Do Heat Pumps Work in Cold Climates?

Heat pumps are a promising HVAC technology with several advantages over traditional heating systems. Heat pumps can achieve electricity savings of over 50% when replacing resistance heaters of the same capacity, and they also have a lower operating cost than propane and oil-fired heating systems.

When compared with gas furnaces, heat pumps have a similar operating cost in many regions. However, heat pumps operate with zero emissions when using electricity from renewable sources, while furnaces are a constant source of carbon pollution.

In spite of their advantages, traditional heat pumps have a technical limitation. They can provide space heating efficiently with moderately cold temperatures, but they suffer a drastic loss of performance under freezing temperatures. However, many HVAC manufacturers have now developed cold climate heat pumps (CCHP), which are capable of efficient space heating at temperatures below 32°F.

How Does a Heat Pump Work?

Air-source heat pumps are based on the same physical principle as air conditioners, but the heat transfer direction is reversed.

• An air conditioner uses the repeated evaporation, compression, condensation and expansion of a refrigerant to capture indoor heat and release it outdoors.
• A heat pump also uses the vapor compression refrigeration cycle, but it captures outdoor heat to be released indoors.Using a suitable refrigerant, heat pumps can extract heat from outdoor air even during cold weather. 
• In both cases, the unit is using electrical power to move heat from a cooler environment to a warmer environment.

Many heat pumps are designed for reversible operation, which means they can operate like an air conditioner during summer. Instead of purchasing different types of HVAC equipment for space heating and air conditioning, you can achieve both functions with a single unit. Just like air conditioners, heat pumps are available in multiple configurations, including: ducted central units, ductless mini-split units and packaged terminal heat pumps (PTHP).

Traditional heat pumps suffer a drastic loss of performance when the outdoor air reaches freezing temperatures. Due to the low outdoor temperature, ice starts to form on the refrigerant coils of their outdoor units. When a heat pump is frosted, its ability to provide space heating is drastically reduced. Excessive frosting can also damage the outdoor unit.

Heat pumps have a defrosting function, which is used to melt ice that accumulates on their outdoor units during cold weather. When a heat pump enters defrosting mode, it reverses its operation just like in air conditioning mode. To prevent the circulation of cold air inside your home, the indoor unit fan is switched off. However, the defrosting function by itself is not enough for the coldest climate zones:

• Since a heat pump reverses its operation while defrosting, it cannot provide space heating simultaneously.
• When the weather is very cold, the heat pump must spend more time defrosting. This means the unit has less time available to provide indoor heating.

Cold climate heat pumps (CCHPs) have several design features that help prevent frosting under subzero temperatures. They can provide space heating reliably and continuously, at temperatures where a traditional heat pump is no longer effective. CCHPs also have a defrosting mode, but they don’t need to use it with the same frequency as a regular heat pump.

What Is a Cold Climate Heat Pump?

According to the National Renewable Energy Laboratory (NREL), a cold climate heat pump is designed to heat homes when outdoor temperatures drop to 5°F or less. As mentioned above, traditional heat pumps suffer a drastic loss of performance when the weather reaches freezing temperatures.

Cold climate heat pumps have several features that improve their performance when operating at low temperatures. Here we will discuss some of the main design features used by HVAC equipment manufacturers.

Variable Speed Compressors

The most basic heat pumps have a single-speed compressor, which switches on and off depending on space heating and cooling needs. Modern heat pumps have variable-speed compressors, which are capable of adjusting their speed according to space conditioning demands.

• Variable-speed compressors have a much lower energy consumption, and they can also operate efficiently across a wide range of temperatures.
• Variable-speed compressors improve heating efficiency during winter and cooling efficiency during summer.
• HVAC systems with this type of compressor have a lower operating cost all year long.

The most efficient heat pumps in the market use variable speed compressors. Thanks to their superior efficiency, they often qualify for financial incentives from government agencies or electric utility companies.

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Low Temperature Heating Systems

Many modern heat pumps have systems that improve their space heating capacity during cold weather, without relying on frequent defrosting. Two examples of this are the Mitsubishi Hyper-Heating technology and the Fujitsu Extra Low Temperature Heating (XLTH) technology.

• Cold-climate heat pump manufacturers equip their outdoor units with built-in heaters or heat recovery systems, which prevent ice accumulation at temperatures below 32°F.
• These heat pumps can operate for longer periods under freezing weather conditions, without having to rely on frequent defrosting.

Cold climate heat pumps are normally designed with additional compressor capacity and larger heat exchangers, to compensate for the increased workload caused by very low temperatures. They also use refrigerants with a low boiling point, which offer better performance in cold climate applications.

Smart Controls and Mobile Apps

Cold climate heat pumps from leading manufacturers include smart controls that optimize their operation under all types of weather conditions. These control systems include self-diagnostic functions: if they detect any issues that need attention from a qualified technician, they generate automatic notifications. Many HVAC manufacturers also offer mobile apps that can be used to monitor and control your heat pump from any location.

Smart controls and mobile apps are offered with all types of HVAC equipment, not only heat pumps. These features can also improve the performance of air conditioners and gas furnaces.

Cold-Climate Heat Pumps: Ongoing Research and Innovation

The US Department of Energy (DOE) launched the Residential Cold Climate Heat Pump Challenge in 2021, in partnership with the US Environmental Protection Agency (EPA) and Natural Resources Canada (NRCan).

The Challenge has the goal of improving the cold-climate heat pump technologies that are commercially available, and it has been divided into two segments: a CCHP optimized for 5°F (-15°C) and a CCHP optimized for -15°F (-26°C). The Challenge is subject to the following timeline:

• Product Prototype = Late 2021 / Early 2022
• Lab Testing = 2022
• Field Testing = Winter 2022-2023 and Winter 2023-2024
• Deployment Programs and Commercialization = 2024

There are 10 HVAC manufacturers participating in the Challenge: Bosch, Carrier, Daikin, Johnson Controls, Lennox , LG , Midea , Mitsubishi, Electric, Rheem, and Trane Technologies. Several utility companies and local government agencies are also participating.

The US DOE published detailed technical specifications for the CCHP designs participating in the challenge. Here are some of the main requirements:

• The challenge is exclusively for residential, centrally-ducted, electric-only heat pumps with capacity between 24,000 and 65,000 BTU/hour.
• The heat pump must use a refrigerant with a low Global Warming Potential (GWP).
• The heat pump must be capable of interacting with the local power grid.
• The heat pump must incorporate electric heat staging.

Heat pump manufacturers can participate in both the 5°F segment and the -15°F segment. The Challenge will bring new heat pump technologies to the market in 2024, which will exceed the performance of current products.

What Are the Advantages of Cold-Climate Heat Pumps?

CCHPs have both economic and environmental benefits. According to the NREL, heat pumps can normally achieve lower heating costs that the following heating technologies:

• Electric resistance heaters
• Oil heating systems
• Propane heating systems

When compared with natural gas heating, the results achieved by heat pumps can vary. Depending on local electricity and gas prices, heat pumps can have lower or higher operating costs than gas furnaces.

CarbonSwitch estimated the typical savings you can expect when switching from traditional heating systems to heat pumps, using NREL data. Here are the results of their analysis:

There is a clear savings opportunity when heat pumps replace electric resistance, propane and oil heating systems. The savings opportunity is smaller when comparing heat pumps to gas heating, but heat pumps are a cleaner alternative when running with renewable electricity.

In addition to lowering your heating costs, cold climate heat pumps can also help you avoid the emissions associated with combustion heating. Here are the carbon dioxide emission coefficients of the main heating fuels, provided by the US Energy Information Administration:

• Propane = 62.88 kg CO2 per million BTU
• Home Heating Oil = 74.14 kg CO2 per million BTU
• Natural Gas = 52.91 kg CO2 per million BTU

To put these figures into perspective, consider that homes in northern US states and Canada can consume over 65 million BTU of natural gas per year. In the case of larger properties and homes located farther to the north, annual gas consumption can exceed 100 million BTU. Propane and heating oil are less common than natural gas, but they have an even higher carbon footprint.

Cold climate heat pumps generate indirect emissions when using grid electricity that was generated by fossil fuels. However, their operating emissions are zero when using energy from renewable sources, and this includes onsite generation with solar panel systems.

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