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Air-to-air vs air-to-water heat pumps: which one should you choose?

Air-to-air or air-to-water heat pump? Which one should you choose for your home?
GreenHome
Air-to-air, air-to-water – two similar-sounding terms for two systems that serve different purposes. One replaces a boiler, the other replaces an air-conditioning system. This guide takes a project-based rather than a technical approach, to help you identify which of the two solutions is best suited to your situation, and in which cases the two complement each other.

  • The essentials

    An air-to-water system replaces a boiler: it supplies radiators or underfloor heating, and produces domestic hot water. An air-to-air system heats and cools the air in rooms via wall-mounted units, without involving the water circuit or producing hot water. In terms of cost, air-to-air systems start at a much lower price: a single-split unit costs around €2,000, compared with €8,000 to €18,000 for a complete air-to-water installation. As for grants, only air-to-water systems are subsidised in Wallonia and Brussels; Flanders subsidises both. Since January 2026, a 6 per cent VAT rate has applied to both technologies throughout Belgium. Many Belgian households end up installing both, each for its specific purpose.

Asking the question ‘air-to-air or air-to-water’ as if it were a choice between two models of the same product often leads to a dead end. Both technologies draw their energy from the outside air in the same way, but they do not provide the same service: one heats an entire house and provides domestic hot water, whilst the other heats and cools individual rooms. The right question, therefore, is not which is better, but which one suits your specific needs.

Air-to-water and air-to-air systems do not replace the same thing

An air-to-water heat pump captures heat from the outside air and transfers it to the water in your central heating system: radiators, underfloor heating and the domestic hot water tank. It completely replaces a boiler, with the same requirements regarding sizing and compatibility with existing heat emitters. Our guide Air-to-water heat pump: how does it work and is it the right choice for my home? explains how it works, the conditions for cost-effectiveness and the full budget, including grants.

Air-to-air systems, on the other hand, blow warm or cool air directly into rooms via one or more wall-mounted units, without passing through any water circuits. They do not produce domestic hot water and are not connected to any radiators. Their key advantage – the ability to switch modes to cool in summer – makes them a solution in their own right rather than simply a cheaper alternative to air-to-water systems. 

Whilst the question relates more broadly to the choice of central heating system itself, including the heat generator and radiators, our guide Heat pump, boiler or underfloor heating: which to choose? sets out this broader context, within which the air-to-water system is one of the possible heat generators.

What exactly is your plan?

This is the key question, far more important than price or efficiency. If the project involves replacing an ageing boiler and heating the whole house with a single, integrated system, an air-to-water system is the only logical solution: it is the only one that supplies all the radiators or underfloor heating and provides domestic hot water at the same time.

Whether the project is more specific – such as air-conditioning a living room that becomes unbearable in summer, heating a flat without an existing hydronic system, or fitting out an extension without altering the existing heating system – air-to-air systems directly meet the need, with installation of a single-split unit taking just one day. It is also the obvious choice in rental properties or blocks of flats where altering the building’s central heating system is not an option.

In between these two scenarios, one situation often arises: a well-insulated house that is already heated by gas to a satisfactory standard, but which has a genuine need for cooling in the summer. In this situation, replacing a boiler that is still working well with an air-to-water system does not always make immediate economic sense, whereas an air-to-air system targeted at the living areas meets the actual need at hand, for a much lower cost.

The scale of the installation work follows the same logic as the budget. A single-split air-to-air system can be fitted in a day: the installer secures the outdoor unit, fits the indoor unit, and connects the two via simple wall penetrations, without affecting the rest of the house. An air-to-water system requires more extensive work, including connection to the existing plumbing system, checking or replacing radiators, and sometimes adapting the electrical distribution board to cope with the required power. This is not a drawback of the air-to-water system; it is the logical consequence of a system designed to heat an entire house rather than a single room.

The starting budget makes all the difference

This is a point that technical comparisons often overlook: with air-to-air systems, you can start small, whereas with air-to-water systems you cannot. A single-split unit for a single room costs around €2,000 including installation – a budget that remains affordable for trying out the technology or meeting a specific need without having to fit the whole house. A multi-split system, which serves several rooms with a single outdoor unit, becomes progressively more expensive depending on the number of indoor units connected.

Air-to-water systems are a different matter altogether. Replacing a central heating system means sizing the installation for the whole house from the outset, with a budget starting at €8,000 and rising to €18,000 depending on the output and whether or not it includes domestic hot water production. There is no ‘low-cost trial’ option for air-to-water systems: it is a fully-fledged central heating investment, not a supplementary heating system.
Illustrative image of a heat pump. A compact, high-performance outdoor unit from the VivAir Lite range.

What if the answer were both?

This is the most common solution in Belgian homes that have both a genuine need for central heating and issues with overheating in a specific room during the summer, such as a south-facing living room or bedrooms under the eaves. An air-to-water system provides background heating and domestic hot water for the whole house, whilst an air-to-air unit, installed only in the room in question, takes over to provide cooling in summer, without having to oversize the entire system for an occasional need.

This approach avoids a common pitfall: installing a reversible air-to-water system to cool the whole house via underfloor heating, when the result (a temperature reduction of 2 to 4 °C, with no draughts) is still milder than that of a proper air-conditioning system. In the room or rooms where there is a real need for cooling, a targeted air-to-air system remains more effective.
Windfree Comfort monosplit pack. Available in the Avant Black version and the Elite version, which features a motion sensor and antibacterial, antiviral and anti-allergy filters.

Bonuses and VAT: the real difference in 2026

From a tax perspective, the two technologies are no longer as different as they used to be. Since 1 January 2026, the reduced VAT rate of 6 per cent has applied to both air-to-water and air-to-air systems, in all homes and throughout Belgium. It is in terms of regional grants that the difference remains clear: in Wallonia and Brussels, only air-to-water systems qualify for energy renovation grants, as they are considered to be fully-fledged central heating systems. In Flanders, both technologies are supported, with more modest amounts for air-to-air systems (€300 to €600 depending on income, compared with significantly higher amounts for air-to-water systems).

This imbalance is a significant factor in the decision to install a full central heating system in Wallonia or Brussels, where air-to-water systems remain the only option eligible for subsidies. It is a much less significant factor for a targeted air-conditioning project, where air-to-air systems have never been eligible for the same subsidies anyway.

Real products to help you visualise your future

At Facq, the difference in price is immediately apparent across the available ranges. In the air-to-air category, Samsung’s Windfree Comfort single-split pack starts at €1,998.36 (including VAT) for one room, the Bosch Climate 3000i pack from €1,549.22 including VAT, and the Innova Aircoheater 2.0 is priced at €2,600.29 including VAT. For a multi-split system, an outdoor unit such as the Bulex VivAIR Lite (€758.67 including VAT) can be combined with several indoor units depending on the rooms to be cooled.

In the air-to-water sector, complete systems are categorised by power output ranges rather than as single packages: the manufacturers distributed by Facq (Bulex, Vaillant, Bosch, Daikin, Mitsubishi Electric) offer single-unit or split-type outdoor units sized according to floor area and domestic hot water requirements, with a total cost of between €8,000 and €18,000 including installation. Mitsubishi Electric’s Zubadan technology and Daikin’s Altherma 3 range remain the benchmark for projects in harsh climates or where a high COP is required.

Frequently asked questions

  • Q1. Can you heat an entire house with an air-to-air heat pump?

    Technically, yes, with a multi-split system covering each room, but this is almost never the most sensible solution: an air-to-air system does not produce domestic hot water, so a separate water heater is required, and the thermal comfort is less consistent than with central heating via radiators or underfloor heating.

  • Q2. Is air-to-air really cheaper than air-to-water?

    At the time of purchase, yes, significantly: a single-split unit starts at around €2,000, compared with a minimum of €8,000 for a complete air-to-water system. But it does not provide the same level of service: it does not heat domestic hot water and does not replace a full central heating system.

  • Q3. Why is the air-to-air system not recognised in Wallonia and Brussels?

    This is because regional schemes are aimed at replacing central heating systems, a role that only air-to-water systems fulfil. Air-to-air systems, regarded as comfort or air-conditioning equipment, do not fall within this framework in these two regions. Only Flanders provides support for them, albeit at a lower level.

  • Q4. Can an air-to-air heat pump be installed alongside an existing boiler?

    Yes, and this is a common setup. The boiler or air-to-water system provides the main heating and domestic hot water, whilst the air-to-air system handles summer cooling in one or two specific rooms, without affecting the rest of the system.

  • Q5. Which solution should you choose for a flat?

    Air-to-air systems, in the vast majority of cases. They can be installed without affecting the building’s central heating system, require no drilling or major alterations, and effectively meet the need for air conditioning in summer – which is often the real priority in flats.