Air-to-water heat pumps extract heat from outdoor air and transfer it into a home’s heating system.
That process works even when the temperature outside falls below freezing.
However, heat pump efficiency in cold weather is not constant. As outdoor temperatures change, so do the conditions under which the heat pump operates.
This is why a heat pump may have one COP at A7/W35 and a different COP at A2/W35 or A−7/W35.
So, what actually happens to a heat pump when winter gets colder?
Heat Is Still Available Below 0°C
A common misconception is that there is no useful heat in outdoor air once the temperature falls below freezing.
In reality, thermal energy remains available well below 0°C.
An air-to-water heat pump absorbs this energy through its outdoor heat exchanger. The refrigerant then carries that energy through the refrigeration cycle.
The compressor raises the pressure and temperature of the refrigerant. Finally, the heat is transferred to the building’s water-based heating system.
The basic process remains the same in cold weather.
What changes is how hard the system has to work.
What Happens to COP as Outdoor Temperature Falls?
COP, or Coefficient of Performance, compares the heat delivered by a heat pump with the electricity used to produce it.
For example, a COP of 4 means that the system delivers approximately four units of heat for every unit of electrical energy consumed under those particular test conditions.
However, COP is not a fixed number.
When outdoor temperature falls, the heat pump generally has to move heat across a larger temperature difference.
This increases the workload on the refrigeration cycle.
As a result, COP will normally decrease as outdoor conditions become colder, particularly when the required water temperature remains high.
That is normal heat-pump behaviour. It does not mean the system has stopped working efficiently.
Why A7/W35 and A−7/W35 Are Different
Heat-pump performance data often uses combinations such as:
A7/W35
A2/W35
A−7/W35
The A refers to outdoor air temperature.
The W refers to the water temperature associated with the test condition.
Therefore, A7/W35 and A−7/W35 describe two very different operating environments.
At A7/W35, the heat pump is extracting energy from relatively mild outdoor air while supplying low-temperature heating water.
At A−7/W35, the outdoor air is much colder.
The heat pump must therefore operate across a greater temperature lift.
This is why buyers and installers should avoid comparing heat pumps using a single headline COP figure.
The test condition matters as much as the COP itself.
Heating Capacity Matters Too
COP is only one part of winter performance.
Heating capacity is equally important.
As outdoor temperatures fall, a building normally requires more heat. At the same time, the operating conditions for the heat pump become more demanding.
A well-designed cold-climate system must therefore balance efficiency with sufficient heating capacity.
This is also why correct heat-pump sizing matters.
Selecting a heat pump based only on nominal capacity under mild conditions may not provide a complete picture of how the system will perform during colder periods.
Installers should consider the building’s design heat load and the manufacturer’s performance data at relevant winter conditions.
Water Temperature Has a Major Effect
Outdoor temperature is not the only variable affecting heat-pump efficiency.
The required flow temperature is also important.
A heat pump supplying water at 35°C generally operates under easier conditions than the same system producing much hotter water.
This is one reason heat pumps work particularly well with low-temperature emitters such as underfloor heating.
However, many European homes use radiators and may require higher flow temperatures.
Modern R290 heat pumps can address this requirement by providing high-temperature water while maintaining heat-pump operation.
Tongyi’s R290 platform, for example, can provide outlet water temperatures of up to 75°C, depending on operating conditions and system configuration.
The important point is that high-temperature capability and maximum efficiency are not the same thing.
The system should always operate at the lowest practical water temperature that can comfortably heat the building.
Why Is R290 Suitable for Cold-Climate Heat Pumps?
R290, or propane, has become increasingly important in modern air-to-water heat pumps.
Its extremely low climate impact is one reason. Under the current European F-gas framework, R290 has a GWP100 of 0.02.
However, R290 also has thermodynamic properties that make it suitable for efficient heating applications.
The refrigerant alone does not determine winter performance.
Compressor design, heat exchangers, refrigerant circuit design, controls and system sizing are also critical.
This is why it is better to evaluate the complete heat-pump system rather than assume that every R290 heat pump will perform identically.
What Is EVI and Why Is It Used in Cold Weather?
Some heat pumps use Enhanced Vapour Injection (EVI) to support operation under demanding conditions.
EVI introduces refrigerant vapour into an intermediate stage of the compression process.
When correctly designed, this can help the refrigeration cycle maintain heating performance across challenging operating conditions.
For cold-climate applications, technologies such as EVI can therefore form part of a broader system strategy.
However, EVI should not be viewed in isolation.
The compressor, refrigerant circuit, controls and heat exchangers must work together as one system.
Frost and Defrost Also Affect Winter Efficiency
Cold weather introduces another challenge: frost.
When the outdoor heat exchanger operates below freezing under suitable humidity conditions, frost can develop on its surface.
Too much frost restricts airflow and reduces heat-transfer performance.
The heat pump must therefore periodically remove it through a defrost cycle.
Defrosting is a normal part of air-source heat-pump operation.
However, unnecessary or poorly controlled defrost cycles consume energy and can reduce seasonal performance.
Modern systems therefore use sensors and control algorithms to determine when defrosting is actually required.
This becomes particularly important in cold and humid climates.
Laboratory COP vs Real Winter Performance
Standardized test points are essential because they allow different heat pumps to be compared under controlled conditions.
However, a home does not operate at one fixed outdoor temperature throughout winter.
Temperatures rise and fall. Heating demand changes. Humidity varies. The required flow temperature can also change.
Therefore, one COP figure cannot describe an entire heating season.
Seasonal metrics such as SCOP provide a broader indication of performance across different operating conditions.
Real-world results are also influenced by system sizing, hydraulic design, controls, building characteristics and user behaviour.
This is why heat-pump selection should consider both standardized performance data and the conditions of the actual installation.
What Should You Look for in a Cold-Climate Heat Pump?
When evaluating a heat pump for colder European climates, do not focus on one headline specification.
Look at the complete performance envelope.
Important factors include:
- heating capacity at low outdoor temperatures;
- COP at several test conditions;
- seasonal efficiency;
- maximum and practical flow temperatures;
- compressor modulation;
- defrost strategy;
- operating temperature range;
- and correct sizing for the building.
Together, these figures provide a much better picture of winter performance.
Cold Weather Does Not Stop a Heat Pump — It Changes the Operating Conditions
Air-to-water heat pumps can continue extracting useful thermal energy from outdoor air even below freezing.
What changes in winter is the difficulty of moving that energy to the temperature required by the building.
As outdoor temperature falls, temperature lift increases. COP may decrease, heating demand rises and frost management becomes more important.
Good cold-weather performance therefore comes from the interaction of refrigerant, compressor, heat exchangers, controls and system design.
For modern R290 systems, understanding those relationships is much more useful than asking whether a heat pump simply “works” below 0°C.
The better question is:
How efficiently can the complete system deliver the heating the building needs under real winter conditions?
#FridayTech | Tongyi Heat Pump
