How a heat pump works: the ‘refrigerant cycle’?

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The ‘refrigerant cycle’ has four phases: evaporation; compression; condensation; expansion.

The main components of a heat pump system include:


A heat pump takes in heat or cold from the air, water or underground and transforms it into heating or cooling for your building or water.

Sources include ambient air, exhaust air, underground heat, groundwater and water.

The energy from these sources is infinite, meaning it is renewable.

This energy makes up about 80% of the energy needed.

The heat pump captures the heat from the ground, or the air or water. This heat is then used by the heat exchanger, known as the evaporator, to turn the refrigerant in the heat pump into gas.


The refrigerant gas then reaches the heart of the heat pump: the compressor. The compressor compresses the refrigerant gas to a high pressure, which leads to a rise in temperature.

Why this works: High pressure heats up gas, just like a bicycle pump that heats up when you are using it.

To drive the compressor, additional energy is needed. This can come from electricity, gas or thermal energy. This makes up about 20% of the total energy needed to run the heat pump. If green electricity is used, for example from solar or wind energy – then the heat pump uses 100% renewables and is carbon neutral.


On the discharge side of the compressor, vapour which is now hot and highly pressurised passes through the second heat exchanger, called the condenser. This heat exchanger allows the refrigerant to release heat into the heating system for the house. As a result, the refrigerant then turns back from a gas into a liquid state.

The heat coming into the house can do so through an air system, like an air conditioning unit, or a water-based system like floor heating or radiators, known as ‘hydronic’. The indoor unit can also contain a hot water storage tank.

Expansion :

The condensed refrigerant then passes through a pressure-lowering device, known as the expansion valve. The now low-pressure liquid refrigerant can then begin the cycle again.

In addition to these main components, a heat pump system may also include auxiliary equipment and controllers such as cooling fans, evaporator fans, electronic controllers, sensors, etc., to ensure the proper operation and efficient performance of the system.

These components work together in a refrigerant cycle to transfer heat from a low-temperature heat source and deliver it to a high-temperature heat sink, enabling heating or cooling functionality.


(Source:European Heat Pump Association (EHPA))