Air-to-water heat pump vs. ground-to-water – which is more cost-effective?

This question comes up almost with every project where a customer is considering a heat pump. And the answer is never simple – it depends on the plot of land, the house, the budget, and what you expect from the system in the long run. Over the years of practice, I have seen both technologies work excellently and poorly – it always depends on the correct selection and installation for the specific situation. This article will help you understand the differences in depth, not just superficially, and will make you an informed customer who knows what they are ordering and why.

Basic principle: where the pump gets the heat

Both technologies operate on the same physical principle – the heat pump "moves" thermal energy from a lower temperature environment to a higher temperature environment, using electricity only for the operation of the compressor and pumps. The difference is in where this heat is taken from.

Air-to-water (shortened to AW or air-source heat pump): The outdoor unit draws in ambient air and extracts thermal energy from it using an evaporator. Air is an inexhaustible source, it is everywhere, and therefore these systems are easy to install – the outdoor unit is placed next to the house, connected to the indoor unit (or directly to the distribution system), and that's basically all.

Ground-to-water (shortened to GW or geothermal heat pump): Heat is extracted from the ground, either via a horizontal collector (horizontally laid pipe at a depth of about 1.2–1.5 m) or via vertical boreholes (usually 80–150 m deep). The ground has a stable temperature at these depths throughout the year – around 5–10 °C for a horizontal collector, and even 10–12 °C for boreholes. This stability is key to the efficiency of the system.

Decisive numbers: COP and SCOP

The most important performance indicator of a heat pump is COP (Coefficient of Performance) – the ratio of the obtained thermal output to the consumed electrical energy. If the pump has a COP of 4, it means that from 1 kWh of electricity, it produces 4 kWh of heat. A more realistic indicator for annual operation is SCOP (Seasonal COP), which takes into account the entire heating season including frosty days.

Here the technologies differ significantly:

• Air-to-water: Summer COP can be 4.5–5.5, but when it is, for example, -15 °C outside, COP drops to 2.0–2.5. The average seasonal SCOP for Central European conditions is around 2.8–3.5.
• Ground-to-water (horizontal collector): The source is more stable, COP varies less. Seasonal SCOP is usually 3.5–4.2.
• Ground-to-water (boreholes): The most stable heat source, COP remains consistent. SCOP can reach 4.0–5.0.

What does this mean in practice? A house with an annual heat requirement of 15,000 kWh at SCOP 3.0 consumes 5,000 kWh of electricity annually. At SCOP 4.0, it only consumes 3,750 kWh – a difference of 1,250 kWh, which at a price of 0.20 €/kWh amounts to 250 € annually. Over 15 years, that is 3,750 €. This is a real difference that must be considered when comparing investment costs.

Investment costs: where the real numbers differ

This is the point where most customers make their first major mistake – they compare only the price of the device itself and forget about the total installation costs. Let's look realistically at what each technology will cost you.

Air-to-water: lower initial investment

The price of the air-to-water heat pump itself for a typical family house (output 8–12 kW) ranges roughly from 3,500 to 9,000 €. The outdoor unit is mounted on a base or hung on a wall, the refrigerant line connects the outdoor and indoor units (usually 5–15 m), and the connection to the heating system in the house is made. Total installation costs including materials and labor are usually in the range of 1,500–3,500 €. Total investment thus: 5,000–12,500 € for a typical family house.

One of the things often overlooked during installation is the protection of the outdoor unit and accessories for reliable operation. For example, self-regulating heating cable for ITEC heat pumps protects the condensate drain from freezing during icy weather – a small detail, but important for reliable year-round operation of the air heat pump.

Ground-to-water: higher initial cost, but with justification

The price of the unit itself is comparable or slightly higher – it ranges from 5,000 to 12,000 €. The difference is made by the ground collector or boreholes:

• Horizontal collector: You need an area about 1.5–2 times larger than the area of the heated house – for a house of 150 m², this is about 225–300 m² of garden. Excavation work and collector material usually cost 3,000–6,000 €.
• Vertical boreholes: A borehole of 100 m depth costs about 3,000–5,000 €, for a house of 150 m² usually 2–3 boreholes are needed. Total borehole costs: 6,000–15,000 €. In addition, permission from the geological survey is required and in some cases also a building permit.

Total investment for ground-to-water with a horizontal collector: 8,000–18,000 €. For ground-to-water with boreholes: 11,000–27,000 €.

The difference compared to air-to-water can be 3,000–15,000 €. This amount must be paid back from savings on operation. With a higher SCOP and longer lifespan of geothermal systems, it is usually more advantageous in the long run – but the payback period can be 8–15 years depending on specific conditions.

Performance in extreme conditions: when air-to-water lags behind

One of the most frequently discussed topics is the behavior of air heat pumps in freezing conditions. Here it is important to be honest: most modern air heat pumps operate without problems down to −15 °C, some premium models even down to −25 °C. But the decreasing temperature of the source (air) directly reduces the output and COP.

In practice, this looks like this: a pump with a rated output of 10 kW at 7 °C/35 °C (A7/W35 – standard testing condition) may deliver only 6–7 kW at −15 °C outside and its COP drops to 2.0–2.2. This is still usable, but in combination with the fact that this is exactly when the need for heat is highest, it can lead to a situation where the pump cannot cover the need alone and auxiliary electric heating (bivalent operation) comes into play. This is much less energy efficient and can significantly increase the cost of operation during harsh winters.

Geothermal pumps do not have this problem. The temperature of the ground collector in January is no lower than in October – it still ranges from 0–10 °C (collector) or 8–12 °C (boreholes). Performance and efficiency change very little seasonally. This is a huge practical advantage, especially for houses in mountainous areas or where winters are long and harsh.

Noise and placement: where problems arise in practice

An air heat pump has an outdoor unit with a fan. Modern devices have a noise level of usually 45–55 dB(A) – comparable to a quieter vacuum cleaner or air conditioning. The problem arises when the outdoor unit is placed near a neighbor's window, a bedroom window, or in a small atrium, where the noise reflects. In row houses with small plots, this is a real problem that needs to be addressed already during the planning of the placement.

Another practical detail: during defrosting (which happens regularly in icy weather), the outdoor unit produces water vapor and condensate. The condensate must be drained – and it can freeze during frost. Therefore, it is important to have a properly designed drain under the outdoor unit, or a heating cable. For this purpose, for example, self-regulating heating cable for ITEC heat pumps is used, which automatically regulates the output according to temperature and prevents the condensate from freezing.

Ground source heat pumps are practically silent – there is no outdoor unit, everything is hidden in the house's technical room. A ground loop collector or boreholes do not create any noise. The only noise is from the brine circuit circulation pump, which is negligible in the technical room.

Space requirements and site conditions

Here, a geothermal solution is more demanding. A ground loop collector requires a large free area that:

• must not be built on or buildable (no terraces, sheds, future extensions)
• must not be permanently shaded by dense vegetation – trees with deep roots can damage the piping and shade reduces heat regeneration from the sun
• must be large enough – for a house of 150 m², the ground area is 225–300 m²
• excavation to a depth of 1.2–1.5 m must be possible (rock, groundwater, engineering networks)

Boreholes are space-saving – it is enough to have space for the drilling equipment (usually 3×5 m per each borehole). This makes them more suitable for smaller plots. However, they are more expensive and require a geological survey, permission and a professional driller.

Air source heat pumps are clearly more convenient in this respect – the outdoor unit occupies an area of about 0.5–1.5 m², and that is enough. They are ideal for the renovation of existing houses in urban areas, where there is not enough garden space or drilling possibility.

Integration with the heating system and accessories

Both technologies can be connected to the same type of heating systems – floor heating, heat air units, low-temperature radiators. Heat pumps work most efficiently with a low temperature difference in the system (e.g., 35/30 °C), which is typical for floor heating. With classic radiators designed for 70/50 °C, it is necessary to either retrofit the system or reduce the requirements and accept a lower COP.

For the proper functioning of a heating system with a heat pump, zoning and hydraulic regulation are essential. Modern systems with mixed circuits require proper setting of pumps and regulators. For TCH with Optimum function, for example, there is available a relay set that converts the 3-10V signal to on/off for the mixed circuit pump – such details decide whether the system regulates efficiently or just roughly.

An important part of every heat pump is also the preparation of hot domestic water (HDW). Both technologies can efficiently heat water in a storage tank, but it is important to remember hygiene standards – occasional heating to 60–65 °C (protection against legionella). When selecting accessories, for example, a circulation set for TCH series ITEC-T, ATHENA-T, LEGEND, CALIBRA and ATLAS can be helpful, which ensures immediate availability of hot water in the entire distribution without long waiting for heating.

For the installation of the circulation set or hydraulic connection, high-quality mechanical connections are also needed. For dual (DUO) systems, a connecting set for TCH series CALIBRA DUO and ATLAS DUO is suitable, which ensures tight and removable connection of units with the distribution.

Lifespan and maintenance costs

Air source heat pumps have more moving and stressed components exposed to the external environment – compressor, outdoor fan, finned evaporator. The lifespan of quality equipment is 15–20 years, and even more with good maintenance. The evaporator is exposed to the external environment and is subject to corrosive effects (salt on roads, chemicals). Once a year, it is necessary to check the condition of the refrigerant, clean the finned evaporator and check the electrical connections.

Ground source heat pumps have a protected primary circuit (collector or boreholes are underground) and a longer expected lifespan – the collector pipe itself lasts 50 years, the internal pump unit 20–25 years. Annual maintenance is minimal – checking the pressure and composition of the brine, filters, circulation pump.

The difference in service costs is not dramatic in trouble-free operation, but in case of a failure, the geothermal system is more sensitive – if the collector fails (mechanical damage, loss of brine), the repair is significantly more expensive and complicated than replacing the outdoor unit of an air source heat pump.

Grants and legislation: current situation

In Slovakia, it is possible to draw grants for heat pumps through schemes such as Green for Households or through regional funds. The amount of the grant depends on the type of technology and power. In some programs, geothermal systems are subsidized with a higher amount, as their higher investment cost is recognized as a barrier to entry. It is advisable to check the current conditions directly on the SIEA website or with an authorized installation partner, as program conditions change.

From a legislative perspective: the installation of vertical boreholes requires notification or a building permit (water law, geological works law). Air-source heat pumps usually do not require any permit, except in cases where the outdoor unit is located in a quiet zone or an area with protected architectural character.

Which solution for which type of building?

Based on practical experience, we can summarize typical scenarios:

Air-water is more suitable if:

• You are renovating a house in a built-up area without a large garden
• You have a limited investment budget and want a quick return
• The house is located at a lower altitude (up to 500 m) and winters are not extremely harsh
• You do not need maximum efficiency, a solid SCOP of 3.0–3.5 is sufficient for you
• You plan to install the system quickly – installation usually takes 2–3 days

Ground-water (horizontal collector) is more suitable if:

• You have a new build or plan major site modifications, where excavation is not a problem
• You have enough free land without buildings
• You want higher efficiency and quiet operation (no outdoor unit)
• You are willing to invest more upfront for lower operating costs

Ground-water (boreholes) is more suitable if:

• You have a small plot or a fully built-up area
• The house is in a mountainous area with long and harsh winters
• You want maximum efficiency and the lowest operating costs without compromises
• You have a sufficient budget and do not want to think about annual maintenance of an outdoor unit
• You plan to use the house long-term (20+ years)

Scalability and smart integration

Modern heat pumps – regardless of type – offer expansion options through modules. For example, the expansion module for the ITEC heat pump allows the addition of further circuits, integration with a photovoltaic system, or the addition of remote management. These modules give you the flexibility to gradually expand the system without the need to replace the entire unit – which is very important from the perspective of a long-term investment.

If you are interested in how an expansion module integrates into a specific installation, see the article Expansion module for a heat pump – when and why you need it in our Knowledge Center.

Both technologies are now compatible with smart home solutions, control via apps, heating scheduling according to electricity tariffs (night rate, blocks of cheap electricity), or integration with photovoltaic panels. Electricity generated from PV panels is primarily used to power the heat pump – and with a higher SCOP (as in geothermal systems), every kilowatt-hour of PV electricity is more efficient.

Practical perspective: three real customer cases

Case 1 – family house in Nitra, renovation, 2022: A house from the 80s, 140 m², garden 200 m², previously a gas boiler. The customer wanted the lowest possible initial costs. We chose air-water 10 kW. Installation took 3 days, total cost including distribution and hydraulics ~11,000 €. After the first year of operation: average SCOP 3.1, annual electricity costs ~900 €. The customer is satisfied, in January at −18 °C the electric booster ran for about 8 days, which increased monthly consumption by ~25 %.

Case 2 – new build near Žilina, 2021: A house 180 m², land 800 m² – enough space. The customer invested in a horizontal collector (320 m²) and a geothermal heat pump 12 kW. Total investment ~19,000 €. Average SCOP after two seasons: 4.0. Annual electricity costs ~730 €. In January, no problem, the system ran without booster heating even at −20 °C (collector temperature did not drop below 2 °C).

Case 3 – house on a slope near Banská Bystrica, 2023: A house 200 m², small plot (garden only 150 m²), rocky subsoil. A horizontal collector was not an option. The customer invested in two boreholes of 120 m each and a geothermal heat pump 14 kW. Total investment including drilling ~26,000 €. Average SCOP after the first season: 4.6. Annual electricity costs ~850 €. The customer is considering PV panels to reduce costs even further.

These cases show that the decision is not only about the technology – it is about the overall context: land, climate, budget, plans. More on what you need to ask yourself before choosing can be found in the article How to choose a heat pump – what to focus on before purchase, where we go even more into the pre-selection criteria.

Most frequently asked questions (FAQ)

Can an air-source heat pump operate at −20 °C?

Yes, but with reservations. Modern premium air-source heat pumps from reputable manufacturers can operate even at −25 °C. The problem is not the operation itself, but a significant drop in performance and COP. At −20 °C, a heat pump with a nominal power of 10 kW may deliver only 5–6 kW in real terms with a COP of around 1.8–2.0. For most houses, this is not enough to cover peak heat demand and the system is supplemented by electric backup coils – which is expensive. For areas with regularly harsh winters, a geothermal solution is significantly more suitable.

Do I need a building permit for a geothermal heat pump with a horizontal collector?

For a horizontal collector installed in your own garden, a building permit is usually not required – a minor construction notification or even that is not always necessary. However, the situation is different for vertical boreholes: boreholes deeper than 30 m fall under the geological works law and water law and require a permit from the relevant authority. We always recommend verifying the situation at the local building office and the relevant geological department before starting work.

Is a geothermal heat pump worth it if the price difference compared to an air-source pump is 15,000 €?

It depends on the specific conditions. If the price difference is 10,000 € and the difference in annual operating costs is 400 € (a common case), the return on investment is 25 years – which is on the edge of economic sense. However, if the operating savings are 700–800 € per year (higher heat demand, harsher winters), the payback period drops to 12–15 years, which is realistic. In addition, geothermal systems have a higher residual value of the property. We always recommend doing an economic analysis for a specific house – not a generic comparison.

What is the difference between monovalent and bivalent operation?

Monovalent operation means that the heat pump covers 100 % of the heat demand without a backup source. Bivalent operation (alternative or parallel) combines the heat pump with a backup source (electric heating, boiler), which comes into play when the heat pump is not sufficient. Air-source heat pumps are mostly designed for bivalent operation – the pump covers 80–95 % of the annual energy demand and the backup heating (typically an electric coil in a storage tank) comes into play during extreme cold. Geothermal systems usually operate monovalently due to the stable heat source.

How long does the installation of an air-source vs. geothermal heat pump take?

Air-source heat pump: installation of the outdoor and indoor unit, connecting the refrigerant line and hydraulics usually takes 2–4 days. Geothermal heat pump with a horizontal collector: excavation work takes 2–5 days (depending on the area and terrain accessibility for machines), collector installation 1 day, indoor installation 2–3 days. Total: 1–2 weeks. Geothermal heat pump with boreholes: drilling itself takes 3–7 days (depending on the number and depth of the boreholes), plus installation ~3 days. Total: 1.5–2 weeks. Drilling is dependent on weather conditions and the availability of the drilling rig.

What if I make the wrong decision and want to change the system later?

Changing from an air-source to a geothermal heat pump is technically possible – the indoor unit and piping remain, only the heat source is replaced (the outdoor unit is replaced by a ground loop). It is an additional investment, but not a catastrophe. Changing in the opposite direction (from geothermal to air-source) is also possible, although the collector or boreholes remain unused – which is a bit of a shame. Therefore, making the right decision from the start is important. Our article How to choose a heat pump – what to focus on before purchase and the performance calculation in the article What heat pump capacity do I need for my house will help you with that.

Conclusion: there is no one-size-fits-all solution

After all we have discussed, it is clear that there is no objectively "better" technology – there is only a technology more suitable for your specific house, plot and situation. Air-to-water is a modern, flexible and cost-effective solution that works well when properly sized and in favorable climatic conditions. Ground-to-water is a more powerful, more stable and operationally cheaper solution that requires a higher initial investment and suitable plot conditions.

If you are considering installation and want to make the right choice at first attempt, we recommend you also study related topics in our Knowledge Center – especially Heat pump installation – procedure, requirements and common mistakes, Connecting and installing heat pump accessories step by step and Integrating hot water circulation with a heat pump – how to do it. The better you understand the whole system, the better decision you will make – and the longer your investment will serve you satisfactorily.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.

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