Today, several types of heat pumps are available on the market: electric ones, which operate through an electrically driven compressor, and absorption ones, which function by means of a burner powered by natural gas or LPG.
Let’s explore the distinguishing features of these two types of heat pumps.
Page Index
- The synergy between electricity and gas for building heating
- What a heat pump is
- How a heat pump is made and how it works
- Heat pump efficiency
- The cold source (from which heat is recovered)
- The hot sink (to which produced and recovered heat is delivered)
- Types of heat pumps
- Applications of heat pumps
- Choosing a heat pump
- Sizing a heat pump
- Installing a heat pump
- Regulatory framework for heat pumps
- Sources
The synergy between electricity and gas for building heating
When exploring the different types of heat pumps, it is crucial—given the long payback period—to understand the energy scenarios likely to unfold over the next 10 to 20 years.
There is growing consensus that the world we are rapidly moving toward (due in part to external factors such as the increasingly limited availability of imported energy) will not be powered by a single energy source like electricity alone, as is often assumed, but rather by a genuine mix of energy sources, including the so-called green gases—gases produced from renewable sources.
Further reading:
- The role of natural gas in the energy transition
- Green hydrogen: the number one vector of the energy transition
What is a heat pump
A heat pump is a device capable of transferring heat from a fluid at a lower temperature to another at a higher temperature.
The name “heat pump” comes from its ability to move heat from a lower to a higher temperature level, reversing the natural direction of heat flow, which in nature always moves from higher to lower temperatures.
The function of a heat pump can be compared to that of a water pump placed between two basins at different elevations: water naturally flows from the higher basin to the lower one. However, it is possible to bring the water back up using a pump that draws water from the lower basin and pushes it to the higher one.
Below is a summary of the main features of heat pumps, highlighting the differences between the two types currently available.
Gas or electric heat pump: brief summary of technical differences
The differences between a gas heat pump and an electric heat pump can be traced back to various technical and economic factors. Both systems use renewable energy from air, water or ground to produce heat, but they differ in power source and operating principle.
Let’s look in detail at the key differences:
| Feature | Gas heat pump | Electric heat pump |
|---|---|---|
| Power source | Natural gas or LPG | Electric energy |
| Operating principle | Uses an absorption cycle with a gas burner | Operates through compression and expansion of refrigerant |
| Energy efficiency | GUE (Gas Utilization Efficiency) ~1.5 | COP (Coefficient of Performance) ~3 |
| Performance in low temperatures | Maintains high efficiency even below −10°C | Efficiency drops below 0°C; auxiliary resistances may activate |
| Energy consumption | Mainly uses gas, reducing electrical demand | Fully dependent on the power grid |
| Ideal applications | Cold climates, buildings with limited electrical availability | Temperate climates, buildings with adequate electric power |
When to choose a gas heat pump
Gas heat pumps are especially recommended in areas with harsh winters, where temperatures often fall below zero. They are also an optimal solution for buildings with limited electric capacity, avoiding the need for costly electrical upgrades.
When to choose an electric heat pump
Electric heat pumps are a good choice in milder climates, where winter temperatures rarely drop below 0°C. They are particularly suited to well-insulated buildings with low-temperature heating systems such as underfloor heating.
Further technical details in the following sections will help determine which solution is best suited to your specific needs.
How a heat pump is made and how it works
The heat pump consists of a sealed circuit containing a special fluid (refrigerant) that changes between liquid and gas (vapour) states depending on temperature and pressure conditions.
The sealed circuit includes:
| Gas absorption heat pump | Electric heat pump |
|---|---|
| a generator an absorber |
a compressor |
| a condenser | a condenser |
| a set of restrictors | an expansion valve |
| an evaporator | an evaporator |
The condenser and evaporator are heat exchangers—special pipes externally in contact with the working fluids (which can be water or air)—inside which the refrigerant flows. When at a high temperature in the condenser, it releases heat to the water or air (high-temperature side), while at a low temperature in the evaporator (low-temperature side), it absorbs heat from the air or water.
During operation, the refrigerant within the circuit undergoes the following transformations:
| Gas absorption heat pump | Electric heat pump |
| Condensation: the refrigerant, coming from the generator, changes from gas to liquid, releasing heat to the external fluid (water or air). | Condensation: the refrigerant, coming from the compressor, changes from gas to liquid, releasing heat to the surroundings. |
| Expansion: by passing through restrictors (properly calibrated narrowings), the pressure and temperature of the liquid refrigerant drop. | Expansion: by passing through the expansion valve, the liquid refrigerant cools and partially evaporates. |
| Evaporation: the refrigerant absorbs heat from the external fluid (air or water) and completely evaporates back into gas. | Evaporation: the refrigerant absorbs heat and evaporates completely. |
| Absorber: the refrigerant is absorbed by the absorbent fluid, returning it to liquid form. Generator: the liquid solution of refrigerant and absorbent is heated in the generator by a gas burner, separating the refrigerant, which evaporates, increasing in temperature and pressure. |
Compression: the refrigerant in gas form and at low pressure, coming from the evaporator, is compressed to high pressure; during compression it heats up, absorbing energy. |
| This series of transformations forms the cycle of a gas heat pump: energy is supplied via the natural gas or LPG burner, the refrigerant absorbs heat from the external medium in the evaporator, and transfers it to the medium to be heated via the condenser. | This series of transformations forms the heat pump cycle: energy is supplied via the compressor to the refrigerant, which absorbs heat from the surroundings in the evaporator and transfers it to the medium to be heated via the condenser. |
Heat pump efficiency
During operation, the heat pump:
| Gas absorption heat pump | Electric heat pump |
|---|---|
| Consumes natural gas/LPG in the generator. | Consumes electricity to power the compressor. |
| Absorbs heat in the evaporator from the surrounding medium (air or water). | Absorbs heat in the evaporator from the surrounding medium (air or water). |
| Releases heat to the medium to be heated in the condenser (air or water). | Releases heat to the medium to be heated in the condenser (air or water). |
The advantage of using a heat pump lies in its ability to deliver more energy (heat) than it consumes, by extracting heat from the environment—i.e. from renewable energy in air, water, or ground.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| The efficiency of a gas heat pump is measured using the Gas Utilization Efficiency (G.U.E.), which is the ratio between the energy delivered (heat released to the medium to be heated) and the energy consumed by the burner. The G.U.E. varies depending on the type of heat pump and its operating conditions, and typically has values around 1.5. This means that for 1 kWh of gas consumed, 1.5 kWh of heat is delivered. It’s important to note that G.U.E. is calculated based on primary energy (i.e. natural gas or LPG), whereas C.O.P. is based on electricity. When calculating the real efficiency of an electric heat pump, we must consider the energy used by power plants to produce electricity. Assuming an average power plant efficiency of 36%, the C.O.P.ep of electric heat pumps becomes: C.O.P.ep = 3 × 0.36 = 1.1. |
The efficiency of an electric heat pump is measured using the Coefficient of Performance (C.O.P.), which is the ratio between energy delivered (heat released to the medium to be heated) and electricity consumed. The C.O.P. varies based on the type of heat pump and operating conditions and typically has values around 3. This means that for 1 kWh of electricity consumed, 3 kWh of heat are delivered. Efficiency increases when the temperature at which heat is released (in the condenser) is lower and the source temperature (in the evaporator) is higher. It should also be noted that the heating capacity of a heat pump depends on the temperature from which it absorbs heat. |
| The gas heat pump can operate down to outdoor air temperatures of −20 °C while still delivering an efficiency close to 1—comparable to a condensing boiler. | When the cold source (air) drops below −5 °C, the heat pump is often switched off or supported by an electric resistance, as its performance drops significantly. |
The cold source (from which heat is recovered)
The external medium from which heat is extracted is called the cold source. In a heat pump, the refrigerant absorbs heat from the cold source through the evaporator.
The main cold sources are:
- AIR: outside the space to be heated, typically ambient outdoor air.
- WATER: from wells, rivers, or lakes, when available near the building and at shallow depths; other sources include water stored in tanks heated by solar radiation.
- GROUND: through specific pipes inserted at variable depths, connected to the evaporator (so-called geothermal probes).
The hot sink (to which produced and recovered heat is delivered)
The air or water to be heated is called the hot sink.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| In the condenser, the refrigerant transfers to the hot sink both the heat absorbed from the cold source and the energy supplied by the burner. | In the condenser, the refrigerant transfers to the hot sink both the heat absorbed from the cold source and the energy supplied by the compressor. |
Heat can be transferred to the environment through:
- fan coils, which are cabinets through which air is circulated over heating elements. These can be wall-mounted, ceiling-mounted or recessed units;
- underfloor coils, where hot water circulates. Heating occurs mainly by radiation and requires lower water temperatures;
- ductwork, which directly transfers the heat produced by the heat pump to different rooms via appropriate air channels and diffusion vents.
Types of heat pumps
Heat pumps are classified based on the cold source and the hot sink they use.
Depending on the fluid used to transfer heat from the cold source to the heat pump and then to the hot sink, four types are possible:
| Cold source | Hot sink | |
|---|---|---|
| AIR | AIR-WATER the heat pump extracts heat from the cold source, which is outdoor air, and transfers it to the hot sink, which is a water circuit (for space heating). | WATER |
| AIR | AIR-AIR the heat pump extracts heat from outdoor air and transfers it to the indoor air (the heated environment). | AIR |
| WATER | WATER-WATER the heat pump extracts heat from the cold source, which is water (from a lake, river, or well), and transfers it to a water circuit for space heating. | WATER |
| WATER | WATER-AIR the heat pump extracts heat from the cold source, which is water (from a lake, river, or well), and transfers it to indoor air (the heated environment). | AIR |
AIR as a cold source has the advantage of being available everywhere; however, the heating capacity of the heat pump decreases as the air temperature drops.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| When outdoor air is used, a defrosting system (automatically activated below 0 °C) is required. During defrosting (which lasts a few minutes), thermal output decreases but does not stop completely. | When outdoor air is used, a defrosting system is required (around 0 °C), which increases energy consumption. During this phase, the heat pump uses heat from the hot sink to defrost the coil, and heating is interrupted for a few minutes. |
WATER as a cold source ensures stable performance regardless of outdoor weather conditions; however, it involves additional costs for water intake and treatment systems.
GROUND as a cold source benefits from more stable temperatures compared to air and provides heat via geothermal probes—special underground pipes.
Horizontal pipes should be buried 1 to 1.5 metres deep to avoid fluctuations in outdoor air temperature and take advantage of solar radiation.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| Requires a land area 1.5 to 2 times larger than the heated surface. This is a more complex system design. | Requires a land area 2 to 3 times larger than the surface of the heated space. This is a costly solution both in terms of land and system complexity. |
Vertical pipes can reach several tens of metres deep. They do not occupy horizontal space but require the drilling of dedicated deep boreholes.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| The length of the pipes depends on the type of soil (on average from 4 to 12 m per kW of thermal energy delivered to the hot sink). | The length of the pipes depends on the type of soil (on average from 7 to 20 m per kW of thermal energy delivered to the hot sink). |
The GUE: the efficiency metric of gas heat pumps
The efficiency of a gas heat pump is measured by the GUE (Gas Utilization Efficiency), a parameter that indicates the ratio between thermal energy delivered and gas consumption.
Unlike the COP (Coefficient of Performance) used for electric heat pumps, the GUE directly evaluates the energy efficiency of gas combustion, taking into account both the heat absorbed from the environment and the heat generated by the burner.
A typical GUE value is around 1.5, meaning that for every kWh of gas consumed, the gas heat pump delivers 1.5 kWh of useful heat. This efficiency remains stable even at low temperatures, making the technology particularly advantageous in cold climates where electric heat pump performance may decline.
Applications of heat pumps
Possible applications of heat pumps include:
Space conditioning
Heat pumps are now widely used for indoor climate control in both residential and industrial sectors, as an alternative to conventional systems that combine chillers and boilers. The same unit, using a simple reversing valve, can switch the roles of the evaporator and condenser, providing heating in winter and cooling in summer (reversible type).
Using heat pumps for both heating and cooling is the most cost-effective solution, as it shortens the payback period of the system thanks to greater energy savings.
Space heating
Heat pumps can also be used solely for heating. In some regions, their use is even recommended, as these devices use renewable energy and are therefore more environmentally friendly than traditional heating systems.
For space heating, systems can be:
- monovalent
- bivalent
| Gas absorption heat pump | Electric heat pump |
|---|---|
| The monovalent configuration is typically used, as the gas heat pump can meet the full heating demand even when using air as the cold source. In fact, it provides heating at outdoor temperatures down to -20°C without the need for auxiliary systems. | - A monovalent configuration is used when the heat pump can fully meet the heating demand. If the heat pump uses outdoor air as the source, this configuration is suitable in regions where temperatures rarely drop below 0 °C. - Otherwise, a bivalent system is needed, combining a heat pump with a backup heating system (such as a traditional boiler) to cover the heating demand when outdoor temperatures fall below 0 °C. |
Choosing a heat pump
When choosing a heat pump, the following must be considered:
- the climatic conditions of the installation site;
- the building’s characteristics;
- the intended use conditions.
Climatic characteristics
Climate is especially important when the cold source is outdoor air.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| Frost may form on the evaporator during winter, reducing heat exchange. To address this, the heat pump has an automatic defrost system that continues to supply heat to the environment (though at reduced capacity) without needing additional energy or systems (such as electric heaters). | Frost may form on the evaporator during winter, impairing heat exchange. To address this, the heat pump includes a defrost system (such as an electric heater or cycle reversal, which temporarily operates the system in cooling mode). |
Building characteristics
Building features also affect the choice of heat pump type. For example:
| Gas absorption heat pump | Electric heat pump |
|---|---|
| - In residential buildings, the heat pump can be installed outdoors with no additional protection. Outdoor installation saves indoor space and avoids noise transmission indoors. - In commercial and industrial settings, higher output is typically required, so outdoor installation (usually with air as the cold source) is used. Backup boilers or heating systems are generally not needed. |
- In residential buildings, the heat pump can be installed in a basement or boiler room. In this case, noise and condensation are not problematic, and proximity to a traditional boiler makes a bivalent setup feasible. - In commercial and industrial settings, higher output generally requires outdoor installation (typically using outdoor air as the cold source), with a backup boiler as integration. |
- In commercial spaces with high internal humidity, such as hair salons, restaurant kitchens, etc., installing a heat pump can be very advantageous, as its cooling and dehumidifying action improves the comfort of the working environment.
Operating conditions
Operating conditions in different environments also influence the choice of heat pump. As previously mentioned, the heat output of a heat pump depends on the temperature difference between the cold source and the hot sink. The greater the difference, the lower the efficiency, and vice versa. Therefore, the selection of a suitable heat pump also depends on the operating conditions of both the cold source (air or water and its temperature) and the hot sink (water for low or medium temperature heating).
Sizing the heat pump
Sizing a heat pump system requires careful assessment of the heating demand. Oversizing the system increases installation costs and reduces the economic benefits of its use. Therefore, it is advisable to rely on a qualified technician for sizing.
Cooling
| Gas absorption heat pump | Electric heat pump |
|---|---|
| A reversible-cycle heat pump has a cooling capacity of about half its heating capacity, providing water at 3 °C in the case of an air-to-water system. | A reversible-cycle heat pump has a cooling capacity slightly lower than its heating capacity, providing cold water at temperatures down to 7 °C in the case of an air-to-water system. |
Space heating
The heating demand depends on the geographical location. Special attention must be paid to heat pumps using air as a cold source, as their output decreases with falling outside air temperatures.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| The heat delivered to the hot sink, in the form of hot water, can reach up to 65 °C (with little impact from defrosting cycles). | The heat delivered to the hot sink, in the form of hot water, usually ranges between 35 and 50 °C (except during defrosting periods). |
Heat pump installation
The heat pump is a device with proven reliability, but proper installation and minimal maintenance are essential for long-term performance. As with sizing, installation should be entrusted to specialised installers.
Here are some practical tips to help users focus on key installation aspects:
- To prevent condensation, make sure that pipes carrying cold fluids are properly insulated.
- The operation of the heat pump is controlled by a thermostat, either directly or indirectly, based on ambient temperature, with start/stop cycles. To reduce cycling and extend the system’s life, it may be useful to install a buffer tank.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| The minimum operating temperature is -20 °C. | It is important to ensure that the minimum operating temperature of the heat pump is not higher than the minimum outside air temperature in the installation location. Otherwise, an auxiliary boiler must be included in the system. |
A condensate drain should be provided for moisture formed on the evaporator, e.g., using a drain pan or discharge pipe.
| Gas absorption heat pump | Electric heat pump |
|---|---|
| Using natural gas does not require any upgrade of the gas meter or changes to the electricity contract. | An electricity contract with sufficient power capacity is needed to operate the selected heat pump (usually above the standard 3 kW limit for domestic users). Initial start-up should be performed by qualified technicians. |
Regulatory framework for heat pumps
It is now widely recognised that heat pump technology offers significant energy savings, particularly for heating. Thanks to their ability to absorb low-temperature heat, heat pumps use renewable energy sources such as air (aerothermal), ground (geothermal), and water (hydrothermal). For this reason, national and international energy-saving regulations recognise heat pumps as alternative heating systems. The recent European RES Directive also officially recognises heat pumps as renewable energy technologies.
Sources
- ENEA publication – Energy Department – Booklet "The Heat Pump"
- Technical documentation on gas absorption heat pumps – Robur SpA
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