A heat pump operates by harnessing heat from the external environment and transferring it into your residence, all the while relying on electricity for this process. The amount of heat conveyed into your home exceeds the electricity consumption of the system.Since a heat pump captures existing heat in the surroundings, it operates without burning any fuel and, consequently, generates no carbon dioxide emissions.

An air-source heat pump (ASHP) has three cycles:

  • The Heating Cycle:pProviding thermal energy to the building
  • The Cooling Cycle: removing thermal energy from the building
  • The Defrost Cycle: removing frost build-up on outdoor coils

In this article we will focus on the heating cycle of an air-to-water ASHP.

Heating cycle

1- Air Intake and Heat Absorption:

  • The ASHP draws in outdoor air using a fan.
  • The outdoor air passes over an evaporator coil containing a refrigerant with a low boiling point.
  • Heat from the outdoor air causes the refrigerant to evaporate, absorbing thermal energy in the process.

2- Compressor Stage:

  • The vaporized refrigerant is compressed by a compressor, raising its temperature and pressure.
  • This compression process increases the energy content of the refrigerant.

3- Condensation and Heat Exchange:

  • The high-temperature, high-pressure refrigerant is then circulated through a condenser coil inside the ASHP unit.
  • As the refrigerant releases heat, it undergoes condensation, transferring the captured thermal energy to a water loop.

4- Water Circulation:

  • The hot water is then circulated through a hydronic system, which can include radiators, fan coils, or underfloor heating pipes.
  • The heat exchangers in these components transfer the heat from the water to the surrounding air in the case of fan coils or directly to the room surfaces in the case of radiators or underfloor heating.

5- Heat Distribution:

  • Radiators, fan coils, or underfloor heating systems distribute the now-heated air or surfaces to the indoor spaces, providing comfortable and efficient space heating.

6- Return to Outdoor Unit:

  • The cooled liquid refrigerant returns to the outdoor unit to repeat the cycle.

Additional notes

The efficiency of the heat pump in transferring heat from the external air to the interior of the is connected to the outdoor temperature. As the outdoor temperature drops, the heat pump’s ability to absorb heat decreases as well.It’s essential to recognize that the majority of air-source heat pumps have a minimum operating temperature, below which they cannot function. For modern models, this minimum temperature typically ranges from -15°C to -25°C. When the temperature falls below this range, an auxiliary heating system must be employed to ensure the building remains adequately heated.

Heat pumps exhibit greater efficiency compared to other heating systems, as they generate more heat than the electricity they consume. This heat-to-electricity ratio is commonly referred to as the Coefficient of Performance (CoP). For instance, if a heat pump boasts a CoP of 4, it produces four units of heat for every unit of electricity it uses.

Every heat pump comes with a published datasheet that specifies its measured CoP. However, this CoP is determined at a specific point in time under controlled test conditions. In real-world scenarios, heat pumps encounter temperature fluctuations throughout the year, such as variations in ground or air temperatures with changing seasons. As a result, the CoP alone doesn’t always provide an accurate representation of the heat pump’s cost of operation and efficiency over the entire year.

To address this, the Seasonal Coefficient of Performance (SCoP) or Seasonal Performance Factor (SPF) is utilized to assess the heat pump’s efficiency across the entire year. Heat pump installers calculate the SPF based on your home’s system design, considering factors like the average local temperatures and radiator size. Before commencing any work, the installer should share this calculation with you, offering a more reliable estimate of operating costs and efficiency compared to the CoP figure.