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.
David Green says:
I have installed ducted ASHP (Bosch – two 5 tons units) at my own home and ductless ones (Mitsubishi – 3 condensers and 8 heads) at a rental property we own in the same town. Both houses are fairly well insulated and have mostly triple-glazed windows. Both systems were installed by a reputable high-end contractor. Both systems are incapable of maintaining the house at 70F at outside temperatures below about 20F and rely on their original oil furnaces as back ups below 20F. This is in Massachusetts which is not as cold as Minnesota.
Don’t get me wrong, I am big fan of both systems but any suggestion that these systems can heat a house on their own throughout a typical cold-climate winter is highly misleading.
Bosch claims their product works down to 5F and Mitsubishi claims theirs works down to -14F. From an engineers perspective they are probably right – the pumps keep rotating down to those temperatures. But from a homeowner’s perspective, “work” means they heat my house, not simply rotate. From a homeowners perspective, neither system works below about 20F. I think you should be careful of any suggestion that “cold climate” ASHP’s can do this.
Bruce Sullivan says:
Thanks for your comment. It raises an important issue.
My experience with a single-head mini-split has been exceptional. My own home was built 5 years ago in a cold climate (Zone 5, 7000 HDD) and has a design heat load of 8,000 btu/hr. Against the recommendation of the heating contractor I purchased the smallest available unit with a rated capacity of 9,000 btu/hr. That doesn’t provide the “safety margin” that many professionals would think necessary. Winter weather routinely dips below freezing and there are often long spells with single-digit temperatures, even dropping into negative territory on occasion. As “insurance” I installed two 500 watt electric wall heaters, but they have never been necessary. My home is always comfortable. I couldn’t be happier with the performance of the equipment.
It’s not possible to diagnose your specific issue without a lot more information. However, I will speculate that your experience is precisely why careful sizing, delivery location, and installation is important. Small rooms often have heating or cooling loads that are much smaller than the capacity of the available indoor units. When a small room isn’t connected to the air flow of the larger building, the temptation is to install an indoor unit to bring heating or cooling to that isolated space. This quickly leads to a system that is oversized, rapidly meets the temperature setting and shuts off before the space is truly warm (or cool). It’s better for any heating system (forced air included) to run for a long time to allow the the temperature of the building and it’s contents to stabilize. It’s another application of the idea of thermal mass. As I understand it, multi-head systems are particularly vulnerable to oversizing.
Many heating contractors and builders have become accustomed to the idea that every room needs a “delivery” point, when in fact, small rooms without much exterior wall or window surface may have very small conditioning needs. Placing a head unit in smaller rooms can harm performance and increase cost. It’s possible to combine smaller rooms into a single heating zone so the capacity of a small indoor unit can serve more than one space. A great solution to provide heating/cooling to a group of rooms with relatively small loads is a horizontal ducted mini split system. As an example, if you have 2-4 bedrooms connected to a corridor, the horizontal ducted system can serve all of the bedrooms and provide you a right sized heating/cooling system. The central hallway allows the trunk duct to be located in a ceiling soffit to keep all the ducts in conditioned space.
This document available from Mitsubishi goes into great depth about these design issues and includes a section on high-performance homes too. I think the takeaway is that mini-split systems must be sized carefully, taking into account room by room needs. Of course, heating equipment only comes in so many sizes, so there will always be a compromise. In many cases, two or more single systems (one outdoor unit connected to one indoor unit) may be the most economical and efficient approach. When in doubt, it’s okay to throw out the safety margin when sizing, and install an electric resistance heater as backup. Mini-splits offer amazing efficiency and flexibility through an evolutionary leap in technology. Designers, builders and heating contractors simply can’t apply old methods to this fundamentally modern technology.
tezm says:
I agree with your comment about not working as well when cold but my experience with my 1987 2.5 ton heat pump worked without backup resistance heat down to 17 degrees. Per some generic data I found the efficiency drops off about 1% per degree in lower temperature, in normal operating ranges. I am not sure how it is when really cold. The house is 3550 sqft and super insulated.
Bruce Sullivan says:
This is good to know. I lived with a standard, central heat pump for several years and also found that it worked pretty well. The efficiency of your building envelope surely supports the high performance of the system.
Fred says:
Ducted Daikin 36,000 BTU with it’s matching Daikin air handler easily maintained 70F at 15F ambient without issues. Diakin is rated at 100% heating capacity at -4F with heating capacity dropping as ambient temperatures fall below -4F. Obviously sizing and proper installation is necessary and most importantly the matching air handler which controls the heat pump operation is also necessary. Some people buy generic air handlers (or use existing ones) for use with low temp. ducted system heat pumps, and that doesn’t work.
TIM DEAN says:
Hope to get a lot more comments on this site. The ones so far are useful but need additional data. Hate to throw gas on the political fire but the only good reason to make the investment for a cold climate heat pump is to eliminate a source of carbon dioxide. I also feel the same about electric cars. I don’t believe the capital costs will be recovered but if everyone pitches in climate change can be mitigated. Also needed in the analysis is how long these heat pumps work before they start to fail. A 95% efficient gas furnace will most likely outlast the original owner.
Joe Emerson says:
First, you are right heat pumps currently last about 15 years on average. Newer ones may last for 20 years or longer if well maintained, but not all gas furnaces have the longevity you suggest. Regarding comparative cost, a heat pump both heats and cools. So you will need to compare the cost of installing and running both a gas furnace and an AC system with the installation and operating costs of a heat pump which does both heating and cooling. Switching from a gas furnace to a heat pump will eliminate your methane emissions and methane is a far stronger green house gas than carbon dioxide – so you will be doing the right thing for the climate. But climate change and cost aside, there are many other good reasons to get a heat pump heating and cooling system: they heat much more evenly, they will cost less to operate than a gas furnace and an AC system, they provide very even heating and cooling – no big variations in temp that occur when a gas furnace or a air conditioning unit cycle on and off. The outdoor unit is quieter than indoor or outdoor AC units.
John says:
Ok, so I’m in Colorado Springs (Zone 5b). I have a gas furnace and a 2-Ton Condenser. I also have 32 solar panels and two electric vehicles. (9.16kwp) My utility company does 1:1 net metering. Right now, I usually break-even on solar credits every year.
I’m trying to do what I can to get off NG and THEN maybe save a few bucks in the LONG run. The cost of NG doubled this season. It’s supposed to be “temporary” but we’ll see.
My 2-ton unit (2016) can’t keep up in the summer. I have a brown stucco house (2016), with great insulation but this thing runs non-stop during the warm Colorado summers, if I let it. I was an HVAC tech for the military so I know a little bit. Sub-cooling and pressures all check out for a 410a unit. I also have a whole house attic fan for the cooler summer days.
I’m looking at a 3-ton Daikin or Amana Heat-pump with matching furnace. Some contractors say “heat pumps don’t work in Colorado” and a couple are happy to take my money.
Opinions?
Joe Emerson says:
It sounds like you are well on the path to creating a zero carbon life. If you have not had an energy audit or your home, I would suggest having one, including a blower door and infrared evaluation, in order be sure you have adequate insulation and air sealing to minimize the burden on heat pump heating and cooling. I would also evaluate the heat exposure that may occur in the summer from the setting sun and consider some way of shading late afternoon sun in the summer. Lastly, I suggest you identify the most experienced heat pump installers in your area and get a couple of estimates. Be sure they do the manual J calculations when sizing the heat pump. A well sized extreme cold weather heat pump should provide all your heating and cooling needs if the home is well insulated, air sealed and protected from afternoon summer sun. Depending on the size and lay out of your home, two or more heat pumps may be more cost effective than one larger one.