Air Source Heat Pump COP (Explained)

The Coefficient Of Performance (COP) is an important factor in determining the efficiency of an air source heat pump.

COP can be used to explain how an air source heat pump will perform at different outside air temperatures, with outdoor temperatures being one of the main factors that can influence efficiency of a heat pump.

The Coefficient of Performance (COP) is the ratio of heat output produced by an air source pump in relation to the electrical energy consumed. The higher the COP, the more efficient the heat pump, and a typical COP can range between 2 and 4.

Our own air source heat pump can reach a COP of over 4 in the most ideal conditions, but in less ideal conditions where outdoor temperatures are lower COP can be reduced because more electrical energy input is required to produce the same level of indoor heat.

Air Source Heat Pump
The COP for our heat pump varies depending on outdoor temperatures but can reach over 4 in the most ideal conditions

In this article we discuss more about COP using our own heat pump as an example, what factors can affect the COP of a heat pump at any given time, and how to maximize COP for a heat pump.

Air Source Heat Pumps Explained

Before explaining more about what Coefficient of Performance is, it’s worth providing a brief explanation on air source heat pumps and how they work to give COP some context.

An air source heat pump (ASHP) is an electrical appliance that sits outside and is used to extract heat from outside air for use indoor for heating, and for domestic hot water if required.

An ASHP is powered by electricity only, and our heat pump takes an electrical supply from inside our house.

Air Source Heat Pump
An air source heat pump uses electricity to help generate heat. If more electricity is used to generate the same level of heat then COP is reduced

A heat pump uses fans to force air over a series of coils in which refrigerant is flowing through. The refrigerant captures heat from the air before passing through a compressor that increases pressure and therefore temperature, allowing a heat pump to maximize heat gains for use indoors.

Heat is delivered indoors through pipes to the indoor heating system.

Air Source Heat Pump Behind
Heat being delivered indoors behind our ASHP through pipes

The way in which heat is provided inside a home can depend on the type of heat pump system being used (air to air or air to water).

For an air to water heat pump system (like ours), a water-based central heating system with heating appliances such as radiators and underfloor heating will be used.

Air Source Heat Pump Plate
Our heat pump is the air to water type, which delivers heat to a central heating system

For air to air heat pump systems, one or more air handling units will be used to heat the air directly inside a home.

An air to water heat pump is also able to provide domestic hot water when combined with a hot water tank.

See our other articles on air source heat pumps explained, how an air source heat pump works, and parts of an air source heat pump explained for more information.

What Is COP And Why Is It Important for Heat Pumps?

The coefficient of performance (COP) provides information on a heat pump’s efficiency. COP is determined by how much heat an air source heat pump can generate for every unit of energy it uses.

For example, an air source heat pump with a COP of three can generate three units of heat for every one unit of electrical energy it consumes. The higher the COP, the more efficient a heat pump can be operating.

Efficiency is an important factor for air source heat pumps because they work on the basis of being as efficient as possible to maximize heat provided from the electricity used, therefore helping to keep running costs as low as possible.

Gas and oil boilers/furnaces can be commonplace in many homes but the efficiency of air source heat pumps and the potential to reduce running costs is one of the advantages of ASHPs that helps with their implementation into homes.

A gas boiler/furnace can be less than 100% efficient while an air source heat pump can typically boast an efficiency of over 300% (COP of over 3).

Air Source Heat Pump Front
The efficiency of air source heat pumps is one of their main advantages

How Is Air Source Heat Pump COP Calculated?

COP for an air source heat is calculated by dividing the heat output of the heat pump by the energy input.

COP = Heat Output / Energy Input

For instance, if a heat pump has a heat output of twelve kilowatts, and an electrical power input of four kilowatts, the COP would be three.

Air Source Heat Pump COP vs Temperature

One of the main influences for the Coefficient of Performance for an air source heat pump at any given time can be the outside air temperature.

As an example, the image below shows the COP for our own air source heat pump at two different outside air temperatures: 2°C (35°F) and 7°C (44°F).

Air Source Heat Pump Plate
The COP for our heat pump at two different air temperatures

At an outside air temperature of 2°C (35°F), our heat pump can be operating at a COP of 3.3, while at an outside temperature of 7°C (44°F) our ASHP can be working at a COP of 4.4.

The reason for this difference in COP is due to the temperature difference between outdoors and indoors.

At lower outdoor temperatures, a heat pump will require greater electrical power input to generate the same level of output heat because the temperature difference is greater.

For example, with our heat pump that has a heating output capacity of 11.2kW, the electrical power input increases from 2.51kW to 3.35kW when outside air temperature is at 2°C (35°F) rather than 7°C (44°F).

Power input required at different air temperatures

As COP is the ratio between heat output and electrical power input, the COP for our heat pump reduces from 4.47 at 7°C (44°F) to 3.34 at the lower outside temperature of 2°C (35°F).

How COP is affected by air temperature and power input requirements

This is just two scenarios outlined in the manual for our Mitsubishi Electric air source heat pump, and COP will vary in between and outside of these two air temperatures.

COP will also vary between manufacturer and model of ASHP, but the above gives an example of how COP can change with varying outdoor temperatures.

How To Maximize Air Source Heat Pump COP

While the COP will be set by the manufacturer and model of air source heat pump through its design in efficiency, there are a few things that can be done to help ensure that a heat pump is running as efficiently as possible (where the highest potential COP can be achieved).

  • Insulate your home. Insulating a home as much as possible can help heat to remain inside for a long as possible, therefore helping to reduce the demand on a heat pump to provide the desired indoor temperatures and using less energy.
  • Use a programmable thermostat. A programmable thermostat can also help improve the COP of an air source heat pump. A thermostat can be used to set heating times and temperatures so that a heat pump doesn’t have to work as hard to meet high user demand, such as when you’re away from home or asleep.
Air Source Heat Pump Water Heater Control Panel
The main control panel for our air source heat pump system
  • Have your heat pump regularly serviced. It’s important to have your air source heat pump serviced by a qualified technician, typically at least once per year. This will help ensure that it runs at peak efficiency and does not use more energy than necessary.
  • More efficient heating system. The indoor heating setup can be just as important as the heat pump when it comes to efficiency and COP. As ASHP systems work at lower heating temperatures than other heating systems, more importance is placed on the internal heating apparatus to release the heat as effectively as possible. For air to water heat pumps, the use of large surface area equipment such as underfloor heating and modern radiators can help improve overall efficiency.
Air Source Heat Pump Radiator
Using an efficient indoor heating setup can help to maximize COP for an air source heat pump

Choosing a higher efficiency heat pump can ultimately help maximize COP through minimizing running costs, but more efficient heat pump units may demand a higher price tag. The upfront costs would need to be compared with the electrical running by an installer to work out how cost-effective a certain heat pump would be.

Benefits Of Maximizing Air Source Heat Pump COP

Improving the COP of your air source heat pump can also provide other benefits, such as:

Lower Energy Bills

Maximizing COP for an air source heat pump means reducing electrical input power, which in turn can reduce running costs by minimizing electricity usage.

However, as outdoor temperatures change throughout the year, energy bills will follow suit.

Fewer Greenhouse Gas Emissions

Using less electricity to power a heat pump can means fewer emissions, depending on where and how the electricity was generated.

Further Reading

Air Source Heat Pump Efficiency Explained

Air Source Heat Pump Maintenance Requirements

How An Air Source Heat Pump Can Work In Winter