Do R290 Systems Work in Freezing Weather? Maximum Supply Temperatures and COP Explained

The word “propane” may bring barbecues and patio heaters to mind.

But in a modern heat pump, propane serves a more technical purpose.

R290 is a natural refrigerant increasingly used in air-source heat pumps because it has a very low global warming potential and can operate across a broad temperature range. It can also help certain air-to-water systems produce higher water temperatures than many earlier heat pump designs.

That sounds promising for older UK homes, especially those with traditional radiators.

However, it raises several important questions.

• Do R290 heat pumps still work when the outdoor temperature falls below freezing?
• Can they genuinely supply water at 70°C or 75°C?
• What happens to their coefficient of performance (COP) in cold weather?

Before we get into more detail, let’s talk about what an R290 heat pump would be.

What Is an R290 Heat Pump?

When we talk about R290, we’re technically talking about the name of the refrigerant used in the system to circulate heat.

Like other refrigerants, it moves through the heat pump’s sealed refrigeration circuit, repeatedly evaporating and condensing as it absorbs and releases thermal energy.

R290 has several characteristics that make it attractive for modern heating systems:

• An extremely low global warming potential
• Strong thermodynamic performance
• The ability to support relatively high refrigerant temperatures
• Compatibility with low-temperature and high-temperature heat pump designs

Because R290 is an A3 refrigerant, it is highly flammable. The refrigerant circuit and installation must comply with the manufacturer’s safety requirements, clearance distances, and relevant industry standards.

This does not mean an approved R290 heat pump is unsafe. However, the appliance must be designed, positioned and installed correctly by a qualified professional.

Do R290 Heat Pumps Work Below Freezing?

Yes.

Many modern R290 air-source heat pumps are designed to continue operating below 0°C.

Certain current air-to-water models advertise operating limits as low as -25°C, although the precise limit varies between manufacturers and individual products.

Manufacturers publish different operating limits for individual products. For example, Panasonic states that its Aquarea T-CAP range can operate at outdoor temperatures as low as -28°C and maintain its rated capacity without backup heating down to -15°C. Certain R290 M Series models can maintain capacity without backup heating at -20°C when supplying water at 35°C.

Vaillant’s aroTHERM plus technical documentation also provides model-specific heating capacity and COP figures under sub-zero test conditions.

However, a heat pump’s minimum operating temperature should not be confused with its heating output, maximum flow temperature, or efficiency at that temperature. A unit may operate in extreme cold while producing less heat or using more electricity.

Three different specifications need to be separated:

1. Minimum operating temperature: the lowest outdoor temperature at which the unit is designed to run.
2. Heating capacity at low temperature: how much heat the unit can actually produce under those conditions.
3. Coefficient of performance: how efficiently it produces that heat.

A heat pump can technically remain operational while producing less heat and consuming more electricity than it would during milder weather.

The installer must assess the manufacturer’s performance tables rather than rely only on the operating-temperature headline.

How Can A Heat Pump Extract Heat From Freezing Air?

Air at 0°C still contains thermal energy.

Even well below freezing, outdoor air remains much warmer than absolute zero, the theoretical point where thermal molecular motion is minimal. A heat pump uses a refrigerant that can evaporate at a temperature lower than the surrounding outdoor air.

Heat therefore moves from the air into the colder refrigerant.

The compressor then raises the pressure and temperature of the refrigerant vapour. This allows the captured energy to be released inside the home.

The process becomes more demanding as the temperature difference between the outdoor air and the required indoor supply temperature increases.

In other words, lifting heat from 7°C outdoor air into a 35°C heating circuit is a relatively gentle climb. Lifting it from -7°C air into water at 65°C is a much steeper thermal hill.

The heat pump may still make the journey, but it will normally use more electricity along the way.

What Happens To COP In Cold Weather?

COP stands for coefficient of performance. It measures how much useful heat a system produces compared with the electricity it consumes.

Jargon Buster: Coefficient of Performance (COP)
A metric used to measure the efficiency of a heat pump, calculated by dividing the useful heat output by the electrical energy input.

A COP of 4 means that the heat pump supplies four units of heat for every one unit of electrical energy consumed.

For example:

• 1kWh of electricity used
• 4kWh of heat supplied
• COP of 4

COP is not a fixed score permanently attached to a machine. It changes according to the conditions under which the heat pump operates.

The two most important temperatures are:

• The temperature of the heat source, such as the outdoor air
• The temperature the heat pump must supply to the home

As the outside air becomes colder, the compressor generally works harder.

The COP therefore tends to fall.

Efficiency can decrease further when the system must produce hotter water or warmer supply air. The Energy Saving Trust recommends lowering a heat pump’s flow temperature where possible, as heating water to a higher temperature makes the system work harder and reduces efficiency.

The flow temperature should still remain high enough to heat the property comfortably.

Why Test Conditions Matter

A manufacturer might publish a COP measured under conditions such as A7/W35.

These ratings mean:

• A7: outdoor air at 7°C
• W35: heating water supplied at 35°C

These are favourable operating conditions commonly associated with underfloor heating or generously sized low-temperature radiators.

A second figure may be provided for A-7/W55:

• Outdoor air at -7°C
• Heating water supplied at 55°C

The COP at A-7/W55 will normally be considerably lower because the heat pump must bridge a much larger temperature gap.

Comparing one product’s A7/W35 figure with another product’s A-7/W55 figure would therefore be like comparing a bicycle rolling along a level road with one climbing a mountain. Both are bicycles, but the workload is not remotely equal.

Always compare COP figures measured under the same conditions.

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COP Is Not The Same As SCOP

COP describes performance at one defined test point.

SCOP stands for seasonal coefficient of performance. It estimates efficiency across a complete heating season, including a range of outdoor temperatures and part-load conditions. It estimates efficiency across a complete heating season, including a range of outdoor temperatures and part-load conditions.

Real household performance may also be described using a seasonal performance factor, or SPF, depending on where the system boundary is measured.

Seasonal figures are generally more useful when estimating energy consumption because a UK heat pump will not spend the entire winter operating at one fixed outdoor temperature.

However, SCOP remains a standardised rating rather than a guarantee of household running costs.

Actual performance depends on factors including:

• Heat loss from the building
• Heat pump sizing
• Radiator or underfloor-heating design
• Required flow temperature
• Control settings
• Hot-water demand
• Defrost operation
• Installation quality
• Occupant behaviour

Energy Saving Trust guidance stresses that heat pump sizing should be based on the home’s heat demand at its local design outdoor temperature, supported by room-by-room heat-loss calculations and manufacturer performance data.

What Is A Heat Pump’s Maximum Supply Temperature?

For an air-to-water heat pump, supply temperature usually refers to the temperature of the water leaving the heat pump.

This is also called the:

• Flow temperature
• Leaving water temperature
• Water outlet temperature

Several modern R290 air-to-water heat pumps advertise maximum flow temperatures of approximately 70°C to 80°C.

For example, Vaillant lists flow temperatures of up to 75°C for its aroTHERM plus range.

Panasonic states that certain R290 Aquarea systems can provide water outlet temperatures of up to 75°C, including up to 75°C at an outdoor temperature of -10°C on specified models. Midea advertises leaving-water temperatures of up to 80°C for parts of its R290 range.

These figures demonstrate what particular systems can achieve under defined operating conditions.

They do not mean that every R290 system can provide 75°C water at every outdoor temperature, or that running the system continuously at 75°C is the most efficient way to heat a home.

Maximum Does Not Mean Optimum

A car may be capable of travelling at 130mph. But that does not mean 130mph is its ideal speed for every journey.

The same principle applies to heat pump supply temperatures.

An Engineer’s View

While an R290 system can theoretically push flow temperatures up to 75°C, doing so constantly will drastically reduce your efficiency and increase running costs. Always aim to upgrade your heat emitters and improve home insulation to get the most out of your heat pump.

– Rhys Powell, Engineer

A high maximum flow temperature provides useful flexibility. It may help with:

• Heating domestic hot water
• Legionella-control cycles
• Older radiator systems
• Short periods of high heat demand
• Properties where a full emitter upgrade is impractical

However, lower flow temperatures usually support better efficiency.

A well-designed system should therefore aim for the lowest practical flow temperature rather than treating the maximum figure as the normal thermostat setting.

What About R290 Air-To-Air Heat Pumps?

An air-to-air heat pump does not heat water for radiators.

Instead, it extracts heat from the outdoor air and releases it directly into indoor air through fan-coil units.

The phrase “maximum supply temperature” therefore means something different.

Air-to-air manufacturers may specify:

• Heating operating range
• Indoor discharge-air temperature
• Heating capacity at defined outdoor temperatures
• COP or seasonal coefficient of performance
• Maximum or minimum indoor and outdoor temperatures

The temperature of the air leaving an indoor unit is not directly comparable with the flow temperature of an air-to-water system.

Warm air at 40°C moving through a room can provide effective heating, but it does not serve the same purpose as 40°C water circulating through underfloor pipes or radiators.

Air-to-air systems can also operate below freezing when designed for cold-weather heating. As with air-to-water models, their capacity and COP usually decline as the outdoor temperature falls.

The UK Government’s review of air-to-air heat pumps notes that real-world performance depends on factors including climate, system sizing, controls, building heat loss and the use of supplementary heating.

Model-specific low-temperature performance is particularly important. Some units are primarily designed as air conditioners with occasional heating, while others are engineered to remain effective during sustained winter conditions.

Does R290 Automatically Make a Heat Pump Better in Cold Weather?

Not automatically. R290 has favourable thermodynamic properties and allows manufacturers to design systems capable of relatively high supply temperatures.

However, refrigerant choice is only one part of the machine.

Cold-weather performance also depends on:

• Compressor design
• Heat-exchanger size
• Refrigerant charge
• Fan operation
• Control software
• Defrost strategy
• Inverter range
• Vapour-injection or other cold-climate technology
• System sizing
• Installation quality

Two R290 heat pumps can therefore deliver different outputs and COP figures at -7°C.

Consumers should compare the complete performance envelope rather than selecting a system purely because the brochure contains the letters “R290.”

What Is a Defrost Cycle?

When an air-source heat pump operates in cold, damp conditions, moisture can freeze on the outdoor heat exchanger.

A light coating of frost may be manageable. A thick layer restricts airflow and insulates the coil, reducing the amount of heat it can absorb.

The heat pump must periodically melt this ice using a defrost cycle.

During defrost, the refrigeration cycle is usually reversed temporarily so that heat is directed towards the outdoor coil.

This can:

• Interrupt normal heat production briefly
• Use additional electricity
• Reduce the COP measured over that period
• Produce visible water or steam around the outdoor unit

Frosting is often particularly relevant at temperatures around 0°C to 5°C when the air contains enough moisture to freeze on the coil.

A cloud of vapour rising from the unit during defrost is not usually a mechanical distress signal.

It is often water vapour produced as the ice melts.

The unit does, however, need suitable drainage so that defrost water does not refreeze beneath it and create a small private skating rink.

Is A Cold, Dry Day Worse Than A Mild, Damp Day?

Not necessarily.

Very low outdoor temperatures increase the temperature lift required from the heat pump.

However, damp conditions close to freezing may trigger more frequent frosting and defrost cycles.

A dry day at -5°C could therefore create a different performance profile from a humid day at 2°C.

This is one reason why a single outdoor-temperature figure cannot describe the complete winter performance of a heat pump.

Humidity, wind, cycling, part-load operation and defrost frequency all influence real electricity consumption.

Will Backup Heating Be Needed?

Some heat pump systems include an electric immersion heater or resistance heater.

This may be used for:

• Emergency backup
• Domestic hot-water sterilisation
• Supporting the heat pump during unusually severe conditions
• Covering demand beyond the compressor’s available output

Backup heat is not automatically a sign that the heat pump has failed.

However, direct electric resistance heating typically delivers approximately one unit of heat for each unit of electricity consumed. Its effective COP is therefore around 1.

Frequent reliance on backup heat can significantly increase running costs.

The heat pump should consequently be sized and configured so that supplementary heating is used only when genuinely required.

Some manufacturers also distinguish between the operating limit and the temperature at which backup heating may begin. Those figures must not be treated as interchangeable.

Why Heat Pump Sizing Matters More During Freezing Weather

A heat pump that appears perfectly adequate at 7°C may struggle when the temperature reaches the local winter design condition.

This happens because two things occur simultaneously:

• The home loses heat more quickly.
• The heat pump may produce less heat or operate less efficiently.

The installer must therefore compare the building’s heat loss with the heat pump’s available output at the relevant outdoor temperature and required flow temperature.

Nominal Output Alone Is Not Enough

A unit advertised as an 8kW heat pump may not necessarily provide exactly 8kW under every condition.

Manufacturer capacity tables show how its output changes across different combinations of outdoor and supply temperatures.

How To Read An R290 Heat Pump Specification

Before choosing a system, look for the following information:

Minimum Outdoor Operating Temperature

This shows the lowest ambient temperature at which the manufacturer permits operation.

Heating Capacity At Low Temperatures

Check the available output at -2°C, -7°C or the design temperature relevant to your location.

COP At Comparable Test Points

Compare products using the same outdoor and supply-temperature conditions.

Maximum Flow Temperature

Confirm whether the quoted maximum is available across the full operating range or only at specific outdoor temperatures.

Capacity Without Backup Heating

Check whether the stated output includes support from an electrical heating element.

Seasonal Efficiency

SCOP offers a broader indication of typical seasonal performance than one isolated COP figure.

Defrost And Drainage Requirements

Ensure the proposed outdoor position allows adequate airflow and safe disposal of condensate and defrost water.

So, Do R290 Systems Work in Freezing Weather?

Yes, correctly selected R290 air-to-water and air-to-air heat pumps can continue providing heat when temperatures fall below freezing.

Some R290 air-to-water models are rated to operate at outdoor temperatures as low as -25°C and can produce flow temperatures of 70°C, 75°C or even higher under specified conditions.

The important qualification is that maximum output, maximum flow temperature and maximum efficiency do not normally occur at the same time.

As outdoor temperatures fall and supply temperatures rise, COP generally decreases.

A high-temperature R290 heat pump can provide valuable flexibility for older homes and existing radiators. Nevertheless, the most efficient installation will still aim to reduce the required temperature lift through appropriate sizing, insulation, emitter design and weather-compensated controls.

Let WarmZilla Assess Your Home

Choosing a heat pump involves more than finding the system with the highest temperature printed on its brochure.

The property’s heat loss, radiator sizes, insulation, pipework, hot-water demand and local winter conditions all influence the required specification.

WarmZilla supplies and installs air-to-air heat pump systems designed to provide efficient heating and cooling throughout the year.

Rather than choosing a system based on a single headline temperature or efficiency figure, WarmZilla can help you explore an air-source solution suited to your property, room layout and heating requirements.

Complete WarmZilla’s online air conditioning survey to begin comparing suitable air-to-air heat pump options for your home.

Frequently Asked Questions

Do R290 Heat Pumps Stop Working At 0°C?

No. Many R290 heat pumps are specifically designed to operate below freezing. The exact minimum temperature and available heating capacity depend on the model.

Can An R290 Heat Pump Produce 75°C Water When It Is Freezing Outside?

Some models can produce water at or near 75°C under specified sub-zero conditions. However, the maximum available temperature varies by product and may decrease at very low outdoor temperatures.

Does A Heat Pump Have A COP Of 3 Or 4 All Winter?

Not necessarily. COP changes with outdoor temperature, flow temperature, demand, cycling, and defrost operation. Seasonal performance is better assessed using SCOP or a properly calculated seasonal performance estimate.