This tech tip was inspired by Alex Meaney and Ed Janowiak’s session at NCI’s High-Performance Summit 2025: “Good Design Isn’t a Buffet: The Importance and Interconnection of Manuals J, S, and D.” Alex is a building science expert who worked at Wrightsoft (now MiTek) for several years before launching his consulting firm, Mean HVAC Consulting & Design, in 2022. Ed is the Manager of HVAC Design Education at ACCA and heavily contributes to HVAC design education at ACCA and beyond. Both have done incredible work in the industry. 

Information about the NCI’s High-Performance Summit 2026 will be available at gotosummit.com. 

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Air-source heat pumps face unique operational challenges that vary dramatically with local climate and building quality. Understanding these challenges is crucial for HVAC technicians when designing systems and selecting appropriate equipment. This article will examine the distinct performance considerations for heat pumps operating in four climate zones: cold and dry, cold and humid, hot and dry, and hot and humid.

The Building Matters as Much as the Climate

Before diving into climatic challenges, we must understand one key concept: it is not accurate to say that a particular heat pump will be a “good” or “bad” fit for a specific climate region alone. A heat pump's suitability has as much to do with the building it is conditioning as the climate in which it is operating. It would be more accurate to say a particular heat pump is a solution for a specific home in a specific region.

The quality of the building envelope—its insulation and airtightness—also plays a massive role in a heat pump’s effectiveness. This is because insulation reduces a home's heating load far more dramatically than its cooling load. The reason is simple: the temperature difference between inside and outside is much greater in winter (e.g., 70°F indoors and 0°F outdoors is a 70° difference) than in summer (e.g., 75°F indoors and 95°F outdoors is only a 20° difference). Insulation helps prevent heat from entering or leaving a structure via conduction, but a significant portion of the cooling load is radiant; we have solar heat gains through windows. Insulation in the walls and ceilings can't reduce those radiant loads.

Consider two homes across the street from each other in a cold climate. One is an older, poorly insulated house with a massive heating load. In most regions of the USA, sizing a heat pump to meet this heating demand would almost certainly make it grossly oversized for its cooling needs, leading to inefficiency and poor humidity control. (It wouldn’t be a big problem in arid climates, but that’s the minority of cases, as about 80% of the US population lives in a humid climate.) The other home is a new, well-insulated, and airtight home. Its heating load is drastically lower, while its cooling load may be only moderately lower. This makes it far easier to find a heat pump that can handle the heating demand efficiently without short-cycling during the summer.

Therefore, while this article focuses on the effects of climate, a technician must always first consider the building's thermal performance. The most advanced heat pump cannot overcome the deficiencies of a poor building envelope.

Air Source Heat Pumps

As demand for efficient air conditioning grows, so does the popularity of air-source heat pumps. However, a one-size-fits-all model isn’t very effective, as the performance and efficiency of these systems are highly dependent on ambient outdoor conditions. To properly analyze the specific design challenges posed by different climates, it is essential to first understand the metrics used to measure heat pump performance.

Key Performance and Efficiency Metrics

Seasonal Energy Efficiency Ratio (SEER2) & Heating Seasonal Performance Factor (HSPF2): These are the most important metrics for modern residential systems in the U.S. They are the official, legally mandated measures of efficiency over an entire cooling or heating season, respectively. The “2” signifies the updated, more rigorous testing standards implemented in 2023, making them the current benchmark for EnergyGuide labels and federal tax credits.

Energy Efficiency Ratio (EER2): This is a crucial metric for cooling. While seasonal ratings provide an average, EER2 measures a unit's efficiency at a single high-stress point (95°F). This makes it the best indicator of how a system will perform—and how much it will cost to run—during the peak of a heatwave. It's particularly important for buyers in consistently hot climates.

Coefficient of Performance (COP): This is the foundational engineering metric for efficiency. It provides an instantaneous ratio of energy output to energy input at a specific moment under specific test conditions. It's a universal, scientific measure used during design and analysis.

Defining Climate Types

The performance of an air-source heat pump is directly related to the environment in which it is working. This article will use four common climate types according to energy.gov. 

Hot-Humid:

A hot-humid climate is typically defined as a region that receives more than 20 inches of annual precipitation and experiences either a wet bulb temperature of 67°F or higher for 3,000 or more hours, or a wet bulb temperature of 73°F or higher for 1,500 or more hours, during the warmest six consecutive months of the year. Florida (e.g., Orlando, Miami) is an example of a hot-humid climate.

Hot-Dry:

Hot-dry climates are characterized by low annual precipitation (less than 20 inches) and average monthly temperatures that stay above 45°F throughout the entire year. Southern Arizona (e.g., Phoenix) is an example of a hot-dry climate.

Cold-Dry:

Cold-dry climates are areas with substantial heating requirements, as indicated by heating degree days (HDD) between 7,200 and 9,000 (at a 65°F base). These climates receive less than 20 inches of precipitation annually and generally feature cold, dry winters paired with hot, dry summers. Montana is an example of a state in a cold-dry climate.

Cold-Humid:

Cold-humid climates are characterized by high heating needs, with 7,200 to 9,000 HDD (at a 65°F base), and receive over 20 inches of precipitation annually. The combination of cold temperatures and high moisture levels during winter presents specific operational difficulties for heat pumps. Maine is a state in a cold-humid climate.

Performance Challenges in Cold Climates

Cold-Dry Climate

In a cold-dry climate, the performance of an air-source heat pump is fundamentally limited by the outdoor temperature. As the air gets colder, the heat pump's heating capacity and efficiency (COP) both drop substantially. This is because the system relies on a temperature difference between the outdoor air and the refrigerant to absorb heat. When ambient air falls to very low temperatures, it becomes increasingly difficult to maintain this differential, causing the compressor to be less effective and its performance to be reduced.

This drop in efficiency defines the thermal balance point: the outdoor temperature at which the heat pump's output perfectly matches the building's heat loss. Below this point, the system may require supplemental heat. In modern, well-insulated homes using cold-climate heat pumps, this balance point can be very low. However, in older homes, a supplemental heating system may be required to meet the load on the coldest days. This supplemental heating may come from integrated electric resistance strips or a gas furnace (in dual fuel systems). 

A distinction should be made between supplemental heat (designed to assist the heat pump in extreme cold) and emergency heat (used only if the heat pump fails).

• Supplemental heat works alongside the heat pump when temperatures drop too low for the heat pump to efficiently maintain the desired indoor temperature. It helps supplement the heat pump's output.
• Emergency heat is a backup heating source that is only used if the heat pump itself is not functioning. It's a completely independent system designed for temporary use until the heat pump can be repaired.

Another thing to consider is that in dry climates, airborne dust and debris can build up on the outdoor unit's fins. This layer of dirt insulates the coils and reduces the system's ability to transfer heat, lowering its efficiency.

Cold-Humid Climate

In addition to the challenges of reduced capacity in extreme cold, cold-humid climates add the significant problem of frost formation on the outdoor coil. When the coil's surface temperature drops below freezing, airborne moisture freezes onto it, forming an insulating layer of ice that blocks airflow and reduces heat absorption.

To counteract this, the heat pump runs a defrost cycle, which temporarily reverses the refrigerant flow to send hot refrigerant through the outdoor coil and melt the ice. However, this cycle consumes energy, stops heating the home, and often triggers inefficient backup electric heat to prevent blowing cold air indoors. Frequent defrosting can significantly reduce the unit's overall seasonal efficiency (HSPF2).

Proper installation is critical to mitigate these issues. The outdoor unit must be elevated to stay clear of snow and allow meltwater from the defrost cycle to drain away, preventing it from refreezing and creating a damaging block of ice at the unit's base.

Cold Climate Recap

The biggest challenge for a heat pump isn't just cold weather, but cold, humid weather. While low temperatures reduce efficiency in any cold climate, humidity adds the problem of frost. Frost on the outdoor unit acts like insulation, preventing the system from absorbing heat. To remove it, the heat pump runs a defrost cycle, which uses energy and temporarily stops heating the house. Consequently, in cold, humid conditions, a heat pump's efficiency is penalized three times: by the low temperature, the energy used for defrosting, and the power needed for supplemental heat. The impact of this triple penalty is directly tied to the building envelope. A home with a high heating load due to poor insulation will struggle significantly when the heat pump's output is compromised, relying heavily on expensive supplemental heat.

Performance Challenges in Hot Climates

Hot-Dry Climates

In hot-dry climates, extreme outdoor heat is the main challenge. At higher temperatures, the heat pump's compressor becomes less effective at raising the refrigerant’s temperature, making it difficult to release heat into the already hot air. This increased workload lowers cooling efficiency (EER) and strains the compressor.

Furthermore, some variable-capacity units are designed to protect themselves in extreme heat. They may automatically reduce their capacity (derate) or even shut down entirely at very high temperatures (e.g., 115°F or higher) to prevent hardware damage. This means that on the hottest hours of the year, when cooling is needed most, the system's output could be compromised. Technicians in these climates must verify the upper operating limits of the equipment they install.

Airborne dust and sand also pose a problem. Debris coats the outdoor coil, acting as an insulator that traps heat in the system. This forces the compressor to work even harder, drastically reducing efficiency and increasing energy use.

Hot-Humid Climates

In hot-humid climates, removing moisture (latent load) is just as important as lowering the air temperature (sensible load). These challenges of dehumidification and proper sizing are fundamental to all cooling systems, whether they are heat pumps or straight cool air conditioners.

Heat pumps dehumidify by condensing water on the cold indoor coil, which requires long run times to be effective. The most critical design mistake in these areas is oversizing the equipment. An oversized unit cools the air too quickly and shuts down before removing enough moisture. This “short-cycling” creates a cool but clammy environment, which is uncomfortable and can promote mold growth.

High humidity forces the system to work harder, which reduces efficiency and increases energy costs. Additionally, the constant moisture on the indoor coil creates an ideal environment for biological growth if not cleaned regularly.

Hot Climate Recap

System design for air-source heat pumps in hot climates hinges on humidity. In dry areas, the primary challenge is dissipating heat against extreme temperatures, necessitating durable equipment and meticulously clean coils. Conversely, in humid regions, moisture management becomes the paramount concern.

The biggest mistake in hot, humid climates is oversizing the unit. An oversized system cools the air too fast and shuts off before it has time to dehumidify. This leaves the air cool but clammy. Therefore, proper sizing is not just about temperature—it's about humidity control. The industry's tendency to oversize equipment is a major problem in these climates.

Solutions for Air Source Heat Pumps for the Various Climate Conditions

Solving heat pump challenges in varied climates requires both advanced technology and proper system design. Modern units include features to handle extreme conditions, but they only work if the system is designed and installed correctly.

Components

Variable-Capacity Compressors: This is a key technology that allows a system to precisely adjust its output to match a home's needs. This category includes inverter-driven units as well as multi-stage systems.

• In Cold Climates: If a heat pump is to be used as a primary heat source in a cold climate, a variable-capacity system is recommended. By running continuously at a lower speed, these compressors extract more heat from cold air and avoid the energy waste of constantly turning on and off.
• In Hot/Humid Climates: The ability to run at a low speed for longer periods mitigates the “short-cycling” problem of oversized single-stage units. However, this is not a silver bullet. Some systems, when running at very low speeds, operate with a warmer indoor coil, which can significantly reduce their dehumidification capability. Proper commissioning is key.

Enhanced Vapor Injection (EVI): EVI is a technology designed for extreme cold. It injects refrigerant vapor into the compressor, allowing it to work harder and more efficiently at very low temperatures. EVI systems can maintain heating capacity at temperatures as low as 5°F and can continue operating at -13°F or below.

System Controls and Integrations

Dual-Fuel Hybrid Systems: In the coldest climates or in less-insulated homes, a dual-fuel system combines an efficient heat pump with a gas furnace. A smart thermostat uses the heat pump during milder weather and switches to the furnace in extreme cold. This “automatic” switchover is not plug-and-play; it requires proper commissioning by the technician to set the outdoor temperature lockout points based on the equipment and the home's balance point.

Advanced Defrost Controls: To save energy in cold, humid conditions, modern heat pumps use “demand defrost” controls. Instead of running on a simple timer, these systems use sensors to detect actual frost buildup, initiating the energy-intensive defrost cycle only when necessary.

Advanced Humidity Control Modes: For hot, humid climates, many variable-speed heat pumps offer a “dry mode” that prioritizes dehumidification, often by running the indoor fan at a lower speed. Be aware that these modes can sometimes cause overcooling, as they may prioritize hitting a humidity target over the temperature setpoint. It is also recommended to check expanded performance data to verify the equipment's latent capabilities instead of relying solely on sales brochures and marketing materials. 

In some cases, it may be necessary to consider ancillary dehumidification to assist in the removal of excess humidity in certain situations. This could include a standalone unit or one that is integrated into the HVAC system. 

Sizing and Installation

Advanced heat pump technology is only effective if the system is properly designed and installed. Poor installation is a primary reason systems fail to achieve their rated efficiency.

Proper Design: Technicians must stop using outdated “rules of thumb” for equipment selection and duct design and instead use the ACCA (Air Conditioning Contractors of America) design manuals:

• Manual J calculates a home's heating and cooling needs.
• Manual S guides you through the process of matching your load calculation to the manufacturer’s performance data (not AHRI’s) to select correctly sized equipment.
• Manual D ensures the duct system is designed to deliver air efficiently.

Quality Installation: The physical installation must be precise, including correct refrigerant levels, proper outdoor unit placement for airflow, and adherence to all manufacturer guidelines.

Climate Challenge Comparison Chart

Conclusion

Ultimately, the performance of an air-source heat pump is a direct result of pairing the right technology with a design tailored to its specific climate and, just as importantly, the building it serves. Technologies like variable-capacity compressors and EVI have provided effective tools to manage environmental extremes.

However, the most significant barrier to widespread, efficient heat pump operation is not technological, but procedural. The difference between a high-performance system and a costly, uncomfortable one often comes down to a technician's commitment to proper system design guided by ACCA’s Manual J for load calculations and Manual S for selection (with the manufacturer’s performance data, of course).

—JD Kelly