Today, there are more opportunities to apply heat pumps in a wide range of climates, including cold-climate zones. All-electric, high-performance heat pumps are among the most effective and popular technologies for decarbonizing heating and cooling, especially as the industry moves toward strategic electrification and a less carbon-heavy grid.  With the diversity of available ducted and ductless products, heat pump solutions are available for low-load, new construction and retrofit projects. Displacement of fossil-fuel-burning HVAC systems is already possible.

The Benefits of Heat Pumps in Cold Climates

Heat pumps provide more energy in the form of heat than they use in the form of electricity. The efficiency of a heat pump is described by its coefficient of performance (COP). The higher the number, the more efficient the system. Combustion-based heating systems such as furnaces and boilers top out with COPs of 0.96 to 0.98. Traditional electrical-resistance systems might achieve a COP of 1, but COP values for some cold-climate heat pumps are greater than 3 at 5°F. This means, even at very low temperatures, some modern heat pumps can provide up to three-times the heat of less efficient alternatives such as baseboard units.

Cold-climate heat pumps create the opportunity to replace combustion-based systems and electrify regions where older heat pumps couldn’t perform. At 47°F, previous generations of heat pumps would start to lose capacity. Homeowners would feel cool air coming through their vents requiring them to manually switchover to fossil-fuel-based backup heat. Additionally, during regular operation, older heat pumps blew air that felt cool to homeowners compared to furnace air temperatures. Performance issues and unmet expectations with conventional heat pumps led some consumers to believe heat pumps couldn’t deliver reliable comfort. 

Modern, cold-climate heat pumps are designed to deliver up to 100% rated heating capacity at 5°F and up to 76% capacity at -13°F. As long as a heat pump can make its refrigerant cooler than the outdoor air, it can capture thermal energy and deliver it as indoor heat. These capabilities limit the amount of time, if any, homeowners need to rely on fossil-fuel-based backup heat. Also, today’s cold-climate heat pumps can supply hot air even at cold temperatures.

The effectiveness of modern heat pumps in cold climates continues to be demonstrated in field studies and reports such as the Cold Climate Air Source Heat Pump Field Assessment published by the Center for Energy and Environment (CEE) in Minnesota. They’re also available with a variety of indoor unit options from ductless wall-mounted indoor units and recessed ceiling cassettes to air handlers and horizontal-ducted units. For all of their impressive qualities, heat pumps can suffer performance issues when misapplied. Here are three key design considerations to help builders and HVAC contractors apply cold-climate heat pumps for comfort and reliable performance.

Tip 1: Consider Dry-Bulb Temperatures and Variable Capacity

HVAC equipment should be sized according to the building’s design loads. These loads use ASHRAE design temperatures and are meant to ensure that mechanical systems can meet a home’s heating and cooling needs for most of the hottest and coldest hours of the year. Design values should not include extreme values such as those that might occur during events like 2018’s polar vortex. Don’t overshoot design temperatures when doing a Manual J® load calculation in hopes of addressing these extreme situations. Avoid guessing and always use accurate local design temperatures.

ASHRAE design temperatures typically occur during 1% of the hours in a year. During most of a year’s 8,760 hours, HVAC equipment operates in partial-load heating or cooling conditions (commonly called the “shoulder season” or “swing season”). It may come as a surprise, but partial-load conditions are prevalent even in heating-dominated regions. For example, Chicago experiences partial-load conditions for 4,002 of those 8,760 hours and Minneapolis, Minnesota experiences partial-load conditions for 3,535 hours.

Modern cold-climate heat pumps have variable-speed compressors designed to slow down and speed up to match system capacity with the load detected by indoor unit sensors. This makes heat pumps more energy efficient than fixed-capacity conventional systems but doesn’t mean we can ignore the heat pump’s range. Each variable-capacity system has a minimum and maximum capacity which is published in the manufacturer’s technical literature. 

When sizing cold-climate heat pumps, it’s important to consider the partial loads of the building. Pay particular attention to system capacity if you design systems for high-performance, low-load buildings. If the peak load for a high-performance apartment at 14°F is around 7,000 BTU/H, a variable-capacity system with a minimum capacity of 9,690 BTU/H would be oversized for most of the year.

Tip 2: Get Comfortable with Load Calculations and Reports

Load calculations are vital when designing any home but are especially important when designing for low-load, cold-climate applications. The variable-capacity traits of modern heat pumps don’t give us an out here. While modeling tools are always evolving, ACCA Manual J® is still our best available tool for residential load calculations. Inaccurate load calculations put us at risk of oversizing or undersizing a system.

All members of a project team, even if they aren’t performing the calculations, should have a basic understanding of a home’s load profile and feel comfortable reviewing summary load reports. Load calculations often go wrong when we don’t verify basic details we may have copied and pasted from previous projects or found in program defaults. If more project team members are prepared to spot abnormalities or errors in the details, we’re more likely to prevent potential problems. Here are a few key inputs to review for every project:

Site location: Program defaults aren’t necessarily the ASHRAE design temperatures or local jurisdiction temperatures. Using the wrong location increases the chances of using the wrong design temperature. Given the importance of using the correct design temperature, always verify the project’s location in the load calculation software. Also, take care to verify the altitude of the home since decreased air density can cause derating at altitudes above sea level. If the exact location isn’t an available option, you can use a nearby location with similar temperature conditions and elevation. 

Orientation: Verify the home’s geospatial orientation because solar gain can have a significant effect on heating and cooling loads. A 90-degree change in orientation, for example, could cause a significant change in required tonnage. This is particularly relevant for projects with large windows and minimal overhangs.

Mechanical system location: Check whether mechanical systems are located in conditioned spaces. This impacts system performance and derating.

Ventilation loads: Anytime it’s hotter, colder, or more humid outside than our interior design conditions, we add to a space’s load. Always verify that loads associated with continuous ventilation are accounted for or considered.

Envelope details: Two homes might look identical from the outside while having significantly different load profiles based upon their respective envelope details. Verify U-values, infiltration and the characteristics of each project’s windows, roof, wall assemblies, insulation, and other components. Even in heating-dominant regions, you can end up with a predominant cooling load if the home has a high-performance envelope.

Tip 3: Don’t Oversize to Cover Extremes

Intentional oversizing is understandable, but this practice can lead to performance, comfort, and efficiency issues. Builders who seek to protect occupants from extreme weather can specify heat pumps equipped with hyper-heating technology and install controls that automate changeover to auxiliary systems when needed. With hyper-heating technology, heat pumps offer significant heating capacity down to -13°F outside. In new construction, builders install electric resistance kits for backup heat knowing that they may operate for only a few hours a year, if at all. In retrofits, the pre-existing furnace or boiler can serve as an auxiliary system.

Cold-climate heat pumps aren’t magic, but to your average homeowner, they can come pretty close. Modern heat pumps are capable of providing reliable comfort at low temperatures with the attention and work of skilled tradespeople. By routinely reviewing load calculation inputs and summary reports, builders can support best practices needed to correctly size systems for low-load, zero energy homes. This will result in better applications and more homeowners willing to adopt these all-electric heating and cooling systems for a decarbonized future.

For more information or advanced training on designing for cold climates, please contact the Mitsubishi Electric Trane HVAC US Performance Construction Team.