How Heat Pumps Move Heat Efficiently

A well-designed heat pump can deliver three to four units of heat energy into a home for every single unit of electricity it consumes, a ratio that seems to defy basic energy accounting until you understand what is actually happening inside the machine. It is not creating heat. It is moving heat that already exists in the outdoor air, ground, or water into the building, the same underlying principle a refrigerator uses to keep food cold, just run in a direction that warms a space instead of cooling one. That single distinction, moving heat versus manufacturing it, is the reason heat pumps have become the centerpiece of building decarbonization policy from the U.S. Department of Energy to the European Union's REPowerEU plan.

Why "moving heat" beats "making heat" on efficiency

A traditional electric resistance heater, the kind found in a baseboard unit or a portable space heater, converts electricity directly into heat through a resistive element, the way a toaster's coils glow when current passes through them. By the laws of thermodynamics, specifically the first law of thermodynamics governing conservation of energy, a resistance heater can never produce more heat energy than the electrical energy it consumes. At best it approaches 100 percent efficiency, meaning one unit of heat output for one unit of electrical input, and no amount of engineering can push it meaningfully past that ceiling.

A heat pump does something categorically different. It uses electricity to run a compressor that moves existing heat from outside air, from underground soil, or from a body of water into a building, rather than generating that heat from the electrical current itself. The electricity is spent on the labor of concentrating and relocating heat, not on producing it, and that is what allows the delivered heat output to exceed the electrical input several times over.

This is not a violation of physics. It is closer to how a pump moves water uphill: the pump does not create the water, it applies energy to move an existing resource against a gradient. A heat pump applies electrical energy to move thermal energy against a temperature gradient, from a cooler outdoor environment into a warmer indoor one, and the "coefficient of performance" describes exactly how much heat gets moved per unit of electricity spent doing the moving.

Even outdoor air well below freezing still contains extractable thermal energy, because "cold" by everyday human standards is still enormously far above absolute zero, the theoretical temperature at which all molecular motion stops (-273.15 degrees Celsius, or -459.67 degrees Fahrenheit). Air at -10 degrees Celsius (14 degrees Fahrenheit) still holds substantial thermal energy in the random motion of its molecules; a heat pump's job is to concentrate and transfer a usable fraction of that existing energy indoors, not to manufacture new heat from nothing. That is the physical basis for why a heat pump can move several times more heat energy than the electrical energy it consumes running the process.

The U.S. Department of Energy states that heat pumps can reduce electricity use for heating by roughly 50 percent compared to electric resistance heating, based on typical efficiency ratings that put heat pump performance at a coefficient of performance (COP) of around 3 to 4 under moderate outdoor conditions (U.S. Department of Energy, Energy Saver program).

The refrigerant cycle that makes this possible

A heat pump relies on a refrigerant, a chemical compound engineered to evaporate and condense at conveniently low, controllable temperatures and pressures. The cycle runs through four core stages, and every one of them matters to the physics of why the system works.

First, the refrigerant, in its cold liquid state, flows through an outdoor coil and absorbs heat from the outdoor air, ground loop, or water source, causing it to evaporate into a low-pressure gas. Second, a compressor pulls in that low-pressure gas and mechanically squeezes it, raising both its pressure and its temperature significantly in the process. Third, the now-hot, high-pressure gas flows through an indoor coil, where it releases its stored heat into the building's air or water loop and condenses back into a liquid as it cools. Fourth, an expansion valve drops the liquid's pressure again, cooling it back down so it can flow back outdoors and absorb more ambient heat, repeating the cycle continuously as long as the system runs.

The compressor stage is the only part of this cycle that consumes significant electricity, and the physics behind it is intuitive if you have ever pumped up a bicycle tire. Compressing a gas raises its temperature, a relationship described by the ideal gas law and felt directly as heat in the pump barrel and valve stem after a few dozen strokes. Run through a continuously repeating refrigerant loop, that same compression principle is what lets a heat pump deliver far more heat energy indoors than the electrical energy spent running the compressor motor.

Refrigerant chemistry itself has been shifting in recent years for environmental reasons unrelated to heating performance. R-410A, the refrigerant that dominated residential heat pumps and air conditioners for two decades, carries a global warming potential (GWP) of roughly 2,088, meaning a kilogram of it leaked into the atmosphere traps roughly 2,088 times more heat over a 100-year period than a kilogram of carbon dioxide. Under the American Innovation and Manufacturing Act, the U.S. Environmental Protection Agency required manufacturers to transition to refrigerants with a GWP of 700 or below starting January 1, 2025 (U.S. EPA, AIM Act implementation). R-32, one of the leading replacements, carries a GWP of roughly 675, about 30 percent of R-410A's climate impact, while newer blends such as R-454B push that figure down to roughly 466. None of this changes the underlying compression-cycle physics; it changes what fluid is doing the compressing and how much damage a leak does if it escapes.

How does the same system work for both heating and cooling?

A reversing valve lets the refrigerant cycle run in either direction. In cooling mode, a heat pump absorbs heat from inside the building and releases it outdoors, functioning exactly like a standard air conditioner or a refrigerator venting heat out its back coils. In heating mode, the same hardware runs the identical cycle in reverse, absorbing outdoor heat and releasing it indoors, which is why a heat pump can replace both a furnace and an air conditioner with one piece of equipment.

Comparing heat pump types: air, ground, and water source

Not all heat pumps draw their heat from the same source, and the source matters enormously for both efficiency and installation cost. Air-source heat pumps, the most common residential type, extract heat from outdoor air using an outdoor unit similar in appearance to a central air conditioner's condenser. Ground-source (geothermal) heat pumps circulate fluid through buried pipes to exchange heat with soil that stays at a relatively stable temperature year-round, several meters below the surface, insulated from swings in air temperature. Water-source heat pumps exchange heat with a pond, lake, or well, offering some of the same temperature stability as ground-source systems where a suitable water body is available.

The core physical advantage of ground-source and water-source systems is that soil and water several meters down stay far more thermally stable across the seasons than outdoor air does. Air temperature can swing 30 degrees Celsius (54 degrees Fahrenheit) or more between summer and a cold winter night, while soil temperature at typical loop depth barely moves. A heat pump exchanging with a stable heat source does not have to work as hard to maintain its output during the coldest part of winter, which is reflected directly in its coefficient of performance.

Heat pump type	Typical COP (heating)	Typical installed cost (USD, single-family home)	Best suited for
Air-source	2.5-4.0, declining in extreme cold	$12,000-$20,000 (central ducted); $3,500-$6,000 per zone (ductless mini-split)	Most climates, retrofits, smaller lots
Ground-source (geothermal)	3.5-4.5, largely stable year-round	$15,000-$40,000+ (including ground loop excavation or drilling)	Larger properties, new construction, severe cold climates
Water-source	3.0-4.5, stable where a water body is accessible	$2,500-$6,000 for the loop portion alone, if a suitable pond or lake exists nearby	Rural or waterfront properties with pond or lake access

Ground-source systems post the most stable year-round COP because underground temperature barely fluctuates, but that stability costs two to three times more to install than an air-source system, a gap that has not closed even as air-source cold-climate performance has improved (EnergySage, 2025; Filterbuy, 2025).

The efficiency gap between these three types has narrowed over the past decade as air-source technology has improved, but the installation cost gap has not narrowed at anywhere near the same pace. That tradeoff, lower upfront cost against somewhat lower and more temperature-sensitive efficiency, is the central decision homeowners weigh when choosing between air-source and ground-source systems.

Why heat pumps lose efficiency in very cold climates

A heat pump's coefficient of performance (COP), the ratio of heat energy delivered to electrical energy consumed, drops as outdoor temperature falls. Two things happen simultaneously as it gets colder: there is less extractable thermal energy in the available air molecules, and the compressor has to work harder to achieve the same "temperature lift," the gap between the cold outdoor source temperature and the warm indoor delivery temperature.

Older heat pump models saw meaningful performance drop-offs below roughly -15 degrees Celsius (5 degrees Fahrenheit), a limitation that for decades meant contractors in genuinely cold regions like Minnesota, Quebec, or the Nordic countries defaulted to gas furnaces or paired heat pumps with backup resistance heating strips for the coldest days of the year. That reputation, "heat pumps don't work in the cold," is rooted in real historical performance data, but it describes older equipment rather than the current generation of hardware.

Newer cold-climate heat pump models, using variable-speed compressors, improved refrigerant chemistry, and enhanced vapor injection techniques that boost refrigerant flow during high-lift conditions, have significantly extended the temperature range where heat pumps remain useful. The U.S. Department of Energy's Cold Climate Heat Pump Challenge, which tested prototype systems at 23 field sites across ten U.S. states and two Canadian provinces, found units successfully operating at temperatures as low as -26 degrees Celsius (-15 degrees Fahrenheit) while still meeting minimum efficiency targets, and by late 2024 into 2025, eight manufacturers, including Bosch, Carrier, Daikin, Johnson Controls, Lennox, Midea, Rheem, and Trane, had passed the program's field inspections with units entering the commercial market (U.S. Department of Energy, Cold Climate Heat Pump Challenge). ENERGY STAR's cold-climate certification requirements specify a minimum COP of 1.75 at 5 degrees Fahrenheit (-15 degrees Celsius) and at least 70 percent of rated heating capacity retained at that same low temperature, compared to standard 47-degree-Fahrenheit (8-degree-Celsius) rating conditions (ENERGY STAR, 2025 Most Efficient specification).

What COP can I actually expect on the coldest day of the year?

Field data from the DOE's Cold Climate Heat Pump Challenge showed prototype units delivering a COP of roughly 2.1 to 2.4 in single-digit Fahrenheit weather, meaning the system still moved two to two-and-a-half times more heat energy than the electricity it consumed, even under genuinely severe conditions, compared to a COP that used to fall close to 1.0, essentially no better than resistance heating, for older heat pump designs at the same temperature (U.S. Department of Energy, PNNL Cold Climate Heat Pump Challenge field results).

Common misconceptions about heat pumps

A frequent misunderstanding is assuming heat pumps "generate" heat the way a furnace or electric heater does, just more efficiently packaged. In reality, the efficiency advantage comes specifically from moving existing ambient heat rather than converting electricity into heat directly, which is the entire physical reason heat pumps can exceed 100 percent apparent efficiency when measured against electrical input alone. A gas furnace can approach, but never exceed, 100 percent thermal efficiency because it is bound by combustion chemistry; a heat pump is not making that comparison on the same terms at all, because it is not combusting or resisting anything to produce its heat.

Another common misconception, and probably the most consequential one for adoption in cold regions, is that heat pumps stop working entirely once winter arrives. Modern cold-climate heat pump models can still extract usable heat from air well below freezing, though their efficiency does decline as temperatures drop, meaning performance varies by climate zone and specific model rather than heat pumps universally failing in winter conditions. A poorly chosen standard-efficiency unit installed in a severe-cold climate will genuinely underperform; a properly specified cold-climate unit, sized and selected for the local design temperature, generally will not.

A third misconception treats geothermal (ground-source) heat pumps as strictly superior to air-source units in every case. Ground-source systems do offer more stable year-round COP, typically in the 3.5 to 4.5 range regardless of the weather outside, because underground temperatures do not swing with the seasons the way air temperature does. But that stability comes at a real financial cost: ground-source installation, including the excavation or drilling required for the underground loop, commonly runs $15,000 to $40,000 or more, roughly two to three times the cost of a comparable air-source system. For many homeowners, especially in moderate climates or on smaller lots where a ground loop is impractical, a modern cold-climate air-source unit delivers most of the efficiency benefit at a fraction of the installed cost.

A fourth, more technical misconception conflates the refrigerant transition (moving from R-410A to lower-GWP options like R-32) with a change in how heat pumps fundamentally work. The compression cycle described above is unchanged; what has changed is the specific fluid doing the compressing and its climate impact if it leaks, a regulatory and environmental question distinct from the thermodynamics of heat pumping itself.

What this means for home heating and building decarbonization

The efficiency argument for heat pumps connects directly to the broader push to electrify building heating as electricity grids add more renewable generation. A gas furnace's emissions are fixed by the chemistry of burning natural gas, roughly one ton of carbon dioxide for every 53 cubic meters (about 1,870 cubic feet) of gas burned, regardless of where or when it is installed. A heat pump's emissions, by contrast, are entirely a function of how the electricity powering it was generated; the same physical unit gets cleaner every year a grid adds wind, solar, or nuclear capacity, without the homeowner replacing any equipment. Combined with a COP of 3 to 4, meaning three to four times less electricity is needed per unit of delivered heat compared to a hypothetical all-resistance electric heating system, this is why heat pumps sit at the center of most national building-decarbonization strategies, including the U.S. Inflation Reduction Act's heat pump tax credits and the EU's phase-down of new fossil-fuel boiler installations.

The economics also connect back to how buildings retain or lose the heat a pump delivers. A heat pump moving three to four units of heat into a poorly insulated home is still fighting a losing battle against rapid heat loss, the same reason a well-sealed building envelope matters as much as the heating equipment itself. That interaction between heat generation efficiency and heat retention is closely related to the physics discussed in why cities create heat islands, where surface materials and building density determine how much heat a structure gains or loses independent of any mechanical system.

The takeaway

Heat pumps achieve their efficiency advantage by moving heat rather than generating it, using a refrigerant cycle where a compressor concentrates existing ambient thermal energy from outdoor air, ground, or water and delivers it indoors. That distinction, moving heat versus making it, is exactly why heat pumps can deliver three to four units of heat energy per unit of electricity consumed under moderate conditions, a ratio no resistance heater or combustion-based furnace can physically achieve. Cold-climate models have narrowed, though not eliminated, the old performance gap in severe winters, and the remaining choice between air-source, ground-source, and water-source systems now comes down mostly to upfront cost against long-term efficiency stability rather than whether heat pumps "work" in the cold at all.

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Read next:

• How green hydrogen is made
• Why cities create heat islands
• Why desalination uses so much energy

FAQ

Q: How can a heat pump produce more heat energy than the electricity it uses?

It does not create heat from the electricity directly; it uses electricity to run a compressor that moves existing ambient heat from outdoor air, ground, or water indoors, which is why its output can exceed its electrical input when measured as apparent efficiency.

Q: What is a coefficient of performance (COP) and why does it matter?

COP measures the ratio of heat energy delivered to electrical energy consumed, typically around 3 to 4 for heat pumps under moderate conditions according to the U.S. Department of Energy, and it declines as outdoor temperatures drop since less extractable heat is available in colder air and the compressor must work harder.

Q: Do heat pumps stop working in very cold winter climates?

Not entirely. Older models saw significant performance drop-offs below roughly -15 degrees Celsius (5 degrees Fahrenheit), but newer cold-climate heat pump models, tested by the U.S. Department of Energy down to -26 degrees Celsius (-15 degrees Fahrenheit), maintain meaningfully higher COP and heating capacity at those same low temperatures.

Q: Can the same heat pump both heat and cool a building?

Yes. A reversing valve allows the refrigerant cycle to run in either direction, so the same equipment absorbs heat from indoors and releases it outdoors for cooling, or absorbs outdoor heat and releases it indoors for heating.

Q: What part of a heat pump actually uses the electricity?

The compressor is the main electricity-consuming component, since it raises the refrigerant gas's pressure and temperature, the step that concentrates the collected ambient heat enough to release it usefully indoors.

Q: Is a ground-source (geothermal) heat pump always better than an air-source one?

Not necessarily. Ground-source systems typically hold a more stable COP of 3.5 to 4.5 year-round because underground temperatures barely fluctuate, but they commonly cost $15,000 to $40,000 or more to install versus $12,000 to $20,000 for a comparable air-source system, so the better choice depends on climate, budget, and available land for the ground loop.

Q: Why are heat pump refrigerants changing from R-410A to R-32?

R-410A has a global warming potential of roughly 2,088, meaning it traps far more heat if leaked into the atmosphere than carbon dioxide does; U.S. EPA rules under the American Innovation and Manufacturing Act required manufacturers to shift to refrigerants with a GWP of 700 or below starting January 1, 2025, and R-32, with a GWP of roughly 675, is one of the leading replacements.