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

Abstract

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 seasonal energy efficiency ratio (SEER), 9.05 heating seasonal performance ratio (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 coefficient of performance (COP) was (0.40±0.11) higher, and the average heating COP was statistically equal. 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 over the same outdoor temperatures. Defrost performance is 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 supplementary resistive heat.