Air Source Heat Pump Efficiency (Explained with Real Example)

Air source heat pumps can be used to both heat and cool a home and can be considered to be the best way to do so for the foreseeable future thanks to a number of benefits, with one of them being efficiency.

On paper, air source heat pumps can be very efficient but factors such as outdoor air temperature can significantly affect how efficient an air source heat pump can be in a real-world scenario.

Air source heat pumps are very efficient heating and cooling appliances as they don’t generate heat like many traditional heating appliances, but rather transfer it from one place to another. However, lower outdoor air temperatures can reduce heating efficiency.

Our home is fitted with a modern air source heat pump and provides both heating and hot water for our home, but isn’t set up to provide cooling.

Air Source Heat Pump

This heat pump is an efficient model that can generate many more times heat energy than the electrical energy consumed. However, as outside air temperatures fall to freezing and below, efficiency can be significantly reduced where more input power is required to generate the same internal temperatures.

We discuss in more detail below all about air source heat pump efficiency and explain exactly how the efficiency of our heat pump can be affected throughout the year.

How Air Source Heat Pumps Work For Maximum Efficiency

Air source heat pumps (ASHPs) are versatile home energy solutions. They can heat your home or cool it, depending on the season and your needs.

Air source heat pumps transfer heat from one place to another using refrigerants as a medium.

They aren’t the most complicated of appliances but need to have effective design to make them as efficient as possible. In heating mode, ASHPs work by:

  • Using fans to force outdoor air over a series of coils in which refrigerant is flowing through.
  • The refrigerant absorbs the heat from the air and turns into a gas due to a low boiling point.
  • An electrical compressor further increases the pressure of the refrigerant and in turn temperature; therefore, helping to generate the most amount of heat as possible.
  • The refrigerant travels inside where the indoor coil exchanges heat and transfers the heat to the internal heating system, to either air or water depending on the system, which is used to help heat a home.
  • The refrigerant condenses back to a liquid as heat is released and the cycle is repeated.
Air Source Heat Pump Back
A series of coils cover the back and side of our air source heat pump to allow it to capture heat as efficiently as possible.
Air Source Heat Pump
Fans force air through the heat pump to capture heat, and a compressor located in the adjacent compartment helps to maximize temperature by increasing pressure

See our main article on how air source heat pumps work for more information.

Air Source Heat Pump Efficiency Explained

Air source heat pumps are different from standard heating solutions because they don’t burn fossil fuels to produce heat; they use electricity only to help transfer heat from one place to another.

In heating mode, an air source heat pump extracts heat from the air outside and delivers that heat indoors.

In cooling mode, an ASHP will work in reverse and extract heat from indoors and expel it outside, therefore effectively cooling a home.

Air source heat pumps are essentially a reverse refrigeration system and are considered efficient for a number of reasons.

Air source heat pumps typically have a high energy efficiency ratio, which refers to the amount of heat that can be generated from electricity. The higher the number, the more efficient a heat pump can be.

Energy efficiency numbers to look out for can include:

Our own air source heat pump gives information on efficiency by providing its COP/EER at varying outdoor air temperatures.

Air Source Heat Pump Plate
The maximum COP achievable by our heat pump for 2 given outdoor air temperatures

At warmer outdoor air temperatures, our ASHP is able to generate the same level of indoor temperatures while using less electricity as the temperature difference is lower.

This therefore gives a higher COP number as more units of heat energy can be generate from every unit of electrical energy. For example, an average air source heat pump can have a SCOP of 2.5, which means that for every 1kW of electricity, 2.5kW of heat is generated.

You can expect to see air source heat pumps with coefficient performance numbers of anywhere between 2 and 4 or over, depending on the model.

The COP or EER can relate to the energy efficiency of a heat pump at any moment in time. The SCOP or SEER provides an overview of the efficiency performance of an ASHP over either the heating or cooling season.

It can be important to understand the distinction between the two because an air source heat pump will use more electricity in colder months of the year.

For example, the COP for our air source heat pump decreases from 4 to 3 as outdoor air temperatures lower from 7°C (A7) to 2°C (A2), because while the output capacity hasn’t changed, the electrical power input increases to deliver the same output water temperature of 35°C (W35).

Air Source Heat Pump Power Input
The COP reduces for heating at lower outdoor air temperatures because power input increases due to their being a greater temperature difference between the outside air temperature and desired indoor temperature

Even though the efficiency of air source heat pumps decreases as outdoor air temperatures decrease, they’re still very efficient heating and cooling appliances and can deliver more heat energy than the electricity energy consumed.

While efficiency can decrease, heat pumps can still work to generate heat for a home at very low outdoor air temperatures.

Our heat pump can work down to -20°C (-4°F) and generate a sufficient level of indoor water temperatures.

Air Source Heat Pump Operating Temperatures
The outdoor air temperatures ranges in which our heat pump is guaranteed to work within

To further indicate how outdoor air temperature can affect ASHP efficiency, the graph below shows how the output temperature from our heat pump drops as outdoor air temperatures fall.

Efficiency of a heat pump decreases as it can become harder to generate the same output temperatures when outdoor air temperatures get very low

At these very low outdoor air temperatures an air source heat pump will be much less efficient compared to when outdoor air temperatures are above freezing and more ideal.

Are Air Source Heat Pumps Efficient?

Air source heat pumps are very efficient heating and cooling appliances. They can generate multiple times more heat energy from the electrical energy consumed, but efficiency can decrease as outdoor air temperatures decrease.

At optimal, warmer, outdoor air temperatures, an air source heat pump can have a very high Coefficient of Performance (COP) or Energy Efficiency Ratio (EER) ratings as the temperature differences between indoor and outdoor air is lower and less electricity needs to be consumed.

However, at lower outdoor temperatures the efficiencies of air source heat pumps can decrease as air temperature differences increase and more input energy is required.

The seasonal energy efficiency of an air source heat pump (SCOP or SEER) should therefore be considered to understand the performance of any heat pump over an entire heating or cooling season where outdoor air temperatures can vary.

Air source heat pumps can be more efficient compared to other fuel-burning heating appliances such as gas, where they can be less than 100% efficient. With a COP/EER of 3, an air source heat pump can be considered to be 300% efficient.

Things To Consider To Help Keep Your ASHP Running Efficiently

If you’re thinking of installing an air source heat pump, you’ll want to consider the following to help ensure that it remains working as efficiently as possible:

  • Installation. Get an expert installer. If you already have your home’s HVAC system installed, ensure it can accommodate an air source heat pump.
  • Cost. Air source heat pumps can cost more to purchase and install than a standard heating system. Research the different types of heat pumps available and get quotes from different contractors to ensure you’re getting the best deal possible, while still considering the most efficient units available.
  • Maintenance. Heat pumps need to be maintained to work efficiently, and you’ll need to have them serviced annually (or at intervals in line with the manufacturer’s instructions). You’ll also want to ensure you have the right home insulation levels to help keep the heat in.
Air Source Heat Pump Side
Blocked air inlets can affect efficiency and performance and so periodic cleaning and/or de-cluttering of a heat pump is typically advised
  • Location and landscape. Air source heat pumps can be more limited in their placement than other heating systems. An area outside a home must be chosen that’s ideally out of direct sunlight and wind, while still being in an open area to allow for sufficient airflow.
Air Source Heat Pump
Ensuring an air source heat pump is located in an open area will help to maximize efficiency
  • Climate conditions. Air source heat pumps are less efficient at lower temperatures. Their efficiency may start to drop when the air outside falls below 0° Celsius (32 °F). For colder climates consider installing an ASHP off the ground or having a hybrid heating system where a backup heating system can provide warmth if outdoor temperatures drop too low.

Last but not least, don’t forget to factor in your home size. Air source heat pumps are often sized based on the size of the house.

Larger houses would require more heat, but that can be solved with underfloor heating, larger radiators or more air handling units. If a heat pump is sized too small it can compromise efficiency if it’s having to work overtime to compensate.

Further Reading

Pros & Cons Of Air Source Heat Pumps

Are Air Source Heat Pumps Expensive To Run?

Air Source Heat Pump Noise

Is An Air Source Heat Pump Cheaper Than Gas?

Is An Air Source Heat Pump Cost Effective?