided at the end of the heating season which greatly improved the performance of the high velocity system; its average heating COP went from 1.8±0.9 to 2.5±1.1 at a 95 % confidence level. The new firmware heating COP averaged 1.05±0.23 higher than the old firmware. The causes for the differences in performance are evaporator-condenser saturation temperatures and power demand of the indoor and outdoor units. Defrost performance is also very different for these two systems yet they consumed equivalent energy per HDD; the conventional system uses a timed-initiate, temperature- terminate algorithm with auxiliary electric resistive heating while the high velocity system uses calculated evaporator parameters with a hot-gas bypass before a full reverse cycle defrost with no auxiliary resistive heat.
Technical Note (NIST TN) - 2101
Report Number


energy use comparison, field test, low load home, net-zero home, two-stage heat pump, variable- speed heat pump
Created July 5, 2020, Updated July 6, 2020