The Air-Water heat pump: how does it work?

This fashionable heating appliance, acclaimed for its use of renewable air energy, often remains a mystery as to its operating principle. This article will help you understand the principle behind Air-Water heat pumps.

An air-to-water heat pump absorbs calories from the outside air thanks to the endothermic nature of a refrigerant's evaporation reaction. It distributes the calories to the water in the domestic network thanks to the exothermic property of the condensation of the same refrigerant.

The above definition is a "complicated" way of explaining things, but it's possible to simplify things, and once you've understood it, you can go into more detail.

The principle of operation of the PAC Air Water popularized.

Fridge analogy

We all have a fridge at home. If you want it to work, it's best to leave the door closed. The reason is simple, and we're all intuitively aware of it. It's because we need to limit the size of the volume to be cooled. You only want to cool the food in the fridge.

But what happens if you open the fridge door? It's obvious that the food will heat up in contact with the ambient air. The fridge is still running because it doesn't know the door is open. Consequently, without knowing it, it's trying to cool a volume far too large for its small compressor - it's trying to cool the whole room.

As a result, his little compressor keeps on working. It looks at the set point it has to reach, and until it gets there, it keeps going. And the condenser at the back of your fridge is working at full capacity. It condenses, it condenses, so it heats up, because condensation is a reaction that releases heat (exothermic).

Your fridge has become a heater: it has in front of it the volume of the room to be cooled as long as its door is open. For it, this is virtually infinite and impossible to cool. So it runs in a loop and heats by condensation.

The same principle applies to heating your home with a heat pump. Except that you don't "open the door" to the volume of the room, but to the entire outside environment, the garden...the planet, in fact. So you have an "infinite" source to cool. As a result, it's possible to heat the house by condensing the water from the domestic network at the condenser: the refrigerant redistributes the calories taken from the outside air.

Counter-intuitive to take calories from cold air

It may seem counter-intuitive to heat a room using energy from the outside air, when it's freezing cold outside in winter (-10 / -15°C). And it's precisely in winter that it needs to heat the most.

But it's important to remember that whatever the air temperature, it still contains calories that can be pumped out. Of course, the colder and drier it is, the fewer calories it contains, which is why heat pumps work a little less well in winter. In other words, they consume more electricity to provide the same amount of heating.

This fact naturally appears counter-intuitive to us because we instinctively know that heat always passes from the warmest to the coldest environment. You know that if you open the window in winter in the middle of the night, you will quickly shiver because all the heat in the house will have gone out the window.

The magic of the heat pump is to achieve the opposite! In other words, heat is transferred from the coldest to the warmest medium. This is where the term "heat pump" comes from, as heat is pumped from the natural environment. Just as a well pump overcomes gravity by moving water from its lowest point of potential energy to a higher point of potential energy, so it fights this natural transition from hot to cold.

More details on the operation of the air-water heat pump

You should have an intuitive grasp of the principle of the air-to-water heat pump by reading the first two paragraphs. Now let's get into the details. What is the mystery behind the heat pump's ability to reverse the natural principle of heat transfer?

The refrigerant: the heat pump's orchestra conductor

The magic of the heat pump is due in large part to the property of refrigerants to evaporate at very low temperatures, thus capturing energy from the natural environment even in winter.

We all know that water, the most common natural fluid, evaporates at around 100°C. But this only applies under standard conditions, i.e. at atmospheric pressure 1013 HPa. It then changes from liquid to vapor, and is said to evaporate. This evaporation reaction is endothermic, meaning that it absorbs energy (calories) from the environment in which it takes place.

However, if you're at the top of Mount Everest, the atmospheric pressure is not at all the same: it drops by almost 3 times (around 350HPa). Water evaporates much faster, in this case from around 70°C. This illustrates the fact that it is possible to alter the pressure of a fluid and thus modulate its evaporation temperature.

It's easy to make the connection with the heat pump. If we can modulate the pressure of the refrigerant gas according to our wishes, then we can evaporate it at the temperature we want, and in this case we want it to evaporate at the temperature of the outside air, in order to capture its calories.

Let's take the example of the refrigerant gas R32, which will soon be found in all residential heat pumps, since R410A is slowly moving towards a programmed ban (2025) and all manufacturers are on board. Its boiling (evaporation) temperature is -51.7°C at atmospheric pressure. If we're in Siberia, that's fine, but we're not in Siberia, so we're going to have to modulate the pressure so that it evaporates when it's -6°C, for example. It's going to have to rise towards 6500HPa for that to happen.

This is why, in addition to the heat exchangers (air evaporator + water condenser), heat pump systems have two components that modulate fluid pressure. These are the compressor and the expansion valve. Placed respectively after and before the evaporator, they enable adaptive regulation of the gas pressure, so that whatever the temperature outside, the gas can evaporate as it should and capture the maximum number of calories (depending on heating requirements).

Special case of the air-water heat pump

For an aerothermal heat pump, heat is collected from the outside air at the evaporator. They are then transferred to the domestic water network via a water-cooled condenser, in which the refrigerant passes on one side, and the water on the other. Heat exchange takes place through the walls of the exchanger, and there is of course no contact between the water and the refrigerant.

Some brands use a plate heat exchanger to manufacture the condenser, while others use a coaxial condenser, which is generally more reliable and durable, and requires less maintenance.

In the case of a split heat pump (separate indoor and outdoor units), the condenser is located in the indoor module. The refrigerant circulates between the indoor and outdoor modules.

In the case of a monobloc heat pump (everything is in the outdoor unit), the condenser is outside the unit and water circulates between the unit and the inside of the house. The water recovers heat from the condenser and exchanges it with the buffer tank in the equipment room, via an exchanger.

Also read on the subject:

How much does a heat pump consume?

What are the disadvantages of a heat pump?

What should I choose between a single or split heat pump?

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.