Why can't an air-to-water heat pump provide air conditioning?

If you're looking for a heat pump to provide your heating, you may be thinking of killing two birds with one stone and using it to air-condition one or more rooms in your home.

An air-to-water heat pump cannot provide air conditioning in the true sense of the word. In fact, it can cool the water in your network thanks to a 4-way valve that makes it reversible, but the dew point principle limits the temperature that can be reached.

Later on, we will clarify this notion of dew point, and then we will see which type of heat pump to use to really air-condition, which models to choose, and if heating with this type of heat pump is efficient.

The air-water heat pump cools but does not air-condition : detail

The difference between air-conditioning and cooling is the drop in temperature compared with the room's original temperature. In fact, air-conditioning allows a temperature difference of around 7°C, whereas cooling allows a maximum of 3 or 4°C.

A heat pump is capable of producing heating thanks to its refrigerant circuit. In some cases, the refrigeration circuit is reversible, thanks to the installation of a 4-way cycle reversal valve, enabling the heat pump to cool the water in the network.

However, as the mains water passes through the pipes of your radiators or underfloor heating system, it must be prevented from falling below a certain temperature, known as the dew point temperature. If the temperature falls below this point, there's a risk of condensation on your radiators or underfloor heating. This requires the installation of a temperature control system that adapts to changing conditions, and follows the evolution of the dew point.

In other words, you risk dripping water on your radiators or floor, which is annoying and not in keeping with the principles of hygiene in the home.

Therefore, if you decide to reverse the cycle with a 4-way valve, you must also limit the temperature by keeping it above the dew point.

If it's summer, there's usually a fair amount of humidity in the air. Let's take the example of a hot day with a temperature of 28°C and a humidity level of 45%. So the dew point is 15°C. You can't run water colder than that. As a result, we won't be able to air-condition, but just cool things down by a few degrees.

So how do you air condition with a heat pump?

To really air-condition with a heat pump, we're going to have to move away from water as the medium for transporting frigories, and opt for air.

So we're going to opt for an air-to-air heat pump that can blow fairly cold air without the risk of condensation on a surface. The air is pulsed into the room in an intelligent way to avoid unpleasant draughts, and mixes with the warm air to cool it. This is how you can talk about air conditioning, because you can pulse really cold air. Even if we're going to limit ourselves so as not to create discomfort.

Depending on the required setpoint, the air-to-air heat pump or air conditioner discharge temperature may be below 10°C. The closer the ambient temperature gets to your setpoint, the higher the supply temperature rises, eventually balancing with the ambient air and stopping when the setpoint is reached.

Which air-to-air heat pump models should I choose to air-condition a room or a space?

Many brands offer wall-mounted air conditioners, but the Japanese are at the head of the pack. Indeed, Mitsubishi, Daikin or other Toshiba dominate the air conditioning market in France. You will see them everywhere, especially in the South.

3 models of air-to-air heat pumps or air conditioners to consider

Mitsubishi Wall Mount Compact+ MSZ-FT

This reversible air conditioner comprises an outdoor unit and an indoor unit. This model is available in 3 sizes: 25 / 35 and 50, with cooling capacities of 2.5 / 3.5 and 5kW respectively. Extremely quiet, this model also provides heating in winter, with outputs ranging from 3.2 to 5kW depending on unit size. The SCOP is between 4.3 and 4.6. These models are eligible for the CEE bonus.

Photo of indoor unit

Diagram of the outdoor unit (to be placed in front of the building, hung on the wall or placed on the ground)

Daikin Emura 3

Daikin is a renowned Japanese brand offering a wide range of air-to-air heat pump and air-conditioning solutions. The most stylish of these is the Daikin Emura 3.

It comes in 3 colors: white, silver and matte black, to suit all interiors.

The cooling power ranges from 2 to 5kW at +35°C for an EER (= cooling capacity) between 3.7 and 4.7. In heating, this air-to-air heat pump also works and gives back between 2.5 and 5.8kW of power at +7°C outside.

In addition to its attractive design, this model is very quiet thanks to an indoor unit fan that optimizes air circulation, producing a sound level of just 19dBA... for the 3 least powerful models, and from 21 to 24dBA for the two largest models FTXJ42 and FTXJ50, which develop 4.2 and 5kW of cooling power.

Toshiba Yukai

The Japanese brand Toshiba produces the Yukai, an air-to-air heat pump that is also very quiet, both indoors (19dBA sound pressure at a distance of 1m from the unit, for the smallest models) and outdoors (42dBA sound pressure at 1m).

The Toshiba Yukai develops between 1.5 and 6.5 kW of cooling power, depending on the model. The EER is up to 4.17 for the smallest models, and 2.89 for the largest (6.5 kW cooling).

In heating, the Yukai gives from 2 to 7kW with COP between 3.3 and 4.2 at +7°C. The SCOP in heating mode is always around 4.5.

indoor unit
external group

See the technical and presentation sheets for these 3 models of air-to-air heat pumps :

Can we heat with an air-to-air heat pump?

An air-to-air heat pump can of course heat, if its cycle is reversible. It can operate in heat pump mode or in air conditioning mode.

So the real question is: Is it advisable to heat with an air-to-air heat pump? What are the advantages or disadvantages compared to an air-to-water heat pump?

Air, like water, is a medium for transporting energy. But its mass heat capacity of 1005 J K-1 kg-1 is more than 4 times lower than that of water, which is 4185 J K-1 kg-1. This makes it a much less efficient means of storing and transporting heat than water.

For these physical reasons, it is often preferable to use an air-water model to provide heating.

Moreover, heating a room with air can generate more discomfort than with water. Air circulation in the room, if poorly controlled, can lead to annoying draughts. And since buildings are subject to all kinds of air leaks and other thermal bridges in their thermal envelope, heat loss is more rapid.

Water is distributed via pipes to radiators or underfloor heating. The disadvantage is that it requires regular de-silting of the system, which can become clogged. But the advantage is that it transports more heat per unit of mass, and diffuses it more pleasantly, notably via the floor.

It should be noted, however, that hydraulic heat distribution is much more expensive than an air-source system. In fact, an air-to-air heat pump requires no special installation to distribute the heat, whereas an air-to-water heat pump requires either radiators, which are often quite expensive if you want quality, or underfloor heating, which requires substantial work: purchase of equipment, pipes, feeders, etc., and pouring of concrete slabs.

Generally speaking, for applications where you're not present all the time, i.e. office buildings or sports halls, you can opt for air-to-air heat pumps, because they're less expensive, comfort is assured, and when you leave you can switch them off. The heat pump will even have the advantage of being able to air-condition, which is a superb luxury and ensures very good working or exercise conditions for a few hours during the day.

But if you need to heat a building in which you live continuously, such as your villa, in which your small family lives all year round. Then it's best to opt for an air-to-water heat pump, which will ensure top-quality heating, and perfect comfort if properly sized and installed.

If you want examples of air water heat pump models, you can browse our top 20 air water heat pumps.

Read also about the differences between air conditioning and water heating:

What are the differences between heat pump and reversible air conditioning

Some reasons not to choose a heat pump for heating

How does a heat pump work?

About the author

Juliena mechanical engineering graduate and specialist in climate engineering since 2009, has become a writer specializing in renewable energies, with expertise in heat pumps and photovoltaic solar panels for individual housing.