Outdoor Temperature in Heat Pump Efficiency
Outdoor temperature is one of the most influential factors affecting heat pump efficiency.
Because a heat pump transfers heat from the environment into a building, the temperature of that environment directly impacts how much electrical energy is required. As outdoor temperatures change, so does system performance.
Understanding this relationship helps you interpret COP, SCOP, and seasonal efficiency values correctly.
What Is Outdoor Temperature?
Outdoor temperature is the temperature of the air outside a building, usually measured in degrees Celsius (°C).
In general weather terms, it describes how warm or cold the outside environment is at a specific location and time.
Outdoor Temperature in Heat Pump Systems
In heat pump efficiency, outdoor temperature is especially important because it often serves as the heat source for air-source heat pumps.
When outdoor temperature:
- Is higher (mild weather) → More heat energy is available in the air → The heat pump operates more efficiently.
- Is lower (cold weather) → Less heat energy is available → The system requires more electrical input → Efficiency decreases.
This directly affects performance values such as COP and seasonal efficiency calculated under EN 14825.
Why Outdoor Temperature Matters
Heat pumps operate by moving heat from a lower temperature source to a higher temperature sink (the heating system).
For air-source systems, the outdoor air temperature is the primary heat source. When outdoor temperature drops:
- Less thermal energy is available in the air
- The compressor must work harder
- Electrical consumption increases
- COP decreases
The larger the temperature difference between the outdoor air and the required flow temperature, the lower the efficiency.
This temperature difference is often called temperature lift.
Outdoor Temperature and COP
Efficiency values such as COP are measured at defined test points under EN 14511.
For example:
- A7/W35 → outdoor air 7°C, flow temperature 35°C
- A2/W35 → outdoor air 2°C, flow temperature 35°C
The same heat pump will typically have a lower COP at A2/W35 than at A7/W35 because colder air increases temperature lift.
This shows how strongly outdoor temperature influences point efficiency.
Outdoor Temperature and Seasonal Efficiency (SCOP)
Seasonal performance values such as SCOP are calculated according to EN 14825.
EN 14825 uses:
- Multiple outdoor temperature bins
- Part-load operation data
- Defined climate profiles (warmer, average, colder)
Because SCOP includes temperature distribution across a heating season, it reflects how the system performs under realistic climate conditions.
In colder climates, SCOP values are typically lower than in milder climates.
Air-Source vs Ground-Source Sensitivity
Air-Source Heat Pumps
Highly dependent on outdoor air temperature.
Efficiency decreases noticeably in colder weather.
At temperatures near freezing, defrost cycles may temporarily reduce heating performance.
Ground-Source Heat Pumps
Use soil or groundwater as a heat source.
Subsurface temperatures are more stable throughout winter.
This typically results in more consistent seasonal efficiency.
Extreme Cold Conditions
Modern air-source heat pumps are designed to operate at low outdoor temperatures, but efficiency decreases as:
- Outdoor temperature drops
- Flow temperature demand increases
In very cold conditions, some systems may activate auxiliary heating. This increases electricity consumption and reduces overall seasonal efficiency.
However, seasonal efficiency calculations already account for typical temperature distributions under standardized conditions.
Outdoor Temperature and Cooling Mode
In cooling mode, outdoor temperature also influences efficiency.
As outdoor temperature rises:
- Heat rejection becomes more difficult
- Electrical input increases
- EER decreases
Seasonal cooling efficiency (SEER) reflects temperature variations across the cooling season.
Climate Categories in European Standards
European seasonal calculations under EN 14825 use three climate profiles:
- Warmer
- Average
- Colder
These categories ensure that seasonal performance reflects realistic regional temperature distributions.
This prevents misleading comparison between products tested under different assumed climates.
Interaction with Flow Temperature
Outdoor temperature does not act alone.
Efficiency depends on the combination of:
- Outdoor temperature (heat source)
- Required flow temperature (heat sink)
Lower flow temperature can partially compensate for low outdoor temperature by reducing temperature lift.
This is why system design and building insulation play a critical role in overall efficiency.
Practical Takeaway
Outdoor temperature directly affects:
- COP at specific operating points
- Seasonal efficiency (SCOP)
- Electricity consumption
- Defrost frequency (air-source systems)
When evaluating heat pump efficiency:
- Always consider climate zone
- Compare identical test conditions
- Understand that colder conditions reduce point efficiency
Understanding outdoor temperature as a performance factor helps interpret efficiency values correctly and avoid unrealistic expectations.
