Explanation of Heat Pumps
When it comes to residential heating and cooling systems, there are few types of heating systems as energy-efficient as heat pumps. But what exactly are heat pumps, and how do they work? In short, a heat pump is simply an electrical device capable of efficiently transferring heat from one place to another – which also means they can cool a space if the heat transfer direction is reversed. Despite the initial counterintuitiveness, heat can be extracted from cold air and added to warmer air.
To explain heat pumps more practically, let’s take a step back and understand the basics of heat transfer. First, for the sake of analogy, let’s imagine heat as a large body of water. Logically, water flows from higher to lower places. Similarly, heat naturally flows from warmer places to cooler places.
So, how do we extract heat from cold places and send it to warm places? Well, it’s a bit like moving water from a lower place to a higher place. All you need is a pump! Heat is defined by the movement of molecules that make up matter. Therefore, in essence, all air with a temperature above absolute zero (-273°C, the temperature where molecules stop moving) contains some amount of heat. So technically, heat can be extracted from the air at any temperature and sent to wherever we want with just a bit of energy to pump the heat to the desired location.
How are Heat Pumps More Efficient than Other Types of Electric Heating?
Let me explain in more depth how heat pumps work more efficiently compared to electric furnaces or baseboard heaters using another analogy I recently heard – think of an elevator in a high-rise building. An elevator uses a motor and a drive system to lift passengers, but gravity is what pulls the elevator car back down to the bottom of the building – so the elevator consumes more electricity when going up (the path of maximum resistance – equivalent to electric furnaces and baseboard heaters) compared to going down (the path of least resistance – equivalent to the compressor in a heat pump).
Coincidentally, it’s interesting to note that researchers at the International Institute for Applied Systems Analysis (IIASA) envisioned a gravity-based system that uses elevators in high-rise buildings to generate and store electricity. The Bullitt Center in Seattle has already operated a retrofitted elevator with regenerative braking, reducing its power consumption by 60% – how cool is that?!
So, are heat pumps commonly used? Are they a proven technology? Heat pumps are quite commonly used in our daily lives. In reality, this exact process of transferring heat from where it’s not needed to where it is needed happens every day in our homes because a refrigerator is a typical form of a heat pump.
The Refrigeration/Heating Cycle in Heat Pumps
In terms of cooling, heat pumps operate much like refrigerators. Heat is extracted from the air that doesn’t need it and transferred outside where the energy is condensed until it’s warm enough to keep everyone comfortable indoors during winter or, conversely, to keep the indoors cool in hot weather.
Heat pumps consist of three main components: the evaporator, compressor, and condenser. Each plays a crucial role in how heat is transferred from one place to another in the heat pump. Heat pumps can absorb and remove heat by using a liquid refrigerant (more on this later), extracting heat from the air.
The four basic steps of a heat pump’s operation are as follows:
The process starts with the refrigerant in a saturated vapor state. This saturated vapor enters the compressor, where it is pressurized, and its temperature increases.
The hot vapor then passes through the condenser, where it is condensed back into liquid form. This condensation results in the refrigerant giving up heat. This is what happens in the back of your household refrigerator, and why the back of the refrigerator gets warm as heat is dissipated.
The liquid refrigerant then passes through an expansion valve, where the pressure is reduced, and the liquid becomes even colder. At this point, the fluid is typically colder than the space that needs to be cooled.
Finally, the partially evaporated cold fluid flows through the evaporator, which is usually a coil or a long tube. A fan then blows air over the coil or tube, cooling the air. This causes the refrigerant to evaporate inside the tube, returning it to its original saturated vapor state.
Essentially, what’s happening is that the refrigerant is forced through a cycle of condensation and evaporation, with significant changes in temperature and pressure. Then, depending on the application, these temperature fluctuations are used to heat or cool a flow of air or water effectively.
How Are Heat Pumps Different from Refrigerators?
Residential heat pumps with reversible operation can provide both heating and cooling by reversing the flow of heat. This is typically not useful for refrigerators! Some models can even be used for efficient, economical domestic hot water heating. The operation of heat pumps in cold weather and their efficiency depend on the type of heat pump installation chosen.
Air-to-air central heat pump systems extract heat from outdoor air and condense that energy until it’s warm enough to keep everyone comfortable indoors during the winter or cool during the summer.
Geothermal heat pumps use the heat stored in water beneath the earth’s surface for heating homes and businesses – though they may present reliability issues due to their cost and complexity, they are typically reserved for larger surface areas.
Ductless heat pumps, as compared to central air-to-air heat pump systems, directly deliver warm or cool air to living spaces in homes through separate air handlers. Typically retrofitted in homes with non-ducted heating, they may best serve as supplemental heating for heating renovations where the original secondary heating system still serves as a backup heat source for the heat pump in case of very cold weather or power outages.
For those already using fossil fuel central air furnaces for heating and in need of replacement, high-efficiency central heat pumps offer an energy-efficient and more sustainable way to stay warm in the winter or cool in the peak of summer.
This explains the scientific principles behind heat pumps, but what makes them so fascinating for residential heating and cooling applications? Which heat pumps are the most efficient and reliable, and which work best in cold climates? Heat pumps come with several advantages, and let’s take a look at some of them now.
Advantages of Using Heat Pumps for Heating:
First, the energy required to operate the compressor and fans or pumps in heat pumps is usually significantly lower than the energy required for generating heat, or “creating” heat. Heat pump performance is typically measured by a “coefficient of performance” or COP. The COP is the amount of heat transferred or moved divided by the energy needed to move that heat. The COP for typical residential heat pumps is usually around 3, meaning for every unit of energy input into the heating system, it transfers 3 units of energy. This is a significant advantage compared to electric baseboard heaters with a COP of 1 (where every unit of energy input is emitted as heat).
Another interesting advantage of heat pumps compared to other residential heating or cooling systems is that they can be configured to provide both heating and cooling simultaneously. The thermodynamic cycle mentioned earlier can be reversed so that it goes from one function to the other. Reversible heat pumps are not just about absorbing heat from the outside to heat the indoors during the winter; they can also absorb heat from indoors and expel it outdoors to cool the indoors during the summer. This helps save on purchase costs and maintenance because one machine can perform the work that used to require two machines. It also helps save basement storage space, as central heat pumps are typically smaller than equivalent gas furnaces and air conditioning combinations.
Other interesting advantages include improved indoor air quality since there’s no burning of fuel and exhaust, and the system always adds fresh air to the home. Heat pumps have a wide range of applications; they can be used to heat incoming outdoor air or act as air-to-water heat pumps to produce residential hot water. Heat pumps can also be combined with geothermal heating and cooling, where heat is either extracted from or returned to the ground.
Heat pumps sound like quite magical devices capable of doing it all. However, they do have some important drawbacks. Firstly, their performance depends largely on the climate. In very cold climates where temperatures frequently drop below -10 degrees Celsius, the efficiency of heat pumps can decrease significantly unless the latest generation cold-climate heat pumps are carefully chosen.
While heat can still be extracted from cold air, only the best heat pumps are suitable for very cold temperatures, for two reasons: 1) In very cold weather, COP often drops significantly, offsetting the efficiency advantage; 2) In cold climate regions like much of Canada and northern states, heating demands for homes are often far greater than cooling demands, to the point where using a heat pump as the sole heat source is impractical, especially in moderately insulated homes.
Can Heat Pumps Work in Cold Climates?
Generally, in cold climates, it’s traditionally advised to use electric baseboards or other forms of heating alongside a heat pump to ensure a home stays warm on the coldest winter days. However, an argument can be made for investing in additional insulation layers in new homes rather than additional heat generation, so that the heat pump can run more efficiently, as less capacity is required.
Additionally, obtaining passive solar heat through well-insulated materials and appropriately designed homes can allow heat pumps to safely provide all the needed heat and comfort for a home even on the coldest winter days. Note that while this system may save money in the long run, the initial cost of heat pumps is often higher than other systems, especially if a secondary backup heating system is also needed.
Lastly, the heat produced by heat pumps is typically not as intense as that from traditional furnaces. For instance, heat pumps usually produce heat between 32 and 37 degrees Celsius, slightly below body temperature. In contrast, a typical fossil fuel gas furnace produces heat close to 50 degrees Celsius, which can feel much more comfortable on cold winter days. Some people find this lower temperature a bit uncomfortable in very cold weather, especially in poorly insulated homes.
So, when shopping for a new residential central heating and cooling system on the market, it’s worth considering heat pumps, especially as many states are banning fossil fuel heating systems. The extent to which they make sense depends on the level of insulation in the house, the climate zone, and the sustainability awareness of future homeowners.
In highly insulated homes, heat pumps can provide all the needed heat and comfort, but in homes built to minimum standards, having a backup heat source may be a prudent choice. As a homeowner or builder, you’ll need to strike a compromise between investing in insulation layers or additional heating systems. Either way, heat pumps are a very efficient source of residential heating and cooling, and in many cases, the advantages outweigh the disadvantages.
We also have a guide on air-to-water hybrid electric heat pump water heaters that leverages this efficiency, significantly reducing the energy required to meet our domestic hot water needs.
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Factors to Consider When Choosing a Heat Pump for Cold Climates
While an official “cold climate” standard for heat pumps is yet to be established, the upcoming Energy Star standard for air-source heat pumps, set to launch in January 2023, will introduce a certification mark specifically for cold-climate heat pumps. This mark will indicate a suitable level of low-temperature performance and efficiency.
In the meantime, the NEEP (Northeast Energy Efficiency Partnerships), led by Dave Lis, maintains a database of heat pump models that excel in cold weather conditions. Many of the models in this database demonstrate effective heating capabilities even at frigid temperatures as low as 5°F, as well as at mild temperatures of 47°F. These heat pumps are specifically designed to withstand the significant temperature fluctuations experienced in the Northeast and Midwest regions throughout winter.
A critical feature to look for in a cold-climate heat pump is a variable-speed compressor, powered by an inverter. While this type of compressor can benefit heat pumps in any climate, its advantages are especially notable in regions with distinct seasonal variations. It enables a single heat pump to operate efficiently and effectively during the harshest winter freezes, the most sweltering summer afternoons, and all the moderate days in between.
Another advantage of a variable-speed system is its ability to maintain a more stable indoor temperature (and humidity level) compared to traditional single-speed HVAC systems. Iain Walker, PhD, a mechanical engineer and building scientist at the Lawrence Berkeley National Laboratory, explains that single-speed setups often result in significant fluctuations in indoor climate as they turn on and off throughout the day.
Dave Lis uses a car analogy to illustrate the difference: Single-speed heat pumps operate at either 0 or 100 mph, while variable-speed systems can run at various speeds in between. On a bitterly cold night in February or a hot, humid day in July, variable-speed systems can operate at high speeds. On milder days in October or April, they can operate at slower speeds, akin to driving through a school zone. The need to reach top gear is rare. Variable-speed models also contribute to energy savings, similar to how consistent, moderate driving improves gas mileage compared to constant acceleration and braking.
Another technology that has enhanced low-temperature performance is flash injection, also known as vapor injection. Standard heat pumps may experience a decrease in heating capacity as outdoor temperatures drop. While a heat pump might effectively heat your home when it’s 40°F outside, it could struggle below 25°F. However, cold-climate heat pumps can utilize a shortcut in their refrigerant loops during low temperatures, enhancing their heating performance in cold weather. Although the efficiency may slightly decrease during this mode and when running self-defrosting cycles (required to melt ice that accumulates on the heat pump’s coils), the energy usage remains considerably more efficient than that of electric resistance or fuel-fired systems, according to Lis.
Similar to “regular” heat pumps, cold-climate heat pumps are available in two main types: ducted and ductless. If your home already has well-functioning ductwork, a ducted heat pump is usually the preferred option. However, if your home lacks ductwork, such as when you currently use radiators for heating or when adding climate control to a garage, attic, or home addition, a ductless system, commonly referred to as a mini-split, is typically the choice. Both types offer efficient and effective performance in cold weather, and the main difference lies in how they deliver heat (and cooling). For more comprehensive information on the various types of heat pumps and their operation, please refer to our heat pump buying guide.
Considerations When Selecting a Heat Pump for Cold Environments
While an official standard specifically for heat pumps in cold climates has yet to be established, the upcoming Energy Star standard for air-source heat pumps, scheduled to launch in January 2023, will introduce a certification mark exclusively for heat pumps suitable for cold climates. This mark will indicate an acceptable level of performance and efficiency in low-temperature conditions.
Meanwhile, the NEEP (Northeast Energy Efficiency Partnerships), under the leadership of Dave Lis, maintains a database of heat pump models that excel in cold weather. Numerous models in this database demonstrate effective heating capabilities even in extreme temperatures as low as 5°F, as well as in milder conditions of 47°F. These heat pumps are specifically designed to withstand the significant temperature fluctuations experienced in winter within the Northeast and Midwest regions.
A crucial feature to consider in a heat pump for cold climates is a variable-speed compressor, powered by an inverter. While this compressor type can benefit heat pumps in any climate, its advantages are particularly notable in areas with distinct seasonal variations. It enables a single heat pump to operate efficiently and effectively during harsh winter freezes, scorching summer afternoons, and everything in between.
Another advantage of a variable-speed system is its ability to maintain a more stable indoor temperature and humidity level compared to traditional single-speed HVAC systems. Iain Walker, PhD, a mechanical engineer and building scientist at the Lawrence Berkeley National Laboratory, explains that single-speed setups often result in significant indoor climate fluctuations as they cycle on and off throughout the day.
Using a car analogy, Dave Lis highlights the difference: Single-speed heat pumps operate at either 0 or 100 mph, while variable-speed systems can operate at various speeds in between. On freezing nights in February or sweltering days in July, variable-speed systems can perform at high speeds. On mild days in October or April, they can operate at lower speeds, similar to driving through a school zone. The need to reach maximum speed is rare. Variable-speed models also contribute to energy savings, comparable to how consistent, moderate driving improves gas mileage compared to constant acceleration and braking.
Another technology that has improved low-temperature performance is flash injection, also known as vapor injection. Standard heat pumps may experience a decrease in heating capacity as outdoor temperatures drop. While a heat pump might effectively warm your home when it’s 40°F outside, it could struggle below 25°F. However, heat pumps designed for cold climates can utilize a shortcut in their refrigerant loops during low temperatures, enhancing their heating performance in cold weather. Although the efficiency may slightly decrease during this mode and when running self-defrosting cycles (required to melt ice that accumulates on the heat pump’s coils), the energy usage remains considerably more efficient than that of electric resistance or fuel-fired systems, according to Lis.
Similar to conventional heat pumps, those suitable for cold climates are available in two primary types: ducted and ductless. If your home already has well-functioning ductwork, a ducted heat pump is typically the preferred choice. However, if your home lacks ductwork, such as when using radiators for heating or when adding climate control to a garage, attic, or home addition, a ductless system, often referred to as a mini-split, is usually the recommended option. Both types offer efficient and effective performance in cold weather, with the primary distinction lying in how they deliver heat (and cooling). For more comprehensive information on the different types of heat pumps and their operation, please consult our heat pump buying guide.
Unveiling the Potential Financial Benefits of Heat Pumps in Cold Climates
As per Lis from the NEEP, the initial surge of cold-climate heat pump adoption in northern New England occurred when heating fuel prices skyrocketed during the early 2010s. Many residents realized that utilizing a heat pump to keep their homes warm was not only cost-effective but also more economical than relying on propane, oil, or wood—the prevalent fuel sources in that region. Moreover, they enjoyed the added advantage of experiencing air conditioning for the first time.
Now, you may question the actual extent of monetary savings achievable with a heat pump or whether it can at least offset your HVAC expenses. A comprehensive study conducted by UC Davis estimates that approximately 32 percent of households in the United States, including those situated in severely cold regions, can reap financial benefits by transitioning to a whole-home heat pump system. Furthermore, if heat pump subsidies become more prevalent, these savings could potentially escalate, especially if the price ratio between fuel and electricity remains as high as it currently stands. For a clearer understanding of the potential savings in your specific state, we recommend consulting the fact sheets provided by the Department of Energy.
The most substantial savings are likely to be enjoyed by individuals who currently rely on “delivered” fuels like propane and oil—similar to the early adopters in New England—as well as those who currently heat their homes using electric furnaces or electric baseboard radiators.
A notable example is Jerome Edgington, a skilled machinist hailing from Leicester, Massachusetts, who installed a mini-split system in his residence towards the end of 2019. In each of the coldest months, he consistently saves an average of around $150 compared to the expenses he would have incurred by utilizing his oil boiler. Considering the persistently high oil prices, which have reached historical peaks since March 2022, Edgington estimates potential monthly savings of approximately $550. Since natural gas is not available in his area and installing a ground-source heat pump would have been exorbitant and complex, he regarded air-source heat pumps as the sole viable alternative.
In terms of cost, using natural gas for heating generally proves to be more economical compared to utilizing a heat pump. Lis explains that as heat pumps continue to improve in efficiency, the cost disparity is gradually shrinking. He emphasizes that there’s not a significant gap between the two options. However, on an individual level, the cost outcome will depend on various factors such as the regional climate, whether the winter is harsh or mild, the specific HVAC equipment used, the quality of installation, and utility prices.
A case study involving Dave Adams, a pastor and home renovator from Fort Wayne, Indiana, further illustrates this point. In 2020, Adams installed a ducted heat pump with excellent specifications for cold climates to replace a broken air conditioner and an aging gas furnace. According to his video, he experienced an increase of approximately $270 in his heating costs compared to the previous year, although a significant portion of that increase occurred during an unusually cold month. However, in terms of cooling, the heat pump demonstrated significantly higher efficiency than his old air conditioner, resulting in his family nearly breaking even on the overall heating and cooling costs during the first year of ownership. Adams details this in a series of YouTube videos, including the one linked.
Now let’s turn our attention to the installation expenses. Cold-climate heat pumps generally carry a higher price tag for both purchase and installation compared to other HVAC equipment, often exceeding the combined cost of a reliable furnace and central air conditioner. However, it’s important to note that exceptions abound, as is the case with most aspects of HVAC systems.
Additionally, it’s important to consider the cost savings associated with choosing a heat pump that doesn’t experience frequent breakdowns. In pursuit of this goal, Consumer Reports (CR) has compiled anticipated reliability and owner satisfaction ratings for 24 leading brands of ducted heat pumps. These ratings are based on extensive data collected from thousands of heat pumps in actual usage scenarios, gathered through member surveys since 2016. CR members have the opportunity to delve into these scores and explore the findings.
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