24 June 2014

Compressor Selection and Equipment Sizing for Cold Climate Heat Pumps

An extensive array of air source heat pump design and sizing options were
investigated to identify solutions to limit heating capacity degradation at -13°F (-25°C) ambient to 25%, compared to the nominal rating point capacity at 47°F (8.3°C). Sixteen
equipment design options were evaluated in one commercial building and one residential building, respectively in seven cities. The energy simulation results were compared to three baseline cases: 100% electric resistance heating, a 9.6 Btu/Wh (2.8 W/W) heating seasonal performance factor (HSPF) single-speed air-source heat pump unit, and 90% AFUE gas
heating system. The general recommendation is that variable-speed compressors and tandem compressors, sized such that their rated heating capacity at a low speed matches
the building design cooling load, are able to achieve the capacity goal at low ambient temperatures by over-speeding. For example, in a home with a 3.0 ton (10.6 kW) design cooling load, a tandem heat pump could meet this cooling load running a single compressor, while running both compressors to meet heating load at low ambient temperatures in a cold
climate. Energy savings and electric resistance heat reductions vary with building types, energy codes and climate zones. Oversizing a heat pump can result in larger energy saving
in a less energy efficient building and colder regions due to reducing electric resistance heating. However, in a more energy-efficient building or for buildings in warmer climates, one
has to consider balance between reduction of resistance heat and addition of cyclic loss.