The energy consumption of various heat pumps is expressed in the Coefficient of Performance , or COP. Regardless of the type, be it an air heat pump or a geothermal heat pump, the efficiency is measured by the COP. But what does this COP actually mean?
The mistake is often made to judge heat pumps based on only one COP value. A high COP does not necessarily imply that a heat pump is economical. In order to make fair comparisons, the integrated value (SCOP) plays a crucial role. It is also important to understand how the heat pump performs at partial load.
Origin of the heat pump
A heat pump extracts heat from water or air, with different systems being distinguished based on the heat source. Virtually any heat-retaining medium (whether or not obtained by solar energy) can serve as a source, such as geothermal heat, surface water, outside air, ventilation air, or residual heat from industrial processes. The heat is upgraded and transferred to air or water for use in space heating or for hot tap water. Electricity is required to start and maintain this process. 
What does the COP of a heat pump mean?
The COP is the ratio between the amount of heat emitted and the electricity consumption (power consumption) of the heat pump. This can best be explained using an example.
For example: for every 5 kWh of heat that a heat pump produces, 4 kWh come from the source (air or water) and 1 kWh from electricity. In other words, for every 5 kWh of heat, the COP in this example consists of 4 kWh of heat from the source and 1 kWh of electricity. The COP here is therefore 5. In a calculation, this looks like this: 
A higher COP implies less electricity consumption and indicates a more economical operation of the heat pump. A heat pump with a high COP has a shorter payback period, and fewer solar panels are needed to compensate for the consumption. This makes the COP a relevant figure. 
Efficiency of a heat pump
In the example mentioned earlier, the COP is 5, where only 1 part electricity is needed for 5 parts heat. This results in an efficiency of 500%, which is extraordinary in the heating sector. For comparison, below are the efficiencies of electric heating and a central heating boiler.
COP and efficiency of electric central heating boiler or electric radiators
With fully electric heating, such as an electric central heating boiler or an electric radiator, the efficiency is not as high. In the best case, a fully electric heating produces 1 kWh of heat for every kWh of electricity it consumes. This is calculated as follows:
The COP is then 1 and the efficiency is 100%. The energy input is equal to the output. More importantly, a heat pump is 5 times more economical with electricity than an electric heater, possibly because a heat pump extracts heat from a source and upgrades it to a usable level. The majority of the energy does not come from electricity, but from the heat of air or water.
COP and efficiency of a gas boiler
A gas-fired central heating boiler has to deal with losses. The heat released when burning gas cannot be fully transferred to the water to be heated, which results in heat loss and a loss of efficiency. In addition, when water is heated in the central heating boiler, water vapour is created that disappears from the chimney together with flue gases. In the case of an HR boiler, the water vapour is condensed, which results in less loss and a higher efficiency. Nevertheless, the efficiency can never be 100% or higher. The average efficiency of a central heating boiler is around 90%, which results in a COP of 0.9. This is more than 5 times less efficient than a heat pump. Some boiler manufacturers claim an efficiency of up to 107%, but this claim is disputed on the following website: https://cvketelkiezen.nl .
COP of a heat pump is not a fixed figure
The COP of a heat pump depends on several factors:
- First of all, the efficiency of the heat pump itself is of influence; the more efficiently the heat pump produces hot water or air, the higher the COP. This includes, for example, the use of inverter compressors and electronically controlled expansion valves.
- In addition, the temperature of the source (air or water) affects the COP, as does the temperature of the produced water/air. The higher the produced temperature, the higher the consumption and the lower the COP. This makes sense; it takes more energy to heat water from 15˚C to 80˚C than from 15˚C to 35˚C.
COP of an air/water heat pump
An air/water heat pump literally extracts heat from the outside air that has been heated by the sun, even when it is freezing. This heat is upgraded to a usable level and released into water for use in space heating in underfloor heating or radiators and/or the production of hot tap water.
This heat pump consists (almost) always of an outdoor unit and an indoor unit. The outdoor unit is outside and sucks in air and blows this air out again, but then colder. The temperature difference between them is the absorbed heat of the heat pump.
Air Water Heat Pump Indoor and Outdoor Unit
You can imagine that when it is colder outside, the heat pump has to work harder to heat water than when it is warmer. After all, a larger difference has to be bridged. This also means that the COP fluctuates at different outside temperatures. The colder it is outside, the lower the COP at the same output temperatures.
This means that the COP of an air heat pump is weather dependent. But also dependent on the season and location. In winter it is colder than in summer and the heat pump has to work harder and the same applies in Scandinavia where it is colder outside than in the Netherlands.
An air heat pump therefore never has one COP (see the graph below). The left axis has two values, namely the COP and outside air temperature. And the bottom axis shows the temperature of the water that is produced.
SCOP Heat Pump
But what is a SCOP of a heat pump? The abbreviation stands for Seasonal Coefficient of Performance . In fact, it is nothing more and nothing less than an average COP over a year, whereby the seasons in a certain region are taken into account.
The SCOP makes it easier to compare heat pumps with each other, and in particular air heat pumps where the seasons have an effect on the efficiency. When it is colder, a heat pump has to work harder and the COP decreases, as described above.
To calculate the SCOP of an air/water heat pump, the average of all COP values in a specific region, such as the Utrecht region (or another part of Europe), is taken in a whole year. This is then done at multiple output temperatures in steps of 5˚C. Then you know what the SCOP is for that heat pump when producing 30˚C water, 35˚C, 40˚C, 45˚C and so on.
The SCOP of a heat pump is lower in Scandinavia than in Spain, because it is colder there. In addition, the delivery system, such as radiators or underfloor heating, can influence the SCOP. More about that in the next chapter.
For modern air heat pumps in the Netherlands, the SCOP is approximately 5.2 at a discharge temperature of 35˚C.
COP and SCOP – Radiators or Underfloor Heating?
The heat emission system, such as radiators or underfloor heating, does indeed influence the COP and SCOP. These values depend on the temperature of the water that is produced.
A traditional radiator is a so-called high temperature heater. For the radiators to function properly, the water flowing through them must be between 60˚C and 80˚C. The radiator takes over the temperature of the water and only from this temperature does convection occur. This means that the air rises and circulates in the room, so that air always passes the radiator to provide the entire room with heat. If this does not happen, only radiant heat is experienced.
Underfloor heating is so-called low-temperature heating. Because the heated surface is much larger than with radiators, the space is heated more evenly. The temperature of the water that flows through the underfloor heating is between 20 and 35˚C, a lot less warm than with radiators. Good insulation is a requirement for low-temperature heating.
Underfloor heating COP Lower
Because the water does not have to be heated as high with underfloor heating, the heat pump can work less hard. This results in a higher COP and SCOP. Low temperature heating is therefore recommended, because the heat pump then uses less electricity. Incidentally, there are also low temperature radiators. In addition, traditional radiators (if suitable in terms of thickness) can be equipped with a Climatebooster, propellers under the radiators, with which the output temperature can be reduced by up to 15˚C. The pipework to this must then be large enough in diameter to be able to transport the larger amount of water that is required.
Compare Heat Pumps with Each Other
In Europe, the measurement method for calculating the COP is established as the standard NEN14511 and NEN14825 at Eurovent. This allows heat pumps to be compared with each other. However, it is still possible that the COP does not provide the complete picture.
When the peak load value (maximum value) of the heat pump is specified, this is the maximum capacity that can be delivered, without any deduction for the conditions in which the heat pump is operating.
Integrated value (integrated or weighted value) is the value that takes into account, among other things, the heat loss during defrosting of the air heat pump. These are actual practical values and are the actual delivered power at specified conditions. The outdoor unit of the heat pump can freeze, especially when it is between -2 and 5°C outside. Every now and then the heating process is automatically reversed in order to defrost the outdoor unit.
This means that the integrated value gives a much more honest picture of how much a heat pump actually consumes. Not every manufacturer provides this value.
Part load and full load
Suppose you can choose between two heat pumps. The first one scores better in terms of COP at -10˚C (full load) but worse at 7˚C (partial load) outside temperature. The second one scores exactly the opposite in terms of COP, so worse at -10˚C but better at 7˚C outside temperature. Which would you choose?
The heat pump we would recommend to you is the heat pump that has the best efficiency at the temperature that occurs most often. In the Netherlands, this is the second heat pump.
The first heat pump will be heavier to achieve better efficiency at lower outside temperatures, at full load. But at partial load the heavier components are in the way (think of a larger pump/fan), which is why this heat pump consumes more.
That is why you often get away with a slightly too small heat pump better than with a heat pump that is too large. Firstly, the consumption is lower in most cases, which makes it the most economical in the end. In addition, the purchase price is also lower. A slightly lower COP at full load (severe frost) you have to accept. In exceptionally cold cases and a small selected heat pump, an electric back-up element can be used that only then provides sufficient heat. Yes, the electricity consumption is then high for a short time. And yes, the rest of the time the electricity consumption is much lower.
COP Tap water
In almost all cases, the COP of tap water is different from the COP of space heating. This has to do with the temperature difference between the two.
Heating water with a heat pump takes more time than with a central heating boiler (because it often has a lower capacity). To prevent you from standing under a cold shower, a supply of hot water is created. The temperature of the water in the buffer tank is usually 55˚C. Every now and then, the water in the buffer tank is heated extra to prevent legionella formation.
The water for space heating is 35˚C for underfloor heating, less warm than the hot tap water. In that case the COP for the tap water is lower than the water for underfloor heating.
With high temperature heating, the COP of the tap water is actually higher than the water for the radiators, which require water between 60 and 80˚C.
COP Geothermal heat pump
With a geothermal heat pump, heat is extracted from water. The heat source can be the ground, surface water or a water source under the ground. This is why people often talk about a geothermal heat pump, ground-bound heat pump or ground heat. These are all the same.
When the heat source is the ground, a closed circuit of pipes runs through it with a mixture of water and antifreeze. The water mixture takes over the temperature of the source. The heated water mixture is then pumped past the heat pump, which extracts heat from it and the heat pump upgrades the temperature to a usable level. The colder return water is pumped back into the source, heated up again by the source and then the circle is complete. This works the same way for a water source under the ground and for surface water, or an open source can be used. In that case there is an open fetch and return pipe.
The temperature of a source in the ground is more constant than the outside air. This means that there are fewer seasonal fluctuations with a geothermal heat pump compared to an air heat pump. The SCOP is therefore not important with a geothermal heat pump. A ground-based heat pump generally has 0.5 to 1 COP point more than an air heat pump. At higher outside temperatures, the air heat pump is actually at an advantage.
Air source heat pump or geothermal heat pump?
If we look at these differences in perspective, the following picture emerges. The average annual gas consumption of an average home is approximately 1600 m3. Of this, an average of 400 m3 is used for hot tap water and 1200 m3 for heating the home. In modern, better insulated homes that meet stricter EPC requirements, the average consumption is 800 m3 of gas per year. In this case, 400 m3 is still used for heating tap water and only 400 m3 for heating the home. With lower energy consumption, there is therefore less to save with a heat pump. This means that earning back 1 COP point is less feasible in a modern home.
About ten years ago, the COPs of air/water heat pumps were not as efficient as they are today, which meant that the efficiencies were considerably lower than those of ground-source heat pumps. Nowadays, intelligent use is made of inverter compressors and electronic expansion valves, which means that the efficiency is almost comparable to that of ground-source heat pumps. At -10˚C, there is now sufficient heating capacity, which was not always the case in the past.
Financially, a ground-source heat pump is €3,000 to €5,000 more expensive than an air/water heat pump due to the required source installation. The savings with a ground-source heat pump compared to an air heat pump, thanks to a slightly better COP, amount to an average of €60 per year. The payback period is therefore unrealistically long; it takes at least 50 years (3000/60 = 50 years) to earn back the investment in a ground-source heat pump compared to an air heat pump. A better alternative is not to invest the money in the ground, but to put it on the roof. By spending the same amount on solar panels, you can compensate for the consumption of the air/water heat pump and enjoy free heating and cooling. More information can be found on the page about air heat pumps or geothermal heat pumps.
Breakthrough point COP versus gas boiler
A heat pump is powered by electricity (costs for private individuals or small consumers: 1 kWh, €0.214), while a central heating boiler works on gas (1 m3, €0.66). In this case, we are actually comparing apples and oranges. To make a fair comparison, we have calculated that there are 9.7 kWh of energy in a cubic meter (m3) of gas. This allows us to determine the price difference. Electricity currently costs €0.214 per kWh, while gas costs €0.66 / 9.77 = €0.068 per kWh. So, gas is considerably cheaper than electricity! Nevertheless, a heat pump uses much less electricity than a gas-fired central heating boiler. The question now is: what should the COP be to be cheaper than gas?
If the COP is better than 2.83, a heat pump is cheaper than a gas boiler. With a SCOP of 5.2 at a discharge temperature of 35˚C, this goal is easily achievable. In fact, this leads to significant savings with a heat pump. In the case of solar panels, you generate the electricity required for the heat pump yourself, which of course has an impact on the payback period, especially when gas consumption is high.
Financial Saving Sustainable Conclusion
The COP provides insight into the consumption of a heat pump; the higher the number, the less electricity the heat pump consumes. However, the COP can give a distorted picture, especially with an air heat pump, because the circumstances are decisive. A SCOP is therefore more accurate, because it is a weighted average over an entire year. When the SCOP is expressed as an integrated value, all variables are included and the result is the most truthful. Despite the fact that gas is cheaper per kWh than electricity, a heat pump still proves to be more economical in consumption due to its economy. The better heat pumps have a SCOP of 5.2.
