Heat Pump Air-Water vs. Ground-Water

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An air-water heat pump and a ground-water heat pump solve exactly the same task - drawing heat from the surroundings and delivering it into the house's heating system - but they do it in such different ways that comparing them by price alone is short-sighted. One takes heat from the air, which cools to sub-zero temperatures during winter; the other from the ground, where the temperature a few metres below the surface is practically the same all year round. This one difference in the heat source then translates into efficiency, price, space requirements, noise, and lifespan. In this article we'll break down both systems in practical terms, using real figures and typical situations from ordinary family houses in Slovakia - not manufacturers' marketing slogans.

One important note up front: we currently don't have specific heat pump models in stock in this category, so you won't find a recommendation for a specific product or a price list in this article. The goal is that after reading it you'll be able to judge for yourself which type suits your house and plot, and know what questions to ask an installation company when you request a quote.

How both systems work - the principle in short

Both technologies are heat pumps in the true sense of the word - they work on the principle of a compressor refrigeration circuit, just in reverse: instead of cooling the interior (like a fridge), they draw heat from the outdoor environment and deliver it into the heating water. The difference lies solely in where the heat is drawn from.

An air-water heat pump draws in outdoor air through a large fan in the outdoor unit; the refrigerant in the evaporator takes the air's energy (even if the air is just -10 °C, it still contains usable heat), the compressor compresses the refrigerant, raising its temperature, and in the condenser this heat is transferred to the heating water. The cooled air is discharged back outside, a few degrees colder.

A ground-water heat pump works with the same refrigeration circuit, only the heat source isn't air but an antifreeze mixture (glycol) circulating in ground boreholes or in a horizontal collector laid below the surface of the plot. This mixture picks up heat directly from the rock or soil, which at our latitudes, at a depth of about 1.5-2 m below the frost line, stays at a practically constant temperature of around 8-12 °C all year round; at greater borehole depths (60-150 m) the temperature is even more stable, usually 10-14 °C.

Below is a simplified diagram of both circuits - notice that only the first step (the heat source) differs; the rest of the technology (compressor, condenser, circulation pump, the house's heating system) is identical.

The key trait: efficiency at different outdoor temperatures

A heat pump's efficiency is expressed as the COP (Coefficient of Performance) coefficient for a specific operating temperature, and as SCOP (Seasonal COP) for the average over the whole heating season. A COP value of 3.5 means that from 1 kWh of electricity fed into the compressor, 3.5 kWh of heat is produced - the rest (2.5 kWh) is drawn "for free" from the surrounding environment.

This is where the biggest real difference between the two systems shows up. An air-water heat pump's efficiency is strongly dependent on the outdoor temperature - the colder it is outside, the bigger the temperature difference the compressor has to "bridge", and the lower the COP. A ground-water heat pump works with almost the same source temperature at all times (that stable ground temperature), so its COP fluctuates only minimally regardless of whether it's -15 °C or +5 °C outside.

Typical indicative values commonly seen in practice for quality modern units working into underfloor heating (outlet water temperature approx. 35 °C):

Outdoor temperature	Air-water COP	Ground-water COP
+7 °C	4.0-4.5	4.8-5.2
0 °C	3.2-3.6	4.6-5.0
-7 °C	2.4-2.8	4.3-4.7
-15 °C	1.8-2.2	4.0-4.4

Over the whole heating season (SCOP), under our climate conditions this averages roughly 3.3-3.7 for air-water and 4.3-4.8 for ground-water. The difference between 3.5 and 4.5 looks small on paper, but converted into annual electricity consumption for a house with a heat demand of 15,000 kWh/year, it means roughly 4,285 kWh of electricity consumed for air-water versus 3,333 kWh for ground-water - about 950 kWh less per year with the ground system, which at typical electricity prices is a noticeable but not dramatic saving (we have a separate detailed article with calculations on this topic, linked below).

The practical consequence: the harsher the winter climate (mountain areas, northern Slovakia, exposed locations with freezing winds), the bigger the real difference in favour of ground-water. Conversely, in milder lowland areas with short frosty periods, the difference is smaller and air-water can be surprisingly close in terms of operation.

Purchase costs - where the biggest difference arises

This is the area where the two systems diverge the most, and where ground-water most often breaks people's budgets. The heat pump unit itself (indoor or outdoor unit with the compressor) doesn't differ that much in price - the difference in the unit alone is typically in the hundreds of euros. The difference comes from the heat source.

For air-water, it's enough to mount the outdoor unit on a reinforced base or brackets by the house wall, connect it with pipework to the indoor unit, and wire the electrics. The whole installation for a typical family house can usually be done in 1-3 days.

For ground-water, you additionally need to build a ground heat exchanger - either vertical boreholes or a horizontal (surface) collector:

Vertical boreholes - drilled to a depth of 80-150 m; drilling costs run roughly around €40-60 per linear metre including material and grouting. For a typical family house with a heat loss of 8-10 kW, 2 boreholes of 100-120 m each are usually made, i.e. 200-240 drilled metres. That alone represents an additional cost on the order of €9,000-14,000 just for drilling, plus the need for the drilling rig to access the plot (not always possible on narrow or already built-up plots).
Horizontal collector - laid flat at a depth of 1.2-1.5 m, cheaper to install (excavation work), but requires a plot area of roughly 1.5-2 times the house's heated floor area, which then must not be built on, paved, or planted with deep-rooting trees.

The result is that a complete ground-water heat pump installation for a typical family house (up to 150 m² of floor area) comes to roughly €18,000-26,000 including earthworks, while a comparable air-water heat pump including installation runs around €9,000-15,000. The difference therefore typically represents €8,000-12,000 against the ground system - and this is the main reason why air-water is by far the more common choice in Slovakia today, both for new builds and renovations.

Space requirements and plot suitability

The air-water outdoor unit only needs a paved area of roughly 1×1 m by the house wall or on a roof/terrace, with sufficient clearance from bedroom windows (for noise and airflow) and free space for air intake and discharge. It's therefore usable on practically any plot, including terraced housing and urban parcels.

A ground-water heat pump places substantially higher demands on the plot. Vertical boreholes need access for the drilling rig (an access path at least 2.5-3 m wide, with sufficiently load-bearing terrain) and adequate spacing between boreholes and from the plot boundaries (usually at least 5-10 m between boreholes so they don't thermally affect each other, plus the neighbour's consent or compliance with the boundary setback under local regulations). A horizontal collector requires an area 1.5-2 times the house's heated floor area, which must remain permanently unbuilt and free of deep-rooting trees.

Noise

The outdoor unit of an air-water heat pump contains a large fan, which is a source of noise - typical values run around 45-60 dB(A) at a distance of 1 m from the unit at full output; better models with a night silent mode can reduce this by a further 5-10 dB. At a greater distance (5-10 m, i.e. toward a neighbour's plot or a bedroom window), the noise naturally drops, but in dense built-up areas or close to plot boundaries, noise can be grounds for a neighbour's complaint or the need for an acoustic study as part of the building permit.

A ground-water heat pump has no outdoor unit with a fan - the whole unit (compressor, circulation pumps) is located inside the utility room. The noise of the indoor unit runs around 35-45 dB(A), comparable to an ordinary fridge or a quiet circulation pump, and the noise is additionally isolated by the house walls. This is one of the clear arguments in favour of ground-water in dense built-up areas, narrow plots, or sensitive neighbour relations.

Lifespan and maintenance

The outdoor unit of an air-water heat pump is directly exposed to the weather (frost, rain, snow, UV radiation, or salty sea air, which we're not covering here), which shortens its lifespan compared to technology protected indoors. The commonly stated lifespan of the compressor and outdoor unit is 15-20 years, after which the unit usually needs replacing (similar to air conditioning). Maintenance includes regularly cleaning the heat exchanger of debris and leaves, and checking the condensate drain and refrigerant.

A ground-water heat pump has its indoor unit protected from the weather, which extends its lifespan to typically 20-25 years. The ground heat exchanger itself (boreholes, collector pipework) has a substantially longer lifespan - well-executed boreholes with certified pipework are rated at 50 years or more, essentially the entire lifetime of the building. This means that when the heat pump is replaced after 20-25 years, with a ground system usually only the unit itself is replaced, while the ground exchanger remains functional - which partly offsets the higher upfront investment over the long term.

Speed and simplicity of installation

From a construction schedule perspective, the difference is significant. Air-water can be installed practically any time of year; installing both the outdoor and indoor units together with wiring usually takes 2-4 working days, which allows flexible planning even for a later addition to an already built and occupied house.

Ground-water requires a separate drilling or excavation phase, best planned before finishing the landscaping around the house (before the lawn, footpaths, or paved areas are laid), since both the drilling rig and excavation equipment need access for heavy machinery. Drilling 2 boreholes usually takes 2-4 days depending on geological conditions (rock drills more slowly than loose soil), plus a further 2-3 days for the heat pump installation itself. For a renovation of an already finished house with a mature garden, implementing a ground system is therefore more demanding both logistically and financially (risk of damaging the existing garden, fences, or paved areas).

Decision diagram - what to weigh up

Below is a simplified process we recommend going through when deciding. It isn't a strict rule, but an order of questions you should ask yourself before approaching an installation company for a specific quote.

In practice, we most often see two typical customer profiles. The first type has a more limited budget, a typical urban or suburban plot, and wants a solution that can be implemented quickly - there we almost always recommend air-water. The second type has a larger rural plot, is planning a house for life (often a new build to a passive or low-energy standard), doesn't mind a higher upfront investment in exchange for lower running consumption and minimal noise - there it makes sense to consider ground-water, ideally already at the house design stage, when the drilling phase can be coordinated with the construction schedule and excavation work.

What both systems have in common

Despite the differences in heat source, the same general recommendations apply to both types. Both technologies work best and most efficiently with a low-temperature heating system (underfloor heating, or oversized radiators or convectors with a low design water temperature of 35-45 °C) - the lower the required outlet water temperature, the higher the COP for both systems. Both systems also require correct sizing according to the house's heat loss (not "by eye" or based on floor area), which should be worked out by a designer based on a heat loss calculation, not estimated by an installation company. Both an undersized and an oversized heat pump - for either type equally - lead to unnecessarily high consumption, more frequent compressor cycling, and a shorter lifespan.

Both systems also require a good bivalent or monovalent heating design (especially for air-water during extreme frosts below -15 to -20 °C, when some older or cheaper models need a backup electric heating element), and both benefit from well-set weather compensation control, which adjusts the heating water temperature to the current outdoor temperature instead of using a fixed value.

Summary - how to decide

To sum up the whole article in a few sentences: an air-water heat pump is today the predominant choice in Slovakia thanks to a lower upfront investment (roughly €9,000-15,000 complete), faster and simpler installation without needing a large plot, but with lower efficiency during hard frosts (COP drops to 1.8-2.2 at -15 °C) and higher outdoor unit noise (45-60 dB). A ground-water heat pump, on the other hand, offers consistently high efficiency regardless of the weather (COP 4.0-5.0 year-round), almost zero outdoor noise, and a longer lifespan for the ground exchanger (50+ years), but at the cost of a substantially higher upfront investment (€18,000-26,000) and the need for a suitable plot with access for heavy machinery.

There's no universally "better" solution - the right choice always depends on a combination of budget, plot size and shape, how long you plan to live in the house, and sensitivity to noise. We recommend having a heat loss calculation done for the house and a non-binding site visit by a professional installation company before making a final decision - especially for ground-water, the real feasibility of boreholes on a specific plot is something that needs to be checked in advance, not after signing the contract.

Frequently Asked Questions

Can a ground-water heat pump be added later to an already existing house with a mature garden?

Yes, but it's more demanding logistically and more expensive than with a new build. Vertical boreholes require access for the drilling rig to the drilling site, which can mean temporary damage to part of the lawn, footpaths, or fencing that needs to be restored afterwards. A horizontal collector is practically unworkable in a mature garden without significant disruption to existing planting. In practice, ground-water is therefore easier to implement with a new build or before finishing the landscaping around the house.

Does an air-water heat pump work reliably even in hard frosts below -15 °C?

Yes, modern units are designed to operate even at -20 to -25 °C, but their efficiency drops significantly at such temperatures (COP around 1.8-2.2), and some models need help from a backup electric heating element during extreme frosts. When designing the system, you always need to account for the lowest typical temperatures in your location and have the sizing assessed by a professional based on the house's real heat loss.

Is it true that a ground-water heat pump is always more advantageous in the long run?

Not automatically. Ground-water's higher efficiency means lower annual electricity consumption, but the difference in upfront investment (typically €8,000-12,000 against ground-water) pays back gradually over many years at typical family house consumption. The exact payback calculation depends on the house's specific heat loss, electricity prices, and whether the house also uses photovoltaics. You'll find a detailed calculation in a separate article on heat pump savings (linked below).

How much space on the plot do I really need for a ground collector or boreholes?

For a horizontal collector, allow for an area of roughly 1.5 to 2 times the house's heated floor area, which must remain permanently free (no paved areas, pool, or deep-rooting trees). For vertical boreholes, it's more about accessibility - you need access for the drilling rig (a reinforced path at least 2.5-3 m wide) and adequate spacing between boreholes (5-10 m) as well as from neighbouring plot boundaries under local regulations.

Is a ground-water heat pump really substantially quieter?

Yes, significantly. The air-water outdoor unit with its fan reaches 45-60 dB(A) at the unit, while the ground-water indoor unit located in the utility room runs around 35-45 dB(A), and is additionally isolated by the house walls. If you're dealing with dense built-up housing, a close neighbour, or a bedroom right next to the planned outdoor unit location, this is one of the strongest arguments in favour of a ground system.

Is it worth combining a heat pump with an existing gas boiler instead of a full replacement?

Such a bivalent solution is possible and makes sense in some renovations - the heat pump covers most of the season, and the older boiler only kicks in during the hardest frosts or as a backup. This solution is more typical with air-water (due to simpler installation alongside an existing boiler) than with ground-water, where the high upfront investment in boreholes generally motivates a full monovalent solution without keeping the old source.

Have a question on this topic?

Not sure what to decide, or dealing with a specific situation in your home? Write to us - we're happy to help.