How Do Heat Pumps Work

Heat pumps are amazing – everyone says that, despite not necessarily knowing how heat pumps work. Unless you are a physics expert, the technology that drives them might seem very vague.

In that sense, they can be as mysterious as the Bermuda triangle.

Let us all go back to the summer or winter of 1856 when Peter von Rittinger developed the first heat pump in Austria. He built it so they could dry out salt from salt marshes… interesting. Since then, there have been a lot of modifications to its mechanism and as expected, its applications have become all the more varied.

We use water heaters, air conditioners, in our houses and offices. Heat pumps can also be used – in some cases via a green homes grant – as a heat source for domestic heating and industrial heating.

Types of Heat Pump

There are different types, each with their specific features and peculiarities.

  • Air to air heat pumps
  • Air to water heat pumps
  • Ground source heat pumps
  • Water source heat pumps

While there is often a renewable heat incentive to use each one, they all work in similar ways, so only the first two will be discussed here.

Air to air heat pumps

The air to air heat pumps is the most common heat pump type. They usually have indoor and outdoor components. They can work as a heating only device or if a reversing valve is incorporated as a heating and cooling device.

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The reversing valve is a component in a heat pump, that changes the direction of refrigerant flow.

Once the direction that the refrigerant is flowing is reversed, the refrigerant cycle of the heat pump is also changed from heating to cooling or in the other direction. Basically, the heat pump system works by circulating the refrigerant through a cycle of evaporation and condensation.

Air to Air heat pump

The major components of an air to air heat pump include the compressor, the reversing valve, indoor heat exchanger, two pairs of expansion valves and their non-return valve, a bidirectional filter drier, sight glass, outdoor heat exchanger and the controller and several temperatures and pressure sensors.



The following description is for the air to air heat pump in heating mode.

Generally, the refrigerant undergoes changes in terms of form, temperature, pressure as it moves from the compressor to the reversing valve to the indoor heat exchanger. It should be noted that the refrigerant leaves the compressor as a vapour at high temperature and pressure and the reversing valve is in a heating system mode suitable for this description.

The cool air that blows over the indoor heat exchanger causes the refrigerant to disperse heat to the room, making the room warm. The loss of heat to the room will cause a change in the form, temperature and pressure of the refrigerant. As such a high-pressure cool liquid leaves the indoor heat exchanger for the non-return valve from where it goes through the filter dryer, the sight glass, then the expansion valve.

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The expansion valve transforms the refrigerant into a liquid and vapour mixture with low temperature and pressure. From the expansion valve, the refrigerant moves to the outdoor heat exchanger. Here the refrigerant gains heat from surrounding outside air because the boiling point of the refrigerant is quite low, it boils, releasing heat energy. The refrigerant moves from the outdoor heat exchanger to the reversing valve. The refrigerant is a vapour at low temperature and low pressure.

The whole process can be repeated starting with the compressor.

How it Cools

A quick look at how the cooling mode works shows the refrigerant – which at this stage is a vapour at high temperature and high pressure – leave the compressor for the reversing valve from where it moves to the outdoor unit. The outdoor unit is built with a fan which blows cool air across the outdoor heat exchanger, in the process it removes heat from the refrigerant (reduces temperature).

indoor heat exchanger


Losing heat means that the refrigerant condenses and leaves the outdoor heat exchanger as a liquid at high pressure and low temperature. It then passes through the non-return valve, then through the slight glass and the filter drier. It then passes through the expansion valve where it emerges as a liquid and vapour mixture accompanied by a decrease in the temperature and pressure of the mixture.

The refrigerant flows into the indoor heat exchanger and a fan blows warm air over the refrigerant transferring heat to it. The refrigerant has an intrinsically low boiling point, as such, it boils and loses heat. The refrigerant enters the reversing valve is a liquid at low temperature and low pressure. From the reversing valve, it is diverted into the compressor and the whole process can be repeated

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Energy efficiency is key in the way air to water heat pump works. As the name implies the transfer of heat is usually between a ‘water system’ and an ‘air system’. Its mode of operation is similar to the air to air heat pumps except that there is no reversing valve.

The refrigerant which is a vapour at high temperature and pressure move from the compressor to the heat exchanger. The cold water enters the heat exchanger and gains heat from the refrigerant which is at high temperature. So the cold water leaves the heat exchanger as hot water, then flows into the hot water storage tank.

The refrigerant loses heat and becomes a liquid with high pressure and reduced temperature. The refrigerant courses through the sight glass and filter drier and reaches the expansion valve. The expansion valve transforms it into a liquid and vapour mixture. From there it moves to the outdoor heat exchanger where it boils on exposure to atmospheric air. It moves to the compressor as a vapour at low temperature and pressure.

For those keen on renewable energy options for their home or office, solar panels are good but they are not the only options available. Heat pumps are also used as energy-efficient central heating options, bringing in air source heat where possible.

So use it wisely.