Air-to-Air Heat Pump: Heating with Air Conditioning Units

Many people initially think of summer, heat and cool air in the living room when they hear air conditioning. Few know that the same technology can also heat efficiently and cost-effectively. Yet modern air conditioning units are full-fledged heat pumps.

Because air conditioning units are so-called air-to-air heat pumps. They extract heat from the outside air and release it directly into the room air. Without radiators, underfloor heating or water-bearing pipework.

This type of heating is not a future experiment, but has been proven in practice for decades. Especially for single-family homes, it offers a simple and flexible solution for gradually saying goodbye to oil or gas. And with manageable effort.

Anyone who already owns an air conditioning unit can usually also use it for heating without any additional effort. And anyone starting from scratch has the option of equipping room by room with split or compact units, also as a supplement to the existing heating system.

Especially exciting: Unlike the classic air-to-water heat pump, which must be connected to the radiators or to the underfloor heating, an air conditioning unit already includes the complete heat transfer system.

Major conversions to the heating system are not needed. A buffer storage tank is also not necessary. And that is exactly what makes air conditioning units a real alternative, especially in existing older buildings.

In this article, you will find out how air-to-air heat pumps work, for which houses they make sense, what they cost and what you should pay attention to when choosing one.

What is an air-to-air heat pump?

Air conditioning units can not only cool, but also heat very efficiently. What may be surprising at first glance is, on closer inspection, a well-designed technology that can work in both directions:

In cooling mode, they extract heat from the room air and transfer it outside. When heating, this process is simply reversed. The heat extracted from the outside air is transported into the house via the indoor unit.

The key point here: It requires no water-bearing pipework and no radiators or anything similar. Heat transfer takes place from air to air.

The difference from a “normal” heating heat pump lies in the heat distribution system. While air-to-water heat pumps have to be connected to existing heating surfaces such as radiators or surface heating systems, an air conditioning unit has its own heat transfer system.

The indoor unit is a modern high-performance radiator, only with air. Thanks to the fine fin structures, it achieves an enormous surface area. Even the smallest commercially available units have a heat exchanger surface of 7 square metres, which means they can work efficiently even with possible low heating temperatures.

Because the entire system is factory-matched, and also heats efficiently when oversized, it does not even require a complex heat load calculation, which would be mandatory for air-to-water heat pumps. You do not have to replace radiators or check pipework. This not only saves time, but also reduces sources of error during planning.

This is a real advantage, especially in existing buildings. Long heating pipes in masonry, poorly insulated pipes and heat losses on the way from the heating unit to the room reduce the efficiency of air-to-water heat pumps. None of that matters here.

Installation is also comparatively straightforward. The technology arrives at the house almost ready for operation. As soon as the units are mounted, electrically connected and connected with the refrigerant line, the system is ready for use.

By contrast, an air-to-water heat pump often requires additional components such as a buffer storage tank, circulation pumps and also hydraulic balancing has to be carried out. Every additional component reduces efficiency. With the air-to-air variant, all of this is eliminated.

Air conditioning or classic heat pump? The small but important difference

Anyone who already owns an air conditioning unit can usually also use it for heating without any problems. This saves fossil fuels and lowers heating costs. Even if the heating output of the air conditioning unit does not match that of the existing oil or gas boiler, it is at least sufficient to cover the basic heat demand.

Especially in well-insulated buildings, rooms can be heated with it all year round. And another advantage is: Anyone who does not yet use air conditioning units does not have to convert everything immediately. The conversion can be carried out step by step. This gradual conversion makes the switch plannable, financially easy to calculate and lets you slowly become familiar with the technology.

Split units are particularly suitable for single-family or two-family houses, but are also increasingly being used in apartment buildings. So-called single units consist of an indoor and an outdoor unit. One single unit is used per room. If several rooms are to be supplied, so-called multi-split systems are a good choice.

In this setup, one outdoor unit supplies up to five indoor units. This reduces the space required outside and helps to better comply with noise limits. For installing single units, you usually only need a normal alternating current connection. For the more powerful multi-split systems, three-phase current is usually required.

Monoblock air conditioning units: Heating and cooling without an outdoor unit

In addition to the split systems with indoor and outdoor unit already mentioned, there are also compact units that are installed entirely inside the apartment. These so-called monoblocks sit on the external wall and exchange heat with the outside air via two air ducts in the wall (drill holes).

Because no outdoor unit is needed, they are also suitable for rented apartments or condominiums with limited outdoor space. Installation only requires two core drillings and a socket. A refrigeration technician is not required for this.

Especially with old night storage heaters or decentralised gas heaters, replacing them with these wall-mounted air conditioning units can be a sensible solution. The monoblocks heat and cool somewhat less efficiently than split systems, but they are still many times better than night storage heaters. In addition, the existing electrical installation can largely continue to be used.

In urban locations, where central heating systems are difficult to implement, these units offer a simple alternative. Even in homeowners’ associations, where larger conversions often fail, individual apartments can be independently converted to air-to-air heat pumps.

Of course, there are also limitations. The units create air movement and make noise. Anyone who is sensitive to drafts or needs absolute silence in the living space should seek good advice beforehand.

7 good reasons to switch to air heating

1. Easy to retrofit: Particularly interesting in existing buildings

What many people do not know: Air conditioning units can also be retrofitted very easily. No water-bearing pipes, no radiators, no underfloor heating are needed. The units arrive at the house preconfigured and, in principle, only have to be mounted on the wall, connected to the refrigerant line and electrically connected.

This is a real advantage, especially in existing buildings. You do not need complex planning, do not have to tear open walls, lay pipes or replace heating surfaces. The question of whether the radiators are suitable for the heat pump does not arise here either. This not only saves effort, but also reduces many sources of error that can occur with an air-to-water heat pump.

2. Dual function: Heating and cooling with one unit

Another advantage: With an air conditioning unit, you get two functions in one device. In summer you can cool with it, in winter you can heat. Technically, it is the same process, only reversed.

This makes the system particularly interesting for rooms that heat up strongly in summer, such as under the roof or with large window areas. In winter, the unit then ensures that it remains pleasantly warm even in freezing outside temperatures. And quickly and reliably.

3. Low investment with high impact

Anyone who thinks heat pump technology is always and necessarily expensive will be surprised by air conditioning units. Good units are available from around 1,000 to 1,500 euros per kilowatt of heating output. Mobile units or pre-filled monoblocks are sometimes available for under 500 euros per kilowatt.

Compared with air-to-water or even ground-source heat pumps, that is a world of difference. Nevertheless, you get a full-fledged heating system. Just compact, decentralised and with manageable effort. If you also use electricity from a PV system, the running costs fall significantly again.

4. Fast response, no sluggish heating circuit

Air conditioning units respond very quickly. After switching them on, it often takes only a few minutes until warm air flows through the room. Unlike a water-based heating system, where the heating water first has to be heated, sent through the pipes and then transferred to the heating surface, everything here is much more direct.

This is particularly practical in the transitional season. You only need a little heat for one or two hours? No problem. Switch on the air conditioning unit, and once the temperature is reached, the unit can be switched off again or the output reduced. All of this works without delay (inertia) and without heat losses in the pipe system.

5. Less CO₂, more energy efficiency

Anyone heating with an air conditioning unit immediately reduces their CO₂ emissions if they previously heated with oil or gas. Because the unit uses the heat from the ambient air and transports it indoors. However, it requires much less electricity than the amount of heat that is transported. Its efficiency is comparable to classic air-to-water heat pumps.

Why is that? Quite simply, because there are no long pipes, no buffer storage tanks, no circulation pumps that consume energy or cause losses. Everything is compact, perfectly matched and very efficient.

6. Air purification included: Good for allergy sufferers

Modern air conditioning units now come with filters fitted as standard. These filter fine dust and other particles such as pollen from the air. In summer, the dehumidification function also ensures that the indoor climate remains pleasant.

In winter, the air is not only heated, but cleaned at the same time. This is especially interesting for allergy sufferers, but also for anyone who simply wants clean air in the room. Without any additional devices.

7. Modularly expandable: Retrofit room by room

You do not have to convert the whole house right away. Many people start with just one unit in the living area. If that works well, more rooms follow later, for example the office, bedroom and children’s rooms.

This makes the conversion plannable and financially manageable. You can familiarize yourself with the technology and say goodbye to oil or gas step by step. Especially in existing buildings, this is often the more practical route.

But does it suit your house? Requirements and limits

Not every house is automatically suitable for an air-to-air heat pump, but in many cases the system fits better than we think.

Passive houses in particular (highly insulated houses) and energy-renovated old buildings benefit especially, because their heat load is significantly reduced. The units can be used where heat is needed, completely without distribution losses via long pipework.

But use can also be worthwhile in uninsulated old buildings, for example as a supplement to the existing heating system. Especially in the transitional season or in rooms with particularly high comfort requirements, air conditioning units are an excellent option.

However, there are also limits of use and disadvantages. Anyone expecting central domestic hot water production may be disappointed. Because the vast majority of air-to-air heat pumps are pure space heating units. For hot water supply, an additional solution is usually needed, for example a hot water heat pump. Only a few air conditioning units also offer a connection to domestic hot water production.

You should also keep the acoustic properties in mind. An air conditioning unit moves air, and that inevitably brings noise with it. Anyone who is sensitive to drafts or noise-sensitive could feel disturbed. However, modern units are usually much quieter than feared.

Whether the investment is worthwhile depends heavily on usage behaviour. Anyone who uses the units in a targeted way benefits from the fast heating effect and high efficiency. Electricity consumption remains manageable, and heating costs comparatively low.

What does an air-to-air heat pump cost?

Anyone who decides on an air-to-air heat pump has comparatively low entry costs. The units themselves start at around 1,200 euros. Depending on size, features and manufacturer, it can also be 1,800 euros. On top of this come installation, possible wall penetrations and electrical connection.

For a single split unit in one room, including installation, you are usually looking at around 2,500 to 3,500 euros. Anyone equipping several rooms at once or opting for a multi-split system should expect total costs of between around 10,000 and 20,000 euros. Practically everything is then included, from the outdoor unit to the last remote control.

Subsidies are also available for air-to-air heat pumps. The funding conditions are basically the same as for air-to-water heat pumps. The basic subsidy for installation is 30% of the eligible total costs.

This base rate can then be increased to a maximum subsidy rate of 70% through various additional and bonus subsidies. However, funding is only worthwhile for larger systems, from a funding amount of more than 10,000 euros. For individual systems, the funding programme is too time-consuming.

In operation, however, the system scores points. Electricity consumption remains significantly lower compared with classic electric heating, especially if the system is inverter-controlled (output-adjusting). Anyone who also uses a photovoltaic system lowers running costs noticeably again.

The required maintenance is manageable. Most of it, such as cleaning the filters and the outdoor heat exchanger, you can do yourself. With good care, the units easily run for fifteen years and longer. And even after that, replacement is usually simple because the technology is modular. Anyone who plans cleverly and generates their own electricity can save really substantial heating costs with the air-to-air heat pump.

Practical examples & tips: How to make the switch smoothly

Before you decide on a unit, it is worth taking a closer look at your requirements. Depending on the building conditions and living situation, different variants may be suitable. Wall-mounted air conditioning units work without an outdoor unit.

They can be installed almost anywhere, even in condominiums or where outdoor conditions are difficult. All that is needed are two holes through the external wall and a socket.

With classic split units, more planning is required. Choosing the right location is crucial, not only indoors, but above all outdoors as well. Because outdoor units should disturb no one if possible and therefore should not hang directly under the neighbours’ bedroom window. In densely built-up areas, this can become a tricky task.

The power connection also plays a role. While a single unit often manages with a normal household connection, larger systems as well as multi-split systems may require three-phase current.

A specialist company should be called in for this. It is also important to match the unit to the room size. Too little heating output leads to continuous operation and higher electricity consumption. Too much output causes frequent cycling and reduces service life.

5 mistakes you should avoid during installation:

• Wrong location: If the indoor unit hangs in a corner, the warm air builds up. The outdoor unit must stand freely.
• Unfavourable air distribution: If the airflow in the room is not planned properly, disruptive drafts may occur.
• Too little output: Anyone who chooses a unit that is too small for cost reasons will later have problems with sufficient heat supply.
• No specialist company: Especially with split units, the refrigerant circuit must be installed by a certified specialist company. This is not where you should cut corners.
• Wrong power connection: Multi-split systems usually need three-phase current. Anyone who checks and clarifies this in advance avoids potential trouble during installation.

Conclusion: Who is the air-to-air heat pump worthwhile for?

The air-to-air heat pump is practically interesting for anyone who wants to reduce their oil or gas consumption as quickly, simply and cost-effectively as possible.

Anyone who wants to convert step by step benefits twice over: The technology can be used flexibly, causes hardly any installation effort and is still highly efficient. The technology can also be used sensibly as a supplement to the old existing heating system.

In short: The air-to-air heat pump is a practical alternative for anyone looking for a simple and uncomplicated alternative to oil and gas heating and who does not want to spend too much money on it.

Image sources:

1. Featured image: Air-to-air heat pump: Heating with air conditioning units (Canva Stock)
2. Indoor unit and outdoor unit of a split air conditioning system (Adobe Stock)
3. Split system on the wall below the ceiling (Adobe Stock)
4. Multi-split systems can supply heat to up to five indoor units (Carsten Herbert)
5. Indoor unit of a split air conditioning system (Adobe Stock)
6. Outdoor unit of a split air conditioning system (Adobe Stock)
7. Installation of an air conditioning unit in the attic (Adobe Stock)