How a heat pump works: a complete guide to this environmentally-friendly heating technology

By David S. Heating and energy savings Jan. 9 2026 (update)

Heat pumps are one of the most efficient and environmentally-friendly heating solutions on the market today. As energy prices continue to rise and the energy transition becomes a priority, understanding how a heat pump works is essential for any homeowner wishing to optimize their heating system. In this comprehensive guide, we explain in detail how a heat pump works, its different types, its benefits and everything you need to know to make the right choice.

Contents

What is a heat pump?

A heat pump, often abbreviated PAC, is a heating system that captures energy naturally present in the environment (air, water or ground) and releases it inside a building in the form of heat. Unlike traditional heating systems, which generate heat by burning fuel, heat pumps simply transfer energy from one medium to another.

The principle is similar to that of a refrigerator, but in reverse: whereas a fridge extracts heat from its interior and rejects it outside, a heat pump captures heat from outside and distributes it inside the home. This technology produces more thermal energy than it consumes electrically, making it a particularly economical and environmentally-friendly solution.

The thermodynamic principle behind the heat pump

The refrigeration cycle in four stages

The operation of a heat pump is based on the thermodynamic cycle, also known as the refrigeration cycle. This process takes place in four main stages, which are continuously repeated:

1. Evaporation: The refrigerant circulating in the heat pump circuit absorbs calories from the cold source (air, water or ground). Even at sub-zero temperatures, the outside air contains thermal energy. When it comes into contact with the evaporator, the refrigerant, which evaporates at very low temperatures, changes from a liquid to a gas, capturing this heat.

2. Compression: The compressor, the heart of the heat pump, sucks in the gaseous refrigerant and compresses it. This compression considerably increases the pressure and temperature of the gas, which can reach 80 to 90°C. This is where electricity is consumed to run the compressor.

3. Condensation: The hot, high-pressure refrigerant passes through the condenser, where it transfers its heat to the home's heating system (underfloor heating, radiators or fan coil units). As the gas loses heat, it cools and condenses, becoming a liquid again.

4. The expansion valve lowers the pressure of the liquid refrigerant, causing its temperature to drop sharply. The cold fluid then returns to the evaporator to start a new cycle.

Coefficient of performance: the key to efficiency

The efficiency of a heat pump is measured by its coefficient of performance, or COP. This coefficient represents the ratio between thermal energy produced and electrical energy consumed. For example, a heat pump with a COP of 4 produces 4 kWh of heat for 1 kWh of electricity consumed.

COP varies according to outdoor conditions: the higher the temperature of the cold source, the higher the COP. That's why heat pumps are particularly efficient in temperate climates. Manufacturers usually quote a COP measured under standardized conditions, but it's important to understand that actual performance depends on operating conditions.

The different types of heat pumps

Air-to-air heat pumps

The air-to-air heat pump draws calories from the outside air and releases them directly into the indoor air via diffusion units. This is the simplest and least expensive system to install.

How it works: An outdoor unit captures energy from the ambient air, while one or more indoor units distribute the heat to the various rooms in the home. These indoor units can be wall-mounted, console-mounted or ductable, depending on requirements and housing configuration.

Advantages : Fast, non-invasive installation, affordable purchase price, summer cooling possible by reversing the cycle, simple maintenance.

Disadvantages : Reduced performance when outdoor temperatures drop below -5°C, air diffusion that can create a draughty sensation, not eligible for certain financial aid.

Air-to-water heat pumps

The air-to-water heat pump also captures energy from the outside air, but transfers it to a water circuit that supplies the central heating system (radiators, underfloor heating) and can also produce domestic hot water.

How it works: The outdoor unit recovers calories from the air, and the system transfers them to a hydraulic circuit that distributes the heat throughout the home. This solution can be connected to an existing heating system, facilitating energy renovation.

Advantages : Compatible with existing central heating systems, domestic hot water production possible, eligible for financial assistance (MaPrimeRénov', prime énergie), decent performance down to -10°C or even -15°C for high-temperature models.

Disadvantages : More complex installation than an air-to-air heat pump, higher investment cost, need for technical space for hydraulic module.

Geothermal heat pumps (ground-to-water)

A geothermal heat pump harnesses the heat contained in the ground or groundwater. As the underground temperature remains stable all year round (around 10 to 14°C), this system offers excellent, consistent performance.

How it works: Collectors buried in the ground (horizontally at a depth of 60-120 cm or vertically up to 100 meters) contain a heat-transfer fluid that recovers calories from the soil. This energy is then transferred to the home's heating circuit.

Benefits: high, stable COP all year round (between 4 and 5), long service life (over 20 years), optimum performance even in extreme cold, silent operation with no visible outdoor unit.

Disadvantages : Very high installation costs due to drilling and earthworks, need for a sufficiently large plot of land for horizontal collectors, prior geological study essential.

Water-to-water heat pumps

This system draws energy from groundwater, a well or a nearby stream. With groundwater maintaining a constant temperature of around 10°C all year round, this solution offers excellent performance.

Functioning: Two boreholes are required: a collection well to extract the water and a discharge well to return it after the calories have been extracted. The heat pump then transfers this energy to the heating system.

Benefits : Exceptional performance with a COP of up to 5, stable performance whatever the season, long system life.

: Very high cost due to the drilling required, administrative authorization required for water exploitation, risk of exchanger clogging, impossible in certain geographical areas.

The essential components of a heat pump

The compressor: the system's engine

The compressor is the heart of the heat pump. Its function is to draw in the low-pressure gaseous refrigerant and compress it to raise its temperature and pressure. It is the only component in the system that consumes electrical energy.

There are different types of compressor: piston, scroll and screw. The latest models feature inverter compressors that automatically adjust their rotation speed according to the home's thermal requirements, optimizing performance and reducing power consumption.

The evaporator: the energy collector

The evaporator is the heat exchanger where the refrigerant captures calories from the cold source (air, water or ground). In this element, the refrigerant evaporates, absorbing heat from the outside environment, even at negative temperatures.

Evaporator design is crucial to optimize heat exchange. In air-to-air or air-to-water heat pumps, the evaporator resembles a car radiator inside the outdoor unit, with fins that increase the surface area for exchange with the air.

The condenser: the heat diffuser

The condenser is the heat exchanger where the refrigerant transfers its heat to the home's heating system. In this element, hot gas under high pressure condenses back into liquid, releasing the accumulated thermal energy.

In an air-to-air heat pump, the condensers are integrated into the wall-mounted indoor units. For an air-to-water heat pump, the condenser transfers heat to the water in the central heating circuit.

The pressure reducer: the pressure regulator

The expansion valve regulates the flow and lowers the pressure of the refrigerant after it has passed through the condenser. This expansion causes a significant drop in temperature, preparing the fluid for a new evaporation cycle.

Modern systems use electronic expansion valves that automatically adapt to operating conditions, optimizing heat pump performance.

The refrigerant: the energy carrier

The refrigerant is the fluid that circulates in a closed circuit in the heat pump, transporting the thermal energy. Its special thermodynamic properties enable it to change state (liquid/gas) at temperatures suited to the system's operation.

Environmental regulations on refrigerants are constantly evolving, due to their impact on the ozone layer and the greenhouse effect. New heat pump models use less-polluting fluids such as R32 or R290 (natural propane).

Installing a heat pump: what you need to know

Preliminary thermal study

Before any installation, a thorough thermal study is essential to correctly size the heat pump. This study takes several parameters into account: the surface area of the home, the quality of insulation, the climate zone, the desired temperature and the type of existing heat emitters.

Correct sizing guarantees optimum performance and avoids over-consumption of electricity. An oversized heat pump will cycle on and off too frequently, while an undersized model will run continuously without being able to meet the home's thermal needs.

Choice of location

The location of the outdoor unit has a direct influence on performance and comfort. The best location is sheltered from wind and weather, away from bedrooms to limit noise pollution, and with good air circulation to optimize heat exchange.

For geothermal heat pumps, a site survey will determine the most suitable type of collector: horizontal if the available surface area is sufficient (1.5 to 2 times the area to be heated), or vertical if the site is limited.

Compatibility with existing heating systems

Low-temperature air-to-water heat pumps work ideally with underfloor heating or low-temperature radiators. If the home has high-temperature radiators, you'll need to opt for a high-temperature heat pump or consider replacing the emitters, which represents an additional cost but considerably improves the system's overall efficiency.

Administrative procedures

The installation of a heat pump generally requires prior declaration of works to the local council, particularly for the outdoor unit. For geothermal heat pumps with boreholes, a specific declaration or authorization may be required, depending on the depth.

If the property is located in a protected or classified area, or in a condominium, additional authorizations may be required.

Heat pump care and maintenance

Mandatory regular maintenance

Since 2020, heat pumps with an output of between 4 and 70 kW have had to be serviced every two years. This maintenance must be carried out by a qualified professional, who will check the refrigerant circuit for leaks, clean the various components, check pressures and temperatures, and ensure that the system is operating correctly overall.

Routine maintenance

Between professional visits, there are a few simple steps you can take to maintain the performance of your heat pump:

  • Clean indoor unit filters regularly (monthly for air-to-air models)
  • Clear the outdoor unit of any obstacles (leaves, branches, snow)
  • Check that supply and exhaust vents are not obstructed
  • Visually check for leaks or abnormal icing
  • Keep radiators and underfloor heating clean and uncluttered

Lifespan and renewal

A well-maintained heat pump can operate efficiently for 15 to 20 years. Geothermal heat pumps generally have a longer service life (over 25 years for collectors), thanks to the absence of an outdoor unit exposed to the elements.

After this time, even if the system is still running, its efficiency gradually decreases and power consumption increases. Replacing it with a newer model allows you to benefit from the latest technological innovations and significantly improve performance.

Economic and ecological benefits

Substantial energy savings

Thanks to their excellent coefficient of performance, heat pumps can considerably reduce heating bills. On average, a heat pump can save between 40% and 70% on heating costs compared with a traditional electric system, and up to 60% compared with an oil or gas boiler.

These savings vary according to the type of heat pump, the quality of the home's insulation, consumption habits and climate zone. A well-insulated home will maximize the benefits of the heat pump.

An ecological and renewable solution

Heat pumps use mostly free, renewable and inexhaustible energy present in the environment. Only the compressor requires electricity to operate. So, for every 1 kWh of electricity consumed, an efficient heat pump delivers 3 to 5 kWh of heat, 2 to 4 kWh of which comes directly from the environment.

CO2 emissions are therefore considerably lower than with fossil-fuel heating systems. With the development of renewable energies in the French electricity mix, the carbon impact of heat pumps continues to fall.

Financial assistance available

The installation of a heat pump is eligible for a number of financial assistance schemes that significantly reduce the initial investment:

  • MaPrimeRénov': government assistance, the amount of which varies according to household income and the type of heat pump installed.
  • Energy bonus (CEE) : paid by energy suppliers as part of the Energy Savings Certificate scheme.
  • L'éco-PTZ: zero-interest loan to finance work without interest
  • Reduced VAT at 5.5%: applicable to materials and labor
  • Local assistance: some local authorities offer additional subsidies

These grants are conditional on compliance with certain criteria: use of an RGE (Reconnu Garant de l'Environnement) installer, compliance with minimum performance levels, and sometimes completion of other insulation work.

Limits and points of attention

Dependence on climatic conditions

Aerothermal heat pumps (air-to-air and air-to-water) lose performance when outside temperatures get very cold. Below -5°C to -10°C, depending on the model, the COP drops, and the heat pump may require an electrical backup to maintain thermal comfort.

In regions with harsh winters, we recommend installing a supplementary heating system, or opting for a geothermal solution that is not affected by these performance variations.

Noise level

The outdoor unit of a heat pump generates a noise level that can vary from 45 to 65 dB(A) depending on the model and operating conditions. Manufacturers have made great strides in recent years with low-noise models, but careful placement of the outdoor unit is still essential to preserve the acoustic comfort of the neighborhood.

Initial investment

Despite financial assistance, installing a heat pump represents a substantial investment: 5,000 to 10,000 euros for an air-to-air heat pump, 10,000 to 15,000 euros for an air-to-water heat pump, and 15,000 to 25,000 euros for a geothermal heat pump.

The return on investment must be calculated over the long term, taking into account the energy savings achieved, the system's lifespan and any financial aid received. Return on investment is generally between 7 and 15 years, depending on the situation.

Conclusion

Today, heat pumps are a powerful, economical and environmentally-friendly heating solution. Its thermodynamic principle makes the most of the free energy in the air, ground or water to heat homes efficiently, while considerably reducing energy consumption and greenhouse gas emissions.

The choice of heat pump depends on a number of factors: the configuration of the home, the existing heating system, the climate zone, the available budget and energy performance targets. A thermal study carried out by a qualified professional will determine the most appropriate solution for each situation.

With coefficients of performance often in excess of 3, heat pumps produce far more energy than they consume, generating substantial savings on heating bills. The financial aid available significantly reduces the initial investment and accelerates the system's profitability.

Regular maintenance by a professional and a few simple everyday gestures guarantee optimum performance and maximum longevity of your system. By choosing a heat pump, you're opting for a proven technology that actively contributes to the energy transition, while improving your thermal comfort and preserving your purchasing power.

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David S.

Member Premium Plus of domeashop since 2019.

Our team of writers has been selected to provide you with quality articles to help you realize your projects. All information is systematically checked and referenced, so you can be sure of our editorial quality.

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Frequently asked questions

The domeashop store offers you a list of questions and answers corresponding to the most frequently asked questions related to the theme and products discussed in this article. If you can't find the answer you're looking for, contact us and our specialists will be happy to help.

Yes, a heat pump continues to operate even at sub-zero temperatures, because even cold air contains thermal energy. However, its performance gradually declines as the temperature drops. Recent models work well down to -15°C, and some even down to -20°C, but their COP decreases with cold temperatures. Below these temperatures, an electric booster can be triggered automatically to maintain comfort. Geothermal heat pumps don't have this problem, as the ground temperature remains stable all year round.

A heat pump's electricity consumption depends on a number of factors: its output, its COP, the surface area to be heated, the home's insulation and outside temperatures. On average, a properly sized heat pump for a 100 m² house consumes between 3,000 and 5,000 kWh per year. Thanks to the COP, for every 1 kWh of electricity consumed, the heat pump delivers 3 to 5 kWh of heat. Consumption is therefore considerably lower than with direct electric heating, which would require 12,000 to 15,000 kWh for the same surface area.

Yes, it's perfectly possible to install a heat pump in an older home, but it requires a thorough preliminary study. Building insulation is a decisive factor: the better the insulation, the more efficient the heat pump. For a poorly insulated home with high-temperature radiators, you'll need to opt for a high-temperature heat pump, or consider improving insulation and replacing the emitters. An air-to-air heat pump can be installed more easily without modifying the existing system, but will not be eligible for the same financial assistance as an air-to-water heat pump.

A reversible air-conditioning system is actually an air-to-air heat pump capable of operating in both directions: heating in winter and cooling in summer. The technical principle is identical, only the direction of the thermodynamic cycle changes. In heating mode, the system captures heat from the outside air and releases it indoors. In air-conditioning mode, it extracts heat from inside and rejects it outside. Most modern air-to-air heat pumps are reversible and offer this dual function, which represents a considerable advantage in terms of year-round comfort.

Compulsory maintenance of a heat pump generally costs between 150 and 300 euros every two years, i.e. around 75 to 150 euros per year. This fee includes a visit by a qualified professional to carry out a complete regulatory inspection, check the refrigerant circuit for leaks, clean the components and make any necessary adjustments. Some installers offer annual maintenance contracts that can include troubleshooting and replacement of wearing parts. These contracts cost between €150 and €400 a year, depending on the services included. Routine maintenance (cleaning filters, clearing out the outdoor unit) is free of charge and within everyone's reach.

Yes, some heat pumps can produce domestic hot water in addition to heating. Air-to-water heat pumps can be equipped with an integrated or connected thermodynamic storage tank, which uses the same principle to heat domestic hot water. There are also independent thermodynamic water heaters that operate on the same principle as a heat pump, but are dedicated solely to hot water production. This solution offers significant additional savings on energy bills, as domestic hot water accounts for an average of 15-20% of a household's energy consumption.

A well-maintained heat pump has an average lifespan of 15 to 20 years for air-to-air and air-to-water systems. Ground-source heat pumps can last even longer: up to 25 years for the indoor unit and over 40 years for the ground-source collectors. This longevity is highly dependent on the quality of the initial installation, compliance with maintenance recommendations and operating conditions. The compressor is generally the component that wears out the fastest, and may require replacement after 10 to 15 years of intensive operation. Inverter technologies, which adapt the compressor's rotation speed, help extend its life by avoiding frequent on-off cycles.

For a horizontal geothermal heat pump, you need a plot of land approximately 1.5 to 2 times larger than the area to be heated, i.e. 150 to 200 m² for a 100 m² house. The land must be unbuildable in the catchment area, free of trees and easily accessible for earthworks. For vertical water collection, the plot can be much smaller because the probes are buried at depth (up to 100 meters), but you need to make sure that drilling is authorized and that the nature of the subsoil allows it. A preliminary geological study is essential to determine the feasibility of the project and choose the type of catchment best suited to the characteristics of the land and subsoil.

Heat pump noise levels vary according to model and operating conditions. An outdoor unit typically generates between 45 and 65 dB(A), equivalent to the noise of a normal conversation at a distance of 1 meter. Manufacturers have considerably improved the acoustics of their appliances in recent years, and many models are now classified as "silent", with sound levels below 50 dB(A). The location of the outdoor unit is crucial: keep it away from bedrooms and property lines, avoid confined spaces that create resonance, and favor anti-vibration support. Geothermal heat pumps are virtually silent, as they have no outdoor unit.

Yes, combining a heat pump with photovoltaic solar panels is a particularly efficient and environmentally-friendly combination. Solar panels generate the electricity needed to run the heat pump's compressor, further reducing energy bills and environmental impact. This synergy is all the more relevant as solar production is at its peak in mid-season, when the heat pump operates at excellent efficiency. We recommend sizing the solar system according to the heat pump's annual consumption. With the addition of storage batteries, it's even possible to aim for total self-consumption and energy autonomy, making your home a true positive-energy house.