How does an air-to-water heat pump work when it’s cold outside?

It may sound surprising, but even cold air contains thermal energy. An air-to-water heat pump uses a refrigerant with a very low boiling point. The refrigerant absorbs heat from the outdoor air, the compressor raises its temperature, and the heat is then transferred to the home’s heating system.

This is the same principle used by all heat pumps: electricity is used to move and concentrate heat rather than generate it directly.

If you would like a more detailed explanation of the process, read our guide on how an air-to-water heat pump works.

As outdoor temperatures fall, two things happen:

• There is less readily available energy in the outdoor air.
• The home's heating demand increases at the same time.

As a result, the heat pump has to work harder. Efficiency is generally lower during periods of severe cold than in milder weather, but that does not mean the heat pump stops being effective. It simply means the system must be properly selected and sized for the property, local climate and heating system.

It is also important to understand the difference between COP and SCOP. COP measures efficiency at a specific operating condition, while SCOP provides a more realistic picture of performance across an entire heating season by taking varying temperatures into account. When comparing heat pumps for Nordic conditions, it is therefore important to look at performance figures for cold climates.

When an air-to-water heat pump extracts energy from outdoor air, the air passing through the outdoor unit becomes even colder. If the air contains moisture, condensation can form and freeze into frost or ice during cold weather. This is completely normal.

To manage this, air-to-water heat pumps have an automatic defrost function. During a defrost cycle, the system temporarily prioritises removing ice from the outdoor unit to maintain proper airflow and heat absorption.

It is also normal to see water beneath or around the outdoor unit, particularly when temperatures are close to freezing. This water comes from the defrost process and should be drained away properly to prevent ice build-up beneath the unit or on nearby walkways.

Many heating systems include some form of backup heating, often electric, which can assist when demand is at its highest. This is not a sign that the heat pump is underperforming. It is a normal part of a practical and cost-effective system design.

Sizing a heat pump to meet a home's absolute peak heating demand during the coldest hours of the year can be unnecessarily expensive. Instead, it is common for supplementary heating to provide support during short periods of extreme cold.

For homeowners, this is typically an electric immersion heater or electric backup element, while larger buildings may use other energy sources.

In other words, a well-designed heating system should not only perform efficiently during a typical winter day. It should also be prepared for those few occasions when winter temperatures reach their lowest levels.

The answer depends on the model, installation and the property's heating requirements. Modern air-to-water heat pumps are specifically designed to operate in Nordic climates, but every home has different conditions.

One example is the CTC EcoAir 700M, a modulating air-to-water heat pump that automatically adjusts its output to match the property's energy demand. Designed for Nordic conditions, it can provide heating and hot water at outdoor temperatures down to -25°C.

However, the more important questions are:

• How much heat does the property require?
• Where in Sweden is the property located?
• Does the property use radiators, underfloor heating or both?
• What flow temperature does the heating system require?
• How much domestic hot water is needed?
• Is there a suitable location for the outdoor unit and proper drainage for condensation water?

An air-to-water heat pump can be an excellent choice, particularly for homes that already have a water-based heating system and where drilling or excavation is not desirable.

In colder parts of Sweden, however, the system must be sized particularly carefully. Long periods of very low outdoor temperatures place greater demands on the heat pump, and in some cases a ground source heat pump may be a more suitable alternative.

For properties with exceptionally high heating demands, ground source heating can sometimes offer advantages because the energy is extracted from the ground, where temperatures remain relatively stable throughout the year.

That is why it is important to compare heating solutions based on the needs of the property rather than focusing solely on the product itself.

Before investing in an air-to-water heat pump, consider the following:

• Does the property have a water-based heating system?
• How much energy does the property currently consume?
• What temperature do the radiators require during cold weather?
• Is the property located in a mild, moderate or very cold climate?
• Where can the outdoor unit be installed without causing disturbance?
• How will condensation water and defrost water be drained away?
• Would you like the option to expand the heating system in the future?

Future flexibility is often worth considering. Our EnergyFlex concept is designed to make heating systems adaptable, allowing additional heat sources, thermal storage tanks, solar thermal systems, pool heating and other equipment to be integrated now or later.

An air-to-water heat pump continues to operate efficiently even when outdoor temperatures fall below freezing, although it must work under more demanding conditions. Performance is influenced by outdoor temperature, the property itself, the heating system and how well the installation has been sized.

The key is choosing a solution that matches the specific requirements of your home. When properly installed and correctly sized, an air-to-water heat pump can provide comfortable indoor temperatures, reliable hot water production and lower heating costs throughout much of the Swedish winter.

If you are unsure whether an air-to-water heat pump is the right choice for your property, the next step is to speak with an installer or dealer who can assess your home's heating requirements, existing heating system and overall conditions. This ensures that the solution is correctly sized and tailored to your needs from the outset.

Does an air-to-water heat pump work in sub-zero temperatures?

Yes. Modern air-to-water heat pumps are designed to operate in freezing conditions. While efficiency generally decreases as temperatures fall, a properly sized system can continue to provide effective heating, often with the support of backup heating during periods of extreme cold.

Why does an air-to-water heat pump need to defrost?

When humid air is cooled in the outdoor unit, frost and ice can form on the heat exchanger. The defrost cycle removes this ice to maintain airflow and ensure efficient heat transfer.

Is it normal to see water beneath the outdoor unit?

Yes. Water is a natural by-product of the defrost process, particularly during cold and humid weather. It should be drained away properly to prevent ice build-up.

Do I need an electric backup heater?

Many systems include electric backup heating as additional support during very low temperatures or periods of unusually high heat demand. This is often more cost-effective than sizing the heat pump to meet the property's peak heating demand on its own.