One of the most interesting features of a heat pump is that it absorbs thermal energy available in the environment – for example, in the outside air, the ground or water – and then raises it to a higher temperature so that it can be used to heat the building.
It uses a closed refrigerant cycle, the four key components of which are the evaporator, the compressor, the condenser and the expansion valve. As the refrigerant circulates through the system, its pressure, temperature and state of matter change continuously as it passes through these components.
But what exactly happens inside the machine? Let’s take a look at the parts of the heat pump, whilst also following the path of the refrigerant.
How is a heat pump constructed?
If we want to understand how a heat pump works, it is worth first distinguishing between the refrigeration circuit and the entire mechanical system. The four basic components of the refrigeration circuit are:
- the evaporator
- the compressor
- the capacitor
- the expansion valve
The refrigerant flows between these, whose function is to transport thermal energy. According to Daikin’s description, during the cycle the refrigerant absorbs heat in the evaporator, the compressor increases its pressure and temperature, it releases heat in the condenser, and the expansion valve reduces its pressure once again. The process then starts all over again.
Of course, a complete heat pump may consist of many more components than this. Depending on the design, it may also contain a fan, a circulation pump, sensors, control electronics, a changeover valve, various safety devices and additional heat exchangers.
Not all heat pumps are built in the same way: an air-to-water system and a ground-source heat pump system require different designs. For more details on the differences, see the A comprehensive guide to heat pump types — air-to-water, air-to-air, geothermal, hybrid as we wrote in our article entitled ….
How a heat pump works in detail: let’s follow the path of the refrigerant
To understand how it works, we must first clarify one fundamental point: A heat pump does not „produce” heat; it actually transfers heat from a medium at a lower temperature to one at a higher temperature. This requires electrical energy, and one of the main consumers of this energy is the compressor.
Thanks to the special thermodynamic properties of the refrigerant circulating in the heat pump, it is capable of evaporating even at relatively low temperatures. It is precisely this property that we make use of during the cycle. Let’s start with the evaporator.
1. Evaporator: this is where the heat pump absorbs heat from the environment
The evaporator is a heat exchanger. During heating operation, energy from the environment is transferred to the refrigerant via this.
The source of energy may depend on the type of heat pump. In the case of an air-to-water heat pump, the heat may be drawn from the outside air; in a geothermal system, from the ground; and in a water-to-water system, from a suitable water source.
A low-pressure, low-temperature refrigerant enters the evaporator. As it absorbs heat from the surroundings, the liquid refrigerant evaporates, leaving the evaporator as low-pressure vapour.
But how can heat be extracted from cold air?
At first glance, it may seem strange that a heat pump is capable of extracting heat even when the outside temperature is below freezing. The explanation is that air at 0 °C or even below zero is not „heat-free”. For heat transfer to occur, the evaporation temperature of the refrigerant must be lower than the temperature of the heat source. In this way, energy can flow from the cold outside air to the even colder refrigerant. Daikin also points out that air-to-water heat pumps are capable of extracting energy from the outside air even in cold weather.
2. Compressor: the „engine” of the heat pump”
The refrigerant leaving the evaporator is already in a gaseous state, but is still at low pressure. In this state, however, it would be unable to transfer heat to the building’s heating system at a sufficiently high temperature. This is where the compressor comes into play.
The compressor compresses the refrigerant vapour. As a result of the compression, its pressure and temperature rise significantly, so that hot, high-pressure refrigerant vapour is discharged from the compressor.
To put it simply, then:
low-pressure steam → compressor → high-pressure, high-temperature steam
The compressor is used to bring the refrigerant to a temperature at which it is able to transfer heat to the building.
Why does a heat pump use electricity?
A heat pump does not, therefore, move heat without using energy. It requires electrical energy to power, amongst other things, the compressor and, depending on the system’s design, the fans, pumps and control systems.
This also helps to understand the concept of COP, which is often mentioned in connection with heat pumps.
If, at a given operating point, a piece of equipment transfers 4 kWh of thermal energy to the heating system whilst consuming, for example, 1 kWh of electrical energy, then its COP is 4. A significant proportion of the heat transferred is energy absorbed from the environment, and we use electrical energy to raise this to a higher temperature.
3. Condenser: this is where the heat is transferred to the heating system
The next stage for the hot, high-pressure refrigerant coming from the compressor is the condenser. This is also a heat exchanger, but in this case the process is reversed: the refrigerant releases heat. In an air-to-water heat pump, this heat is transferred to the water circuit, which then heats the building via, for example, underfloor heating, radiators or fan coils. If the system is designed correctly, the heat pump can also be used to produce domestic hot water. As the refrigerant releases its heat, it cools down and condenses: the vapour turns back into a liquid.
Why is a low flow temperature beneficial?
When it comes to the energy efficiency of a heat pump, it matters how high a temperature it needs to raise the water to. A low-temperature underfloor heating system can heat a building using a significantly lower flow temperature than a conventional radiator system designed for higher temperatures. The more favourable the temperature conditions on the heat source and heat sink sides, the more efficiently the heat pump can operate. The European Commission’s consumer guide also highlights that, with a low-temperature heating system, the heat pump’s performance and efficiency are generally better.
4. Expansion valve: the refrigerant pressure drops again
The refrigerant leaving the condenser is in a liquid state, but is still under high pressure. In order for it to be able to absorb heat at a low temperature in the evaporator, its pressure must be reduced. This is the function of the expansion valve.
As the refrigerant passes through the valve, a sudden drop in pressure occurs, causing the temperature of the refrigerant to fall significantly. After passing through the expansion device, a cold, low-pressure mixture of liquid and vapour continues on towards the evaporator.
In modern systems, an electronic expansion valve can also be used, which can regulate the flow of the refrigerant and the superheat more precisely. This brings us back to where we started. The refrigerant re-enters the evaporator, absorbs heat from the environment, evaporates, and then reaches the compressor. This cycle repeats as long as the building has a heating requirement.
How a heat pump works in 4 steps
To summarise the process briefly, the operation of a heat pump can be broken down into four basic steps:
- Evaporator: the refrigerant absorbs heat from the surroundings and evaporates.
- Compressor: compresses the gas, thereby increasing its pressure and temperature.
- Condenser: the refrigerant transfers heat to the heating system whilst it condenses.
- Expansion valve: reduces the pressure of the refrigerant, thereby enabling it to absorb heat from the environment once again.
The process therefore repeats itself over and over again:
Evaporator → compressor → condenser → expansion valve → evaporator
What happens to the refrigerant in the meantime?
It is even easier to get an overview of the phase changes of the refrigerant in a table:
| Section | Press | Temperature | Typical condition | What’s going on? |
|---|---|---|---|---|
| Vaporiser | Low | Low | Liquid-vapour → vapour | It absorbs heat from the environment |
| After the compressor | Tall | Tall | Steam | Its pressure and temperature are rising |
| Capacitor | Tall | It is falling from a high level | Vapour → liquid | It transfers heat to the heating system |
| After the expansion valve | Low | Low | Liquid-vapour mixture | Its pressure and temperature are dropping |
Selecting the appropriate refrigerant is therefore a technical issue that affects the system’s operation, design and maintenance alike. Among the latest heat pumps, we are increasingly encountering units that operate using R290, i.e. propane, as the refrigerant. The European Commission classifies R290 as one of the climate-friendly, non-fluorinated alternative refrigerants for heat pumps. We set out the technical and maintenance implications of this in detail in the Why R290 is the refrigerant of the future? - A change of mindset in servicing in our article entitled ….
Where does the heat that the heat pump uses come from?
The basic principle of the cooling circuit described above is the same in many different types of heat pumps. However, there is a significant difference in where the system extracts the heat that it raises to a higher temperature.
In air-to-water and air-to-air heat pumps, the outside air serves as the heat source. In geothermal systems, energy is extracted from the ground. The choice of heat source has a significant impact on the system’s design, capital costs and operational characteristics.
And where is the water or glycol in all this?
It is worth clearing up a common misunderstanding here: the refrigerant and the water or water-glycol mixture circulating in the heating system are not the same. The refrigerant circulates within the closed cooling circuit described above, between the compressor, the condenser, the expansion valve and the evaporator. The heating water, on the other hand, forms part of the hydraulic system: it carries the heat transferred via the condenser on to, for example, underfloor heating or radiators.
This can be particularly important in the case of monoblock heat pumps, as in certain designs the heating water circuit extends all the way to the outdoor unit. In the event of a power cut or circulation problem during periods of sub-zero temperatures, frost protection is therefore a particularly important consideration. This is explained in detail in the Glycol or water - can a monobloc heat pump freeze? We explore this in our article entitled […].
What determines the efficiency of a heat pump?
It’s now easier to understand why It is not possible to determine, on the basis of a single piece of data alone, how cost-effectively a heat pump will operate. The colder the heat source, the greater the „temperature gap” the system must bridge in order to produce heating water at a higher temperature. This is why factors such as the outside temperature, the required flow temperature, the design of the heat exchangers, the control of the compressor, and the correct sizing of the entire system are all important.
The design of radiators and other heat emitters is also important in terms of the energy efficiency of heat pumps, as the building’s heat demand and the output of the heat emitters determine the required flow water temperature. The heat pump and the associated mechanical systems as a whole determine how the heat pump will actually operate.
What can go wrong with the main components of a heat pump?
As the operation of a heat pump is the result of several interlinked processes, a fault can have numerous causes. Problems may be caused, for example, by a compressor fault, a fault with the fan or one of the circulation pumps, a problem with a sensor or the control system, a malfunctioning expansion valve, a dirty heat exchanger or a fault in the refrigerant circuit.
| Heat pump component | Role | What happens to the refrigerant? |
|---|---|---|
| Vaporiser | Absorption of ambient heat | It absorbs heat and evaporates |
| Compressor | Increasing pressure and temperature | The steam condenses and heats up |
| Capacitor | Heat transfer to the heating system | It releases heat and condenses into a liquid |
| Expansion valve | Pressure reduction | Its pressure and temperature drop |
In practice, of course, different manufacturers and product ranges may implement this same basic principle using different technical solutions. If we want to navigate our way through specific pieces of equipment, the Daikin Altherma overview: Altherma 3, 3H HT, M – which one is the right choice in which situation? In our article entitled […], we present a number of solutions designed for various different areas of application.
Frequently asked questions about the components and operation of heat pumps
What are the most important parts of a heat pump?
The four fundamental components of the cooling circuit in a conventional vapour-compression heat pump element the evaporator, the compressor, the condenser and the expansion valve. The refrigerant circulates between these components, absorbing heat from the environment, raising its temperature and transferring it to the heating system. The compressor It is often referred to as the „heart” of the system, as it increases the pressure and temperature of the refrigerant.
What does the expansion valve do?
The expansion valve reduces the pressure of the liquid refrigerant coming from the condenser. As a result of the drop in pressure, the refrigerant cools down and passes into the evaporator as a low-pressure mixture of liquid and vapour, where it can absorb heat once again.
Why is a heat pump able to extract heat even from cold air?
This is because even air at sub-zero temperatures contains thermal energy. If the vapourisation temperature of the refrigerant is even lower than this, heat can flow from the outside air into the refrigerant.
What is the difference between how a heat pump and an air-conditioning system work?
The basic refrigerant cycle is very similar; indeed, a split air-conditioning unit that is also suitable for heating functions as an air-to-air heat pump in its own right. The main difference lies in the medium from which the system absorbs heat and the medium to which it transfers heat, as well as the specific application and operating conditions for which it is designed.
Why does a heat pump work well with underfloor heating?
Thanks to the large heat-emitting surface area of underfloor heating, it is capable of heating a suitably designed building even at a relatively low flow temperature. This is advantageous for the heat pump, as it has to bridge a smaller temperature difference, which generally results in better energy efficiency.
The operation of a heat pump is based on a simple principle
To understand the structure of a heat pump, we therefore need to be familiar with the operation of four key components. The evaporator absorbs heat from the environment, the compressor increases the pressure and temperature of the refrigerant, the condenser transfers the heat to the heating system, and the expansion valve reduces the pressure once again. The cycle then begins again from the start.
The basic principle seems simple, but designing a system that works well is a complex building services engineering task. The heat source, the heat emitters, the flow water temperature, the refrigerant, the hydraulic design and the equipment itself all influence how the heat pump will perform in everyday use.
For this very reason, when selecting a heat pump, it is worth asking not only what type of unit to buy, but also how that particular unit fits into the building’s overall mechanical systems.