The energy consumption of various heat pumps is expressed in terms of the Coefficient of Performance , or COP. Regardless of the type, whether an air-source heat pump or a geothermal heat pump, efficiency is measured by the COP. But what does this COP actually mean?
The mistake is often made to evaluate heat pumps based on just one COP value. A high COP doesn't necessarily mean a heat pump is energy-efficient. To make fair comparisons, the integrated value (SCOP) plays a crucial role. It's also important to understand how the heat pump performs under 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 generated by solar energy) can serve as a source, such as geothermal energy, 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 domestic hot water. Electricity is required to initiate and maintain this process.
What does the COP of a heat pump mean?
The COP is the ratio of the heat output to the heat pump's electricity consumption (power consumption). This is best explained with an example.
For example: for every 5 kWh of heat produced by a heat pump, 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. Calculated as follows: 
A higher COP implies lower electricity consumption and indicates a more economical heat pump. A heat pump with a high COP has a shorter payback period, and fewer solar panels are needed to offset the energy consumption. This makes the COP a relevant figure.
Efficiency of a heat pump
In the aforementioned example, the COP is 5, requiring only 1 part electricity for 5 parts heat. This results in an efficiency of 500%, which is exceptional in the heating sector. For comparison, the efficiencies of electric heating and a central heating boiler are shown below.
COP and efficiency of electric central heating boiler or electric radiators
With fully electric heating, such as an electric boiler or an electric radiator, the efficiency is not as high. At best, a fully electric heater 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 equals the output. More importantly, a heat pump is five times more efficient with electricity than electric heating, possibly because a heat pump extracts heat from a source and upgrades it to a usable level. Most of the energy comes not from electricity, but from the heat of air or water.
COP and efficiency of a gas boiler
A gas-fired boiler experiences losses. The heat released when burning gas cannot be fully transferred to the water being heated, resulting in heat loss and reduced efficiency. Furthermore, when water is heated in the boiler, water vapor is created, which escapes up the chimney along with the flue gases.
In the case of a high-efficiency boiler, the water vapor is condensed, resulting in lower losses and higher efficiency. However, the efficiency can never be 100% or higher. The average efficiency of a central heating boiler is around 90%, resulting in a COP of 0.9. This is more than five 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 and foremost, the efficiency of the heat pump itself is important; 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-to-water heat pump literally extracts heat from the outside air, warmed by the sun, even in freezing temperatures. This heat is upgraded to a usable level and transferred to water for use in space heating (underfloor heating or radiators) and/or for producing hot water.
This heat pump (almost) always consists of an outdoor unit and an indoor unit. The outdoor unit is located outside and draws in air and blows it out again, but at a colder temperature. The temperature difference between them is the heat absorbed by the heat pump.
Air Water Heat Pump Indoor and Outdoor Unit
You can imagine that when it's colder outside, the heat pump has to work harder to heat water than when it's warmer. After all, there's a greater difference to overcome. This also means that the COP fluctuates with different outdoor temperatures. The colder it is outside, the lower the COP at the same output temperatures.
This means that the COP of an air-source heat pump depends on the weather. It also depends on the season and location. In winter, it's colder than in summer, and the heat pump has to work harder. The same is true in Scandinavia, where it's colder outside than in the Netherlands.
An air-source heat pump therefore never has just one COP (see the graph below). The left axis shows two values: the COP and the outside air temperature. The bottom axis shows the temperature of the water being produced.
SCOP Heat Pump
But what exactly is a heat pump 's SCOP? The abbreviation stands for Seasonal Coefficient of Performance . Essentially, it's nothing more or less than an average COP over a year, taking into account the seasons in a given region.
The SCOP makes it easier to compare heat pumps, especially air-source heat pumps, where seasonal variations affect efficiency. When it's colder, a heat pump has to work harder, and the COP decreases, as described above.
To calculate the SCOP of an air-to-water heat pump, the average of all COP values over an entire year is taken in a specific region, such as Utrecht (or another part of Europe). This is done at multiple output temperatures in 5˚C increments. This tells you the SCOP for that heat pump when producing water at 30˚C, 35˚C, 40˚C, 45˚C, and so on.
The SCOP of a heat pump is lower in Scandinavia than in Spain because it's colder there. Furthermore, the distribution system, such as radiators or underfloor heating, can affect the SCOP. More on this 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 distribution system, such as radiators or underfloor heating, does indeed influence the COP and SCOP. These values depend on the temperature of the water being produced.
A traditional radiator is a so-called high-temperature heater. For proper operation, the water flowing through it must be between 60°C and 80°C. The radiator adapts to the water temperature, and only at this temperature does convection occur. This means that the air rises and circulates in the room, constantly passing over the radiator to heat the entire room. If this doesn't happen, you only experience radiant heat.
Underfloor heating is what's known as low-temperature heating. Because the heated surface area is much larger than with radiators, the room is heated more evenly. The water temperature flowing through underfloor heating is between 20 and 35°C, significantly cooler than with radiators. Good insulation is essential for low-temperature heating.
Underfloor heating COP Lower
Because the water in underfloor heating doesn't need to be heated as high, the heat pump can work less hard. This results in a higher COP and SCOP. Low-temperature heating is therefore recommended, as the heat pump then consumes less electricity. Low-temperature radiators are also available. In addition, traditional radiators (provided they are suitable in terms of thickness) can be fitted with a Climatebooster, propellers under the radiators, which can lower the output temperature by up to 15°C. The pipework leading to this must be large enough in diameter to transport the larger amount of water required.
Compare heat pumps with each other
In Europe, the measurement method for calculating the COP is established as the NEN14511 and NEN14825 standards by Eurovent. This allows heat pumps to be compared. However, the COP may 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, heat loss during defrosting of the air heat pump. These are actual practical values and represent the actual power output under specified conditions. The outdoor unit of the heat pump can freeze, especially when the outside temperature is between -2 and 5°C. Periodically, the heating process is automatically reversed to defrost the outdoor unit.
This means the integrated value provides a much more accurate picture of how much a heat pump actually consumes. Not every manufacturer provides this value.
Part load and full load
Suppose you have a choice between two heat pumps. The first has a better COP at -10°C (full load) but worse at 7°C (partial load) outside temperature. The second has the opposite COP, worse at -10°C but better at 7°C outside temperature. Which would you choose?
The heat pump we recommend is the one that offers the best efficiency at the most common temperature. In the Netherlands, this is the second heat pump.
The first heat pump will have been more powerful to achieve better efficiency at lower outdoor temperatures, under full load. But at partial load, the heavier components get in the way (think of a larger pump/fan), causing this heat pump to consume more energy.
That's why you're often better off with a slightly undersized heat pump than one that's too large. Firstly, energy consumption is generally lower, making it the most economical option. Furthermore, the initial investment is also cheaper. A slightly lower COP at full load (in severe frost) is a trade-off. In exceptionally cold conditions and with a small heat pump, an electric backup element can be used to provide sufficient heat only then. Yes, electricity consumption will be high for a short time. And yes, the rest of the time, electricity consumption is much lower.
COP Tap water
In almost all cases, the COP of domestic hot water is different from the COP of space heating. This is due to the temperature difference between the two.
Heating water with a heat pump takes longer than with a central heating boiler (because it often has a lower output). To prevent you from taking a cold shower, a reserve of hot water is created. The water temperature in the buffer tank is usually 55°C. Periodically, the water in the buffer tank is heated to prevent Legionella bacteria from forming.
With underfloor heating, the water for space heating is 35°C, so it's cooler than the hot tap water. In that case, the COP for the tap water is lower than for the 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
A geothermal heat pump extracts heat from water. The heat source can be the ground, surface water, or a subsurface water source. Therefore, it's often referred to as a ground-source heat pump, ground-source heat pump, or geothermal energy. These are all the same thing.
When the heat source is underground, a closed circuit of pipes containing a mixture of water and antifreeze runs through it. The water mixture adopts the temperature of the source. The heated water mixture is then pumped past the heat pump, which extracts heat from it and heats the water to a usable level. The colder return water is pumped back into the source, reheated by the source, and the cycle is complete. This works the same way for underground and surface water sources, or an open source can be used. In that case, there are open flow and return lines.
The temperature of a ground source is more constant than the outside air. This means there are fewer seasonal fluctuations with a geothermal heat pump compared to an air-source heat pump. Therefore, the SCOP is not important for a geothermal heat pump. A ground-source heat pump generally has 0.5 to 1 COP point higher than an air-source heat pump. At higher outdoor temperatures, the air-source heat pump actually has an advantage.
Air heat pump or geothermal heat pump?
When we look at these differences in perspective, the following picture emerges. The average annual gas consumption of a typical home is approximately 1600 m³. Of this, an average of 400 m³ is used for hot water and 1200 m³ for heating the home. In modern, better-insulated homes that meet stricter EPC requirements, the average annual consumption is 800 m³ of gas. This still uses 400 m³ for heating hot water and only 400 m³ for heating the home. Therefore, with lower energy consumption, there are fewer savings to be made with a heat pump. This means that recouping 1 COP point is less feasible in a modern home.
About ten years ago, the COPs of air-to-water heat pumps weren't as efficient as they are now, resulting in significantly lower efficiencies than those of ground-source heat pumps. Nowadays, intelligent use of inverter compressors and electronic expansion valves, among other things, makes the efficiency almost comparable to that of ground-source heat pumps. At -10°C, there is now sufficient heating capacity, which wasn't always the case in the past.
Financially, a ground-source heat pump is €3,000 to €5,000 more expensive than an air-source heat pump due to the required installation of the source. The savings with a ground-source heat pump compared to an air-source heat pump, thanks to a slightly better COP, are on average €60 per year. The payback period is therefore unrealistically long; it takes at least 50 years (3,000/60 = 50 years) to recoup the investment in a ground-source heat pump compared to an air-source heat pump. A better alternative is to invest the money on the roof instead of underground. By spending the same amount on solar panels, you can offset the energy consumption of the air-source heat pump and enjoy free heating and cooling. More information can be found on the page about air-source heat pumps or geothermal heat pumps.
Breakthrough point COP versus gas boiler
A heat pump is powered by electricity (cost for private individuals or small-scale users: 1 kWh, €0.214), while a central heating boiler runs on gas (1 m³, €0.66). This is essentially comparing apples and oranges. To make a fair comparison, we've 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 significantly less electricity than a gas-fired central heating boiler. The question now is: what does the COP need to 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 COP of 5.2 at a heat pump output temperature of 35°C, this target is easily achievable. In fact, this leads to significant savings with a heat pump. With solar panels, you generate the electricity required for the heat pump yourself, which naturally affects the payback period, especially when gas consumption is high.
Financial Saving Sustainable Conclusion
The COP provides insight into a heat pump's energy consumption; the higher the figure, the less electricity the heat pump uses. However, the COP can be misleading, especially with an air-source heat pump, as conditions are crucial. Therefore, a SCOP is more accurate, as it is a weighted average over an entire year.
When the SCOP is expressed as an integrated value, all variables are taken into account and the result is the most accurate. Although gas is cheaper per kWh than electricity, a heat pump still proves to be more economical to use due to its efficiency. Better heat pumps have a SCOP of 5.2.
