This article appeared in the Summer 2023 issue of This Old House Magazine.
Thinking of adding or replacing an AC system? Maybe you want to take advantage of government rebates and tax credits available for installing energy-efficient HVAC equipment powered by electricity. The good news: In both cases, there’s never been a better time to consider heat pumps, especially easy-to-install air-source heat pumps that deliver conditioned air to your home.
While the basic technology has been around for decades, these heat pumps have evolved significantly, delivering heating even in cold climates up to 50 percent more efficiently than other conventional types of heating systems, and cooling with the same efficiency as new air-conditioning equipment. That’s because rather than creating energy, a heat pump transfers energy, which is essential for understanding heat pumps. In summer, it works just like an air conditioner, pulling warmth out of the indoor air and dumping it outside.
In winter, it does the opposite, gathering heat outdoors—even on an ice-cold day—and bringing it inside. Because the system is powered by electricity, it’s cleaner and greener than home-based combustion systems that rely on oil, natural gas, or propane. And if you generate your own electricity with rooftop solar panels, it can make your heating and cooling nearly carbon neutral.
Many cold-weather air-source heat pumps now reliably work down to -15 degrees F, and there are water-source systems that supply heating, cooling, and domestic hot water. Government incentives can lower the cost of upgrading with heat pumps considerably, too, thanks to last year’s Inflation Reduction Act (for more details, see Home Finances, page 44). Here’s what you need to know in order to make the switch to a heat-pump-based HVAC system.
Inside the Cooling Cycle
When a forced-air system powered by a heat pump is in AC mode, heat from indoor air transfers to a coil of cold refrigerant in the air handler. The blower pushes out that cool air, which moves through the ductwork and is delivered via supply registers (return registers cycle indoor air back into the system). That refrigerant is compressed in the heat pump, concentrating the heat so that it becomes hotter than the outdoor temperature; that heat then moves to the less hot outdoor air. The whole system reverses in winter, so warm refrigerant heats up interior air, then gets expanded, dropping its temperature so low that it’s colder than the outside air; it picks up heat outside, which gets compressed and becomes warmer than the inside air, and the warmth transfers into the cooler home.
Types of Heat Pumps
Heat pumps are defined by their heat source and transfer medium. Here’s an overview of how air-source systems work, as well as the fundamentals of ground-source technology
Air-to-air heat pumps
Air-to-air heat pumps are the most affordable type and easiest to retrofit into a house. While they have been in use for decades in milder climates, new technologies within just the last five years make them a viable option even in the Snowbelt. No longer limited to locales with winter lows above 40 degrees F, today’s cold-climate heat pumps can heat a home efficiently when the outdoor temperature dips as low as −15 degrees F. Here are the main components of an air-to-air heat-pump system:
Closed refrigerant loop (A)
This liquid has a low boiling point, absorbing heat when it boils, or evaporates. The refrigerant travels in a closed circuit of pipes, absorbing heat from the house’s interior (for cooling) or exterior (for heating), expelling it on the other side. In AC mode, the indoor coil acts as the evaporator; in heating mode, it’s the outdoor coil.
Compressor (B)
This super-pressurizes refrigerant, concentrating the heat absorbed into it and boiling it into a gas. In
cooling mode, the refrigerant becomes hotter than outside air and the heat is released outside. In heating mode, it becomes hotter than the inside air and is used to warm the house.
Outdoor coil (C)
The refrigerant passes through a coil with a fan blowing over it. In cooling mode, the hot, pressurized refrigerant expels heat outside. In heating mode, it is depressurized, becoming colder than outdoor winter temperatures and picking up heat outside.
Expansion valve (D)
Once refrigerant has discharged its heat, the expansion valve releases the pressure, converts it back into liquid form, and drops the temperature so drastically that it can once again absorb heat—from the indoor air in summer and the outdoor air in winter.
Reversing valve (E)
This changes the direction of the refrigerant flow so it can heat as well as cool. Picture something akin to a window AC unit installed backward so it becomes a heater.
Indoor coil (F)
Refrigerant passes through a coil in the air handler, where a fan blows indoor air across it. In heating mode, warm refrigerant heats the indoor air. In cooling mode, the cold refrigerant absorbs heat from indoor air (and also causes moisture to condense out of it).
Air-to-Water Systems
In an air-to-water system, the source is the same, but heating and cooling are delivered through water. That’s a bit more efficient because water is a better medium for moving heat, says Ross Trethewey, TOH home technology expert. It also means you can retrofit a heat pump into a house with a hydronic heating system. A heat pump can also feed hydro-air systems (which use warm or cold water instead of refrigerant in the air handlers) and indirect domestic hot water tanks or tankless water heaters. In the summer, the heat pump reverses back and forth as needed to provide both domestic hot water and whole-house cooling.
“You’re getting three for one,” says Ross. “A single system is providing heating, cooling, and domestic hot water.”
Common in Europe, air-to-water heat pumps are still unusual here—but that’s going to change, says hydronics engineer John Siegenthaler, who has been designing air-to-water systems in upstate New York for about 10 years. For now, it can be difficult to find contractors familiar with the systems; you’ll likely pay about $15,000 to $20,000 for one if the house has existing compatible heating, cooling, and domestic hot water systems—maybe $30,000 if those need upgrading, or for a complete system in a new house. These are the main components of an air-to-water system that differ from an air-to-air heat pump system:
Closed water loop
Copper, cross-linked polyethylene (PEX), or polyethylene pipes containing water—mixed with antifreeze in cold climates—carry heat into the house in winter and out of the house in summer.
Heat exchanger
This part of the heat pump transfers warmth from the refrigerant to the cool refrigerant to the water loop in winter, and the other way in summer. A stack of stainless steel plates has refrigerant traveling in the gap between two plates water between the next two, and so on. While the two fluids never mix, heat moves through the metal from the hot refrigerant to the warm water in winter (and vice versa in summer). The heat exchanger also acts as the condenser, cooling refrigerant vapor back into a liquid state.
Buffer tank
This holds warm or cooled water at the ready so the heat pump doesn’t need to cycle on and off as often—and so the HVAC and water systems never need to wait for the heat pump. In some cases, the buffer tank doubles as a domestic water preheater, with a coil containing domestic water running through it; the warmed water then goes through an electric tank-style or tankless water heater when hot water is needed.
What About Geothermal?
Deep underground, the earth is a steady 50 to 60 degrees F all year round, providing a reliable heat source and heat sink. Ground-source, or geothermal, systems take advantage of this in a number of ways: Usually a closed loop of antifreeze and water travels down into the ground 100 to 400 feet or more, or horizontally across a large property 4 to 6 feet below the surface, or at least 8 feet below the surface of a pond or lake. A heat exchanger moves the heat between that transfer fluid and a loop of refrigerant. Geothermal is even more efficient than an air-source system since there is no outdoor fan unit, but installation is costly—$13,000 to $36,000, depending on the system, site conditions, and available land—and it can disrupt the landscape significantly. About 50,000 geothermal systems are installed in the United States each year. By comparison, an estimated 12 million air-source heat pumps will be installed in the United States by 2030
Things to Keep in Mind
While heat pumps have many benefits, there are some fine points to consider. When making the decision of whether to upgrade to a heat pump or not, its important to know the differences between a heat pump and a furnace.
You might want a backup plan.
If you retain your original heating system, you have redundancy in case of a problem with the electrical grid or the equipment itself. Plus, you can hedge your energy bets—if electricity rates shoot up, you can fire up the old natural gas furnace, for instance. Removing an old oil tank or radiators (and patching wood floors where they were disconnected) are examples of added costs that can result. On the other hand, if you don’t keep your old heating equipment, you have only one HVAC system to maintain and to replace when it reaches the end of its useful life. In that case, a generator or whole-house battery tied to rooftop solar panels could keep you covered in case of a blackout.
Heat pumps respond slowly.
A natural gas or oil-burning furnace heats up quickly, providing heat on demand. Not so with a heat pump. Heat pumps consume less energy in part because they operate constantly, at a lower level. That means their ramp-up time is slower. “It can take some time for a heat pump to get a house up or down to temperatures, but they are very good at keeping it there,” says Ross.
Defrost mode requires a secondary heat source.
When the ambient outdoor temperature reaches around 32 degrees F and the heat pump has been heating for a long period, the outdoor coil may start to ice up. To prevent the coil from turning into a solid block of ice, cold-climate heat pumps go into defrost mode, which temporarily stops heating the house to melt the ice off the coil. This defrost cycle can happen a couple of times an hour under heavy demand. To prevent the “cold-blow phenomenon,” in which the system actually releases cold air during defrost, your installer should use a backup heat source like electric duct heaters installed in the duct system, electric baseboards, or a traditional fossil-fuel-burning furnace or boiler.
Heat pumps can cause significant vibration.
This can occur throughout the house, both through the house’s framing (especially if, as with many older houses, the exterior walls are 2×4 construction) and through the piping. You can limit this problem by having the outdoor unit installed on the ground rather than attached to the side of the house.
You’ll need to clear any snow.
A heat pump needs plenty of airflow around and through it, or it won’t be able to operate. Even with a snow stand elevating it a foot or more off the ground, you’ll need to manually clear snow from the top if you want heat.
Choose the Right System
Your current HVAC setup is one factor in choosing a heat-pump system for your home.
Best for houses with ductwork
A ducted heat-pump system can connect to existing ductwork—whether full-size or high-velocity miniducts (return register, shown above)—delivering heating and cooling through the existing infrastructure, with little or no retrofitting needed. These systems send refrigerant to a large central air handler that delivers conditioned air through a network of ducts.
Best for houses without ducts
If there is no existing ductwork, a ductless heat-pump system (a.k.a. mini-split), may be simpler, more efficient, and less costly to install. This system pumps refrigerant to compact indoor units placed within individual areas of your home, providing room-by-room zoning.
Best for multiroom additions, finished basements, and attics
A short-run ducted system delivers heated and cooled air through ductwork, but only to a small portion of the house. This can be ideal for a multiroom addition, such as an ADU or in-law suite.
Best for affordability
In a packaged system, both heating and cooling coils are located in the outdoor unit, with a small duct delivering the warmed or cooled air inside. This is a bit less efficient than the more common split systems (where there’s an outdoor unit and an indoor unit) but is less costly upfront because installation is less labor-intensive.
Money Matters
Even though their most significant benefit comes in the heating season, heat pumps are really enhanced air conditioners and the most common time to install a heat pump is when it’s time for a new air-conditioning system. Whenever you do it, a heat pump can pay you back in multiple ways.
How much does a heat pump cost?
Air-to-air heat pumps typically cost $3,500 to $7,500 installed (not including adding or altering ductwork), but if you’re replacing your air-conditioning system anyway, opting for a heat pump might add only 3 to 10 percent ($250 to $800) to your total costs. Air-to-water heat pumps cost about 30 percent more because the technology isn’t as well established in the United States, and therefore you’ll be paying a premium for both the products and installers. In any case, you can get a big chunk of your investment back—so long as you opt for a high-efficiency system—through a variety of incentives. You can take 30 percent of the total cost, up to $2,000, as a tax credit. And the Inflation Reduction Act provides a state-issued rebate of up to $1,750 on heat-pump water heaters and up to $8,000 on heat-pump space heating and cooling for households earning less than 80 percent of the state’s median income (for more on these programs, see page 44). There are also likely local incentives: Take a look at the National Database of State Incentives for Renewables and Efficiency, at dsireusa.org, for state and utility rebates.
How much can it save on utility bills?
Air-source heat pumps are three to five times more efficient than natural-gas furnaces (for cooling, they offer the same efficiency options as new conventional air conditioners). So unless you live in a state with very high electricity rates (looking at you, New England) or very low natural-gas rates (hello, Idaho and Utah), you will see hefty savings on your heating bills. Of course, powering your system with rooftop solar could eliminate your heating-and-cooling power costs entirely—and if your existing heating system uses electricity, propane, or oil, a heat pump will slash your heating costs no matter your electrical rates.
How long will a heat pump last?
Like a new central AC system, the life expectancy for air-source heat pumps is 10 to 15 years (a bit shorter if you’re immediately on the coast, as salt air can cause parts to corrode)—and you can expect a 10-to-12-year warranty on parts; the labor warranty typically runs out after about 3 years. You can prolong your system’s life by having it serviced regularly, and some companies include no-cost labor on repairs with their service contract.
Recommended Technology
As with any complex household system, you’ll have lots of chances to spend more on extra features, from smartphone system controls to HEPA-level filtration. Here are the most important tech upgrades, according to Richard and Ross Trethewey.
Variable-speed, or inverter-driven, compressor
By operating at only the speed necessary for the conditions, these reduce energy use—and thereby prevents
the system from cycling on and off frequently, which helps maintain consistent temperatures, improves
dehumidification during the cooling season, and increases indoor air quality because the air filter is
engaged for longer periods of time. A good idea in any climate, an inverter is most needed in areas that
experience big swings in temperature between the seasons, with both freezing winters and scorching summers. Variable-speed systems also start and stop gradually, leading to quieter operation.
Enhanced vapor injection
This is the tech that allows cold-climate heat pumps to pull warmth out of ambient outdoor temperatures of −15 degrees F or colder. It’s a highly engineered device that pipes a portion of the refrigerant through a heat exchanger before mixing it back in at the compressor.
Dual-fuel, or hybrid, system
Rather than using electric resistance heaters to prevent the cold-blow phenomenon when outdoor temps hover around freezing, these systems use a more efficient natural gas furnace. “You could also just keep your old furnace—or possibly your boiler—and set up the two independent systems as a dual-fuel system with a single set of controls operating them,” says Ross.
High-efficiency system
A heat pump’s efficiency is determined by dividing the Btus of heat it moves by the electricity it uses—this produces a Seasonal Energy Efficiency Ratio (SEER) rating for cooling and a Heating Seasonal Performance Factor (HSPF) rating for heating. New in 2023, the minimum allowable SEER rating is 15 in the South and 14 in the North, but you can get systems rated as high as SEER 33. The range for HSPF is 8.8 to 14. You’ll pay more for higher-efficiency equipment, but that investment should pay for itself several times over during the life of the system. It may make your project eligible for bigger incentives, too, and, with rooftop solar panels, could even make it possible to power your house’s full electrical needs.
Consider Heat Pump Appliances
Not changing out your HVAC system? You can take advantage of heat-pump technology—and rebates—in other ways.
Heat-pump clothes dryers
A built-in air-to-air heat pump on the bottom draws heat out of the air in the room, concentrates it, and uses it to warm the air blowing over damp clothes. That air then passes through a second coil, containing the cooled refrigerant, which causes the moisture to condense; the resulting moisture gets pumped to a drain. There is no vent to the outside. These dryers recycle all the heat produced, resulting in at least 28 percent less energy used. Just be aware that they operate more slowly—in some cases, a load can take twice as long—and they require more frequent lint removal since it all stays in the machine. They cost about twice as much as conventional dryers.
Heat-pump water heaters
Available as both on-demand and tank-type domestic water heaters (the latter, shown at left), these house an air-to-water heat pump that draws heat out of the ambient air—typically warmed by a nearby furnace or other mechanicals in the utility area—concentrates it, and transfers it to the water. These heaters use up to 60 percent less energy than conventional heaters but cost about three times more. There are some limitations on where they can go, too: They require at least 1,000 cubic feet of space around them, and the ambient air temperature has to stay between 40 and 90 degrees F for the heat pump to work. That’s perfect for a basement utility room, and if the furnace and other equipment keep things toasty warm, it makes the water heater even more efficient.
Installation Best Practices
Hire an experienced pro
As with any home-improvement hire, get referrals and check references before hiring an installer. Any HVAC contractor can install a heat pump, but you want someone with years of experience with the type of heat pump you’re installing— air-to-water isn’t prevalent in the market yet, for instance—and the specific brand, too, since the specifications and especially the controls differ widely between products. Check manufacturers’ websites for certified installers in your area. Also, about 80,000 installers across the country use measureQuick, a network of automated sensors that checks your system right after installation, gauging the temperature and speed of the air and refrigerant flow, for example. The benefit: You get a diagnostic report in real-time, certifying the system was designed and installed properly.
Improve insulation and seal air leaks first
Adding insulation in the attic and at the sill above the foundation, weatherstripping doors and windows, and even blowing insulation into the walls is a good idea before any HVAC upgrade because it will literally shrink the size of the equipment you need. With a heat pump, it’s even more important. “Burning oil and natural gas gives you lots of heat really quickly,” says Ross. “Heat pumps work slowly and methodically, so if you have large drafts or your house doesn’t retain the heat, the system can struggle to keep you comfortable.” Plus, in cold climates, heating loads are a lot bigger than cooling loads. “The difference between indoor and outdoor temperatures on a hot day might be 20 or 30 degrees,” says Ross. “On a cold day, it can be 60 degrees or more.” So a heat pump that provides enough heat may be a bit oversize for its cooling needs; improving the house’s envelope shrinks that differential, allowing you to rightsize a system for both heating and cooling.
Inspect and repair ducts and any equipment you’re retaining
Older ducts may not have been sealed or insulated to today’s standards—or insulation may have degraded and ductwork rusted or separated at the seams. Your contractor should check and fix the ductwork before upgrading your system. Inaccessible runs can be misted with a liquid-rubber sealant from the inside. Keeping your boiler or furnace? Have it serviced and filters changed.
Run a heat-loss calculation
In the snowbelt, it’s the heat calculation that matters. But if winter, where you live, means long sleeves, not parkas, your contractor will size the system for your cooling needs instead. A heat pump that’s too small won’t get the house warm enough, and one that’s too big will cycle on and off so much it won’t be able to properly dehumidify the space and will have a shorter life span.