Photo by Paul Scheckel
Shortcomings of fossil-fuel heating continue to mount, including combustion safety, fuel price volatility, air quality, and climate pressures. Conventional fossil-fuel heating systems generate heat by fuel combustion, which transforms chemical energy into heat energy. They require on-site fuel storage and periodic fuel deliveries (unless you’re connected to a natural gas supply line), a chimney, a combustion air supply, a heat distribution system, and a reliable service technician.
Alternatively, electric resistance heating systems are clean and simple to use with a low installation cost. They transform the electric utility’s energy-generating fuels into heat for your home. Unfortunately, electricity is often (though not always) the most expensive energy source available.
Residential heat pumps are gaining in popularity as advances in technology increase efficiency and reduce system costs. Though heat pumps operate on electricity to heat air or water, they’re much more efficient than conventional electric heaters. Heat pumps don’t generate heat; they move it from one place to another. Because heat can be moved in and out of a building, a single appliance can be used for both heating and cooling your home.
This article provides a brief overview of the various types and operations of heat pumps that can heat and cool your home and provide domestic hot water, giving you more options to consider when it comes time to install your next home-conditioning system. Given space limitations, I’ve left many details out of this discussion. I encourage you to seek local recommendations and talk to several contractors about the systems they prefer and why. Regional weather conditions; available human and natural resources; contractor training and ability to service equipment; budget; the age of your home; and personal comfort needs will all influence your decision.
Heat pumps aren’t an option for my off-grid home; the amount of electricity they use will crash a battery bank in a single day during winter or summer. However, as a professional energy consultant, I’ve analyzed a number of heat pump installations and interacted with those who live with them. We’re still in the early-adoption phase of this technology in terms of installer training and user-friendly operation.
Heat pumps aren’t your grandparents’ heating system. Fossil fuel systems are often oversized, and they deliver more heat than is typically required, even during the worst weather conditions, which might only be a week or two out of the year. That means the system is oversized for the vast majority of weather conditions, a situation that negatively affects performance and our accustomed comforts. Heat pumps aren’t so forgiving; they require a careful determination of home heating and cooling loads, and an equally meticulous selection and setup of equipment to effectively and efficiently meet design needs.
Contractors with a history of installing fossil-fuel heating systems will need to shift into a new mindset to understand the nature of heat pump design, installation, and performance. These are critical pieces in getting the best performance from any heating or cooling system. Heat pumps are far more reactionary to load matching than fossil-fuel systems are. A poorly designed heat pump won’t meet the heating or cooling needs of the home, won’t perform well, and won’t reduce your energy costs. Most homeowners will need to learn how best to optimize operation for maximum comfort and efficiency. Heat pumps move heat much more slowly than electric or fossil-fueled space conditioning systems can generate heat. This different mechanical behavior requires a change in our behavior when interacting with the machines. A properly designed and professionally installed heat pump in an efficient home, coupled with a renewable supply of electricity, can provide substantial cost, comfort, and environmental benefits. It’ll pair well with a grid-connected solar electric power system to offset electric use in a net-metering situation.
How Heat Pumps Work
Heat pump operation relies on physical gas laws that translate into the refrigeration — or heat pump — cycle. When gas vapor is put under pressure, its volume decreases and the temperature increases. When allowed to expand, the opposite occurs. In a heat pump system, the gaseous refrigerant (typically Freon-based, but some new models use carbon dioxide) is pressurized by the compressor, and then moves through a condenser, where it cools and condenses into a liquid. The pressurized liquid refrigerant then expands, dropping its pressure and forcing it to cool dramatically. As the cold gas travels through the system’s evaporator coil, it absorbs heat from the zone it’s in, such as the inside of the refrigerator. Heat flows naturally “downhill” from hot to cold, from high energy to low energy. This is important to remember, because cold isn’t added, but rather heat is absorbed from the warmer zone. The warmed refrigerant moves again to the compressor, where the cycle repeats.
Types of Heat Pumps
Heat pumps that move heat between some combination of air, water, and earth are available. Technically, anything with a temperature above absolute zero (minus 460 degrees Fahrenheit) contains heat energy. However, there are practical and cost-effective limitations to extracting and harnessing useful amounts of heat energy. With less heat available in the reservoir zone, the heat pump has to work harder to maintain the controlled zone temperature, and its efficiency decreases.
“Ground-source,” “geothermal,” “earth-coupled,” or “geoexchange” often refer to the same heat pump system. “Geoexchange” is becoming a more commonly accepted term to distinguish this technology. This system uses the modest and consistent temperatures available at the sun-warmed surface of the earth (the uppermost 20 feet), rather than higher-temperature underground geothermal sources. Geoexchange takes advantage of the earth being a fairly constant and reliable source of heat at depths just below the soil frost level.
A heat exchange loop is required for energy transfer between a working fluid and the heat reservoir. Heat is transferred between the earth and the working fluid, and then the fluid is moved to the heat pump appliance, where the refrigeration cycle delivers the conditioned air to the house. In heating mode, heat is extracted from the ground and transferred to the house. In cooling mode, the reverse occurs. The extracted heat can also be used to heat water before being returned to the reservoir.
Illustrations by Carolyn Lang
Geoexchange systems offer reliable and predictable performance, integrating well into conventional heating and cooling distribution systems. However, they can be complex and relatively expensive to install. With increased complexity comes the potential for increased maintenance. Excavation is required to lay pipe, and deep wells or boreholes may need to be drilled, depending on the heat exchange approach used, which is in turn based on local conditions and available resources. In addition, pumping water is an energy-intensive activity. These systems are generally best suited for larger homes or commercial facilities with greater energy requirements.
The operating efficiency of a geoexchange heat pump is typically between two and four times greater than electric resistance heat, and on par with the most efficient air conditioners. However, take care when figuring energy use, as not all the energy-consuming components — such as electricity required for water pumping — are included in the efficiency rating.
Geoexchange heat pump systems can work two different ways:
Open-loop heat exchange systems take water from a pond or drilled well and pump it into the house, where heat is extracted from the water by the heat pump. The cooled water is then returned to the pond or well. This system has the advantage of being relatively simple, but the aquifer or reservoir needs to be large enough to serve the need, and be sufficiently replenished to maintain a temperature that maximizes system efficiency and doesn’t adversely unbalance the aquifer’s environment.
Illustration by U.S. Department of Energy Office of Science and Technical Information
Closed-loop heat exchange systems circulate the working fluid (either anti-freeze or the refrigerant used by the heat pump) in a continuous flow between the reservoir and the heat pump. The reservoir can be soil at a depth below the frost level, single or multiple boreholes, a drilled well, or a body of water.
Illustration by U.S. Department of Energy Office of Science and Technical Information
Air-to-air, or “air-source” heat pumps (ASHP) have been gaining popularity in the United States in recent years, having been proven efficient and reliable in Europe for quite some time. ASHPs are a two-part space-conditioning system with an outdoor unit containing the compressor, and an indoor unit that hangs on the wall or attaches to the ceiling to deliver the conditioned air. These are sometimes called “mini-split” or “ductless” heat pumps, because they’re physically and energetically small and have no ducted distribution system. Ducted mini-splits are also available; they extract heat from the air in one zone and move it to another, using refrigerant lines between the indoor and outdoor units. During the heating season, heat is extracted from outdoor air and pumped indoors. The reverse is true during cooling season. Technological advances make them suitable for heating efficient homes in cold climates, and able to extract heat at temperatures down to minus 15 degrees. However, colder temperatures reduce both efficiency and heat output. Many models have electric resistance backup to provide heat for when it’s too cold for effective heat pump operation.
Modern cold-climate ASHPs can be a good solution for small or efficient homes. They’re versatile in the sense that one outdoor unit can support one or more indoor units when carefully designed and sized. These systems are relatively affordable to install and easy for a homeowner to maintain. ASHPs can be considered zonal heating, meaning that one indoor unit delivers heat to only one or two rooms. If the building is well-insulated and air-sealed, it’ll be more efficient at holding the heat, making the heat pump more effective at heating more of the house. An ASHP isn’t the right choice for a drafty, old farmhouse. Its heated air delivery temperature is low compared with a conventional furnace. This means that an ASHP requires a long time to heat up a cold house. For this reason, temperature set-back of ASHPs isn’t recommended.
Illustration by Len Churchill and Energy Star
Heat pump water heaters (HPWH) extract heat from indoor air and move it to a water storage tank for a domestic hot water supply. A standard electric water heater is about 90 percent efficient, while a gas water heater might be 70 to 80 percent efficient. A HPWH is more than 200 percent efficient when in heat pump mode. This may sound implausible, because nothing in nature can be more than 100 percent efficient, but this is the benefit of moving, rather than generating, heat energy. As heat is removed from the area in which the HPWH resides (the basement, for example) and transferred to the water inside, the surrounding space cools off. The result is that efficiency will drop and the room may cool below comfort level. Because it takes longer to move heat than it does to generate it, HPWHs can be switched into electric resistance mode if speedy temperature recovery is desired. Note: Heat pump compressors can be loud, which can be an important consideration in some cases.
Air-to-water heat pumps (AWHP) are becoming more widely available. They extract heat from the outdoors and move it to a storage tank indoors. These large-capacity units can provide domestic hot water, as well as space heating, using conventional hot water baseboards or other radiant-heat distribution systems. However, the hot water delivery temperature is lower than a conventional boiler, requiring additional heat distribution to meet the heating load. Being a water-based system, AWHPs can’t provide cooling.
Pump It Up!
Heat pumps are becoming a more popular way to increase home comfort, reduce energy costs, and lower a home’s carbon footprint. They can be used for stand-alone space conditioning in a single room, to supplement conventional heating systems, or to provide total comfort in efficient homes. It’s important to understand the benefits and limitations of heat pumps; how to select the proper technology to meet your goals; how to properly size and install the system according to heat pump best practices; how to design the distribution system to work with lower delivery temperatures; and how to use your new space-conditioning equipment for maximum effectiveness and efficiency. Remember, efficiency comes first, so reduce the energy requirements of your home to maximize comfort and minimize cost.
What’s a Heat Pump
A heat pump is a mechanism that moves heat energy from one place to another. Heat is moved from a reservoir of abundance to a dumping zone. Imagine a heat pump as a sponge that absorbs spilled water (such as heat) from the counter, and then squeezes the water down the drain.
You already have a heat pump in your kitchen: the refrigerator. Inside the refrigerator is a reservoir of air that’s warmer than desired. The components making up the refrigerator’s internal heat pump system work together to remove heat from where it’s not needed or wanted (the reservoir of abundance) and squeeze it out of the refrigerator into your kitchen. When the kitchen gets too hot, another heat pump, the air conditioner, is called into action, moving heat energy from the house to the outdoors. But these examples of heat pumps can only move heat in one direction. Modern space-conditioning heat pumps are bidirectional, able to provide either heating or cooling as needed.
Heat Pumps in Cold Climates
As an energy efficiency professional, I support new, super-efficient homes that are designed to be heated with a mini-split air-source heat pump (ASHP). When the oil furnace in my 60-year-old ranch died, I installed a ductless mini-split. Since then, I’ve focused on helping people understand what’s possible with heat pumps in old homes. Heating with heat pumps is drastically different from heating with a centralized fossil-fuel system. If your family isn’t ready to embrace that difference and learn to live with the quirks of a heat pump, wait until there are systems that deliver a more familiar heating experience. Here are the main points I highlight when talking about heat pumps with clients.
Efficiency and operation cost. Modern heat pumps are efficient, but the total energy cost savings depends on what the alternatives are. If you have low-cost electricity or high-cost fuel, heat pumps are a winner. To maximize efficiency and savings, the equipment you choose needs to suit your personal situation, and the system design has to work with the strengths and weaknesses of heat pumps.
Photo by Paul Scheckel
Comfort. Heat pumps slowly produce heat with a relatively low delivery temperature. A boiler might send 200-degree-Fahrenheit water around your house, whereas an air-to-water heat pump (AWHP) delivers 100-degree water. Older homes aren’t designed for low-temperature heating and will need air sealing and insulation improvements. Even with these improvements, you’ll never have a hot spot where you can warm your hands after an afternoon of sledding.
Operation. Installing a new heat pump includes adjusting your routines. When making the change, people have the hardest time remembering to not set the temperature back and to leave the bedroom doors open for adequate room-to-room comfort. In our house, we have a whole routine to prep for super-cold temperatures (below minus 10 degrees). People who don’t want to be that involved in their heating system probably won’t be happy with a heat pump.
Design. Because heat pumps are energy-light devices, the design margins are small; you have to get it right, or the occupants will be unhappy. Don’t oversize the system. Select efficient, ancillary components, such as pumps for geoexchange and AWHPs. Locate ductless indoor units where they won’t blow directly on people. Consider localized sound impacts. Visit homes that use heat pumps prior to installing your own system.
Paul Scheckel is the author of The Homeowner’s Energy Handbook. He’s a hands-on, off-grid homesteader, and a Vermont-based efficiency and renewable energy consultant with Parsec Energy Consulting. Learn more about his work at Parsec Energy.