The air-source electric heat pump (ASHP) has gained significant popularity in the space heating community due to its high-rated efficiencies in contrast to gas-fired systems. Despite its heating capacity derating disadvantages, many manufacturers have optimized ASHP systems to maximize comfort and capacity for cold climates specifically. However, ASHP performance as a function of outdoor air temperature is not very clear due to the impact of many independent variables in field demonstration sites. This paper describes a comprehensive research on ASHP capacities as a result of outdoor and indoor air conditions, and efficiencies at the component and heating system levels derated by short-cycling and indoor comfort conditions.

ANSI/AHRI 210/240 is an ASHP rating test method developed for manufacturers to determine heating capacities and efficiencies as a function of outdoor air temperatures. These rated heating capacities are considered by Heating, Ventilation and Air Conditioning (HVAC) designers when right-sizing heating equipment for HVAC systems. This comprehensive research demonstrates ASHP heating capacities are strongly correlated to air enthalpy rise between the outdoor and indoor air conditions. Therefore, the heating system efficiency and comfort are affected by over- or under-sizing auxiliary heating equipment. Additionally, this research demonstrates auxiliary heating equipment manages over 80% of the heating loads below freezing temperatures based on ASHP operating performance in laboratory evaluation and field demonstration.

An alternative performance metric has been developed to characterize ASHP considering indoor and outdoor conditions to better represent its heating capacities and efficiencies in the seven International Energy Conservation Code (IECC) zones and moist divisions. This metric could potentially allow HVAC designers to right-size ASHP and auxiliary heating equipment based on the actual heating capacity in order to minimize energy consumption at the heating system level. Additionally, this paper presents performance curves and modeling approaches that have been developed to calculate energy consumption for building energy modeling applications.