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Seasonal performance factor: efficient heat pump operation

Photo: © Andrey Popov /

Can I improve the efficiency of my heat pump heating or is the system already operating within its optimum range? The seasonal performance factor (SPF) provides an answer to this question. It is a measure of how much electricity is needed to provide heat, typically over a full year. In practice, the higher the rating, the lower the heating costs. But what constitutes a good seasonal performance factor? How is it calculated and what can be done if the value is far below the average? We answer the most important questions here.

Ensuring economical heat pump operation

Many of us are familiar with the concept of efficiency when it comes to heating systems. This is the ratio of output to input, and a high value is good. A heat pump's seasonal performance factor is a similar measure. While the output can be equated with the heating energy delivered, the input corresponds to the amount of power used. Unlike a heat pump's coefficient of performance, which is derived from only one operating point, the seasonal performance factor covers a period of time, generally a whole year or a heating season.

Different methods for determining the seasonal performance factor

When it comes to determining the seasonal performance factor, various methods have become established. The most reliable method is the calculation based on measured values, which experts also refer to as the seasonal performance factor (SPF). However, this method only provides reliable results after the first heating period. In order to be able to estimate the seasonal performance factor before the heat pump is installed, there are additional theoretical methods. These are defined in VDI 4650 and are important, for example, for applying for subsidies.

Theoretical calculation of the coefficient according to VDI 4650

If no information on heat output or electricity consumption is available at the time of planning, the seasonal performance factor can be determined according to the requirements of VDI 4650. Using a simulation on an hourly basis or a simplified method based on various performance figures, planners and specialist installers determine the so-called Seasonal Coefficient of Performance (SCOP). However, since standard climate data and room temperatures are used for the calculation, the result may deviate from reality.

By the way:  coefficient of performance calculators for heat pumps are available online.   These query important influencing factors and help to estimate the amount provisionally. This is particularly interesting for planning when it comes to comparing different heating systems or heaters.

Calculation of the seasonal performance factor with measured values (SPF)

In addition to the theoretical calculation of the seasonal performance factor, the characteristic value can also be determined using real measured values. These are provided by heat meters and electricity meters, which record the amount of heat emitted and the amount of electricity consumed.  

Important: For consistent results, the correct system limits must be selected. If, for example, the meters only record the electricity consumption of the compressor, the reading at the end of a year is lower than if the consumption of the entire heating system is measured. This in turn affects the seasonal performance factor of the heat pump. In practice, experts take into account the amount of electricity consumed for control, compression and heat source development as well as the electricity consumption of a heating rod, if one is present.

Simple equation for determining efficiency  

Once the meters are installed and in operation, the seasonal performance factor can be calculated. For this, you should read the meters at regular intervals and note the values. Then calculate the amounts of energy transferred and drawn in a year. To do this, subtract the previous year's values from the current year's values. You can then use these results and the following equation to calculate the seasonal performance factor:

  • Heat pump seasonal performance factor = heat transferred / power consumption

Since all amounts are expressed in kilowatt hours, the result is a dimensionless quantity. This can be used in the following equation to estimate power consumption and heating costs:  

  • Power consumption = heat transferred / heat pump seasonal performance factor  

If there is no other heat generator involved, the heat transferred can be equated to the building's heat demand. You can find this out by looking at old billing data or the documents provided by your energy consultant. Then divide the heat demand by the estimated performance factor for your heat pump to get the power consumption in kilowatt hours. Multiplied by the current electricity price, this gives you your possible heating costs for a year.  These two examples illustrate how the seasonal performance factor is calculated:

The heat meter shows a reading of 12,000  kilowatt hours and the electricity meter 4000  kilowatt hours. Plugged into the above equation, this results in a performance factor of three.

An older building requires 15,000  kilowatt hours of heat per year and the intention is to retrofit a heat pump. An expert checks the local conditions and calculates a seasonal performance factor of three. A figure of 5000  kilowatt hours for power consumption is ascertained using the second equation. This is multiplied by the current electricity price to produce a provisional estimate of the heating costs.

Improve your heat pump's seasonal performance factor: Tips

In some cases, the result can be improved by lowering the temperature swing.  This is the temperature differential between the heat source and the flow temperature. If it is low, the compressor has to do less work to get the same amount of heat from the system. This reduces power consumption and increases the seasonal performance factor.  

Ensure maximum efficiency as early as the planning stage

In order to achieve a high seasonal performance factor and save heating costs, it is important to take a holistic approach right from the planning stage. This applies above all to the choice of the right type of heat pump.

  • If consumers choose an air-water heat pump, for example, the initial investment is somewhat lower. However, due to the low outside air temperatures in winter, higher electricity consumption is to be expected.  
  • Brine/water heatheat pumps access a heat source with a higher temperature level and achieve a better seasonal performance factor. They therefore consume less, but cost more to purchase.

Optimising the seasonal performance factor in existing buildings - tips from the expert

If your own heating system is already running and the seasonal performance factor is significantly below the theoretically determined value, the efficiency can also be increased retrospectively. Our Viessmann heat pump expert Egbert Tippelt explains how this can be done. He has been working with heat pumps for almost four decades and knows exactly what is important. According to his expertise, these measures are possible when optimising:

  • Adjust the heating curve: By setting the heating curve correctly, you optimally adapt the heating control to your building. The flow temperature drops, your heat pump consumes less electricity and you save on heating costs.  
  • Hydraulic balancing: During hydraulic balancing, experts adjust the heating system so that it supplies all rooms with the exact amount of heat they need. In practice, this usually leads to lower flow temperatures and thus also to a higher seasonal performance factor.
  • Replace heating surfaces: A large savings potential is hidden in the area of heating surfaces. If these are very small, they require high temperatures to be able to transfer the necessary heat. Replacing them with larger radiators, low-temperature or heat pump radiators or panel heating systems helps to reduce the heat sink temperature (heating flow temperature). This is because you can achieve the same comfort with lower heating water temperatures and operate the heat pump more efficiently.

An interesting solution: the smart thermostats from Viessmann. These not only ensure greater convenience when setting the room temperatures. They also automatically optimise the heating characteristic and system hydraulics to save the seasonal performance factor of the heat pump and save heating costs.

Improve thermal insulation with renovation

It is also advisable to work on the thermal envelope of the building. If you have windows replaced or walls and roofs insulated, the heat requirement is reduced. The heating system can operate with lower flow temperatures and the coefficient of performance is higher. The increased efficiency then leads to lower electricity consumption and lower heating costs.

Which measures are suitable for whom in order to optimise the heat pump and the seasonal performance factor depends on the individual building. Specialist installers and energy consultants take a close look at the building. They identify potential savings and then propose favourable renovation solutions.

Other indicators of environmental heating efficiency

In addition to the seasonal performance factor, there are a number of other indicators of heat pump efficiency. These include the coefficient of performance (COP) and seasonal space heating energy efficiency. The following overview explains what distinguishes these indicators from each other and when they are used.

  • Coefficient of performance (COP): A heat pump's coefficient of performance determined under laboratory conditions can often be found in the manufacturer's documentation. It is defined as the ratio of heat transfer to power consumption and is measured at a particular operating point. The operating point in question can be seen from the indices used, which consist of two letters and two numbers. The letters stand for the media – air (A), brine (B) and water (W) – and are supplemented with a numerical value for the media temperature. For example, a COP of A5/W35 means the coefficient of performance at an outside temperature of 5 degrees Celsius and a flow temperature of 35 degrees Celsius. 
  • Seasonal space heating energy efficiency: The seasonal space heating energy efficiency describes the environmental impact of the heating system. To calculate this value, experts multiply the SCOP by the primary energy factor of the European electricity mix. This measure is an eligibility criterion for heat pump subsidies and is therefore of particular relevance.

Of all of the indicators presented, only the seasonal performance factor says something about the actual efficiency of a particular heat pump. This is because it is based on real measured values, whilst all of the others are arrived at under laboratory conditions.

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