This webinar explored the technical considerations involved in rightsizing packaged air-to-air heat pump equipment for heating operation, with particular attention to Dedicated Outdoor Air System (DOAS)–adjacent and comfort heating applications. While the discussion focused on packaged heat pumps used for comfort heating and cooling rather than pure DOAS systems, many of the principles apply directly to DOAS and ERV systems where heating capacity, low-ambient performance, and supplemental heat selection are critical.
Historically, rooftop units (RTUs) were sized by treating cooling and heating as largely independent functions. Engineers selected a nominal cooling capacity to meet the cooling load and then chose a gas furnace (low, medium, or high) to meet the heating requirement. With the industry shift toward air-source heat pumps, heating performance is now directly tied to the refrigeration system, forcing designers to reconsider how equipment is sized.
Oversizing a heat pump to meet peak heating load can result in:
- Larger cabinet sizes
- Higher first cost
- Increased electrical infrastructure requirements
- Greater embodied carbon in equipment materials
The webinar emphasized that meeting 100% of peak heating load with the heat pump alone is often unnecessary and inefficient, particularly when those extreme conditions occur for a small fraction of annual operating hours.
A core technical challenge with air-source heat pumps is that available heating capacity decreases as outdoor air temperature drops, while building heating load simultaneously increases. This divergence becomes especially important in cold climates. Using Chicago weather data as an example (noted to be similar in shape to Denver’s temperature distribution), the presenter illustrated that:
- Temperatures below 15°F account for approximately 4% or less of annual operating hours
- Heating operation below 20–25°F represents less than 10% of total heating hours
This bin-hour perspective reframes equipment sizing: instead of designing for the coldest possible condition, designers can optimize for the majority of operating hours and rely on supplemental heat during infrequent extreme conditions.
The webinar then presented a conceptual graph plotting:
- Outdoor air temperature on the x-axis
- Heating load and heating capacity on the y-axis
- Annual operating hours as a cumulative curve
If a heat pump is sized to meet 100% of peak heating load, it becomes significantly oversized during moderate outdoor temperatures. By selecting a smaller nominal heat pump capacity, the system operates closer to optimal efficiency across most of the heating season. A transition temperature, often around 20–25°F, was identified as a practical point where heat pump capacity matches heating load above this temperature and supplemental heat is required below it. Since temperatures below this transition represent a small percentage of annual hours, this approach reduces first cost while maintaining comfort.
Below the transition temperature, heating demand is met by auxiliary or supplemental heat, which may include natural gas heat, electric resistance heat, hot water coils (via boiler or district heating). The appropriate supplemental heat source depends on project goals, available utilities, and decarbonization requirements. The webinar stressed that supplemental heat is not a failure of heat pump design, but an intentional strategy to balance capital cost, efficiency, and operational performance.
The heating capacity curve shown in the analysis is linear for simplicity, but real-world operation is more complex due to defrost cycles.Between roughly 20°F and 40°F, outdoor coils may frost under certain humidity and dew point conditions. When frost accumulates, the unit periodically enters defrost mode, temporarily reversing the refrigeration cycle to melt ice from the coil.
Key impacts of defrost include:
- Reduced net heating output during defrost cycles
- Increased reliance on supplemental heat
- Decreased overall heating efficiency
For accurate energy modeling and rightsizing decisions, defrost penalties must be accounted for, particularly in climates with frequent freeze-thaw conditions and higher winter humidity.
As outdoor temperature decreases, the coefficient of performance (COP) of an air-source heat pump declines. At lower ambient temperatures, the energy required to extract heat from the air increases significantly. The webinar emphasized that there is a point where the COP becomes low enough that alternative heat sources may be more energy- or carbon-efficient. In addition to this, the source of grid electricity (coal, gas, renewables) materially affects emissions outcomes. This reinforces the importance of identifying a rational transition temperature rather than forcing the heat pump to operate deep into low-COP conditions.
Using Chicago as an example of a grid with significant fossil fuel–based electricity generation, the analysis showed that:
- An all-electric heat pump plus electric resistance heat can result in higher carbon emissions during cold conditions
- A heat pump with natural gas supplemental heat may yield lower overall carbon emissions for the coldest 10% of operating hours
This highlights that decarbonization strategies must consider both equipment efficiency and grid carbon intensity. What is optimal in one region or utility territory may not be optimal in another.
Electric resistance heat introduces additional design constraints, particularly:
- Increased Minimum Circuit Ampacity (MCA)
- Higher Maximum Overcurrent Protection (MOCP)
- Potential conflicts with existing electrical infrastructure in retrofit projects
Modern equipment platforms allow designers to limit electric coil operation during heat pump operation, preventing the stacking of compressor and full electric heat loads. This capability can significantly reduce electrical service upgrades, especially in retrofit applications.
Finally, the webinar stressed that retrofit projects must consider existing structural capacity, available electrical service, cabinet size limitations, utility availability (gas vs. all-electric mandates). In some projects, decarbonization goals or energy codes may eliminate gas or boiler-based heating options entirely, forcing fully electric solutions despite efficiency tradeoffs. In others, hybrid systems may offer the best balance of performance and emissions.