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Last updated: 08 September 2021

Air Source Heat Pump Performance

What Are SPF and COP?

Historically, air source heat pumps have been the most widely used due to their relatively low cost, easy maintenance, inexpensive installation and eligibility for incentives. However, they are also prone to limitations, based on the way they function.

To heat the inner space of the home, air source heat pumps absorb outside air and move it inside the building. That limits their performance to a certain extent when the outside temperature drops below freezing.

To assess air heat pump performance, there are a couple of indicators that give information about heat efficiency and seasonal temperature fluctuation dependencies. In a sense, the SPF and COP parameters will help you comprehend air source heat pump limitations and adjust your preferences according to your specific needs.

The interactive graphic below illustrates how the process works. You can click each point to read about the steps in more detail:

1 1: An outdoor unit draws in heat from the ambient air with fans, and then blows it over a heat exchanger coil. 2 2: The heat is transferred to a cold coolant flowing in the coil. Due to its very low boiling point, the heat only turns the coolant into a slightly warm vapour. 3 3: The slightly warm coolant vapour passes through a compressor that increases its temperature and compresses it to a denser vapour. 4 4: This hot coolant vapour is then transferred to the indoor unit(s) through internal piping systems. 5 5: The indoor unit blows out the heat from the coolant to the indoor air with fans, thereby warming up the room. 6 6: Once the coolant has transferred some of its heat to the air, it cools back into a liquid. 7 7: The liquid coolant passes through an expansion valve, which forces it to let go of more of its heat and mix with air, thereby creating a liquid vapour mixture. 8 8: The cold liquid vapour coolant mixture is then circulated back to the outdoor unit, to be heated again.
ASHP for Heating

Seasonal Performance

Because air source heat pumps move the outside air to heat the internal space of the home, they are dependent on the seasonal fluctuations of the outside temperature. To reflect that dependence, the performance of heat pumps is rated with a Seasonal Performance Factor (SPF). SPF shows the average Coefficient of Performance of a heat pump over a full heating season.

More specifically, the SPF represents the total useful heat that has been generated annually from the heat pump and then divided by the annual electricity consumption. Electricity consumption is related to all other heating solutions that are part of the overall heating system, e.g. underfloor heating.

However, because heat pumps are a renewable heating solution, the heat produced is always higher than the one consumed. For example, if the SPF is 3, the system is expected to give 3 units of heat energy for each one used.

According to  EU legislation, SPF of 2.5 indicates the minimum performance that makes the heat pump eligible for the Renewable Heat Incentive (RHI).

Efficiency Performance

The efficiency of a heat pump is measured by their Coefficient of Performance (COP). That is the ratio of heat produced per unit of electricity consumed when pumping the heat. Efficiency performance tends to decrease during periods of severe cold.

For comparison, in mild weather COP can be about 4.0, but when the temperature drops to 0°C, the COP can also decline to 2.5. On average, the COP of typical heat pumps has a seasonal variation of about 2.5-2.8. However, there are heat pump models having higher performance in mild climates.

When selecting a heat pump, keep in mind that the higher the ratio, the more efficient the unit and the more efficient the heat delivery. A COP value of 3 means that you get 3kWh of heat output for every 1kWh of electricity used to run the pump. Higher COP values represent a heat delivery that is relatively more efficient.

For example, a COP value of 4 means that you get 4kWh of heat output for every 1kWh of electricity used to run the pump. Keep in mind, that the COP mentioned by manufacturers is measured under test conditions and you should treat it with caution.  

Take into consideration also that the type of air source heat pump will affect the range of COP. Air to air heat pumps have usually higher COP than air to water heat pumps, but this doesn't always mean that they can cover all your heating needs more effectively. In the video below, we compare the two types based on their efficiency, as well as cost, installation and possible savings you can make. 

Air to Water VS Air to Air Heat Pump: A Comprehensive Comparison

Improve the Air Source Heat Pump Performance

When the outside temperature drops below 5°C, remember to remove the ice that accumulates on the evaporator, due to the moisture generated in the air condensing. Because the ice decreases the efficiency of the coil by reducing its ability to transfer heat to the refrigerant, remove it and switch on ‘defrost mode’ by reversing its action. To melt the ice, direct the hot gas to the outdoor coil and switch off the evaporator fan.

No matter that this usually is not mentioned on the manufacturer’s COP, when you remove the ice, the SPF will also decrease. This way, you will minimise the amount of wasted energy and improve the operating efficiency of the heat pump by 5-10%.

Remember That…

To achieve optimal heating of the space in your home, remember that before installing a heat pump you need to insulate the building. That will improve the living conditions, despite temperature fluctuations.

Before you start installing the heat pump, remember to check your eligibility for government grants and incentives, that will considerably reduce the financial burden of purchasing due to the cost of air source heat pumps and installing them. Make sure that your building complies with the building limitations and requirements of local authorities.

In the end, remember that by installing a sustainable heating solution like an air source heat pump, you can reduce the carbon footprint of your household on the environment and make a personal contribution for the preservation of the life on the planet.

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