Air Source Heat Pump Temperature: What Temperature Does an Air Source Heat Pump Work At?
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Quick Answer
Most modern air source heat pumps work below freezing. Depending on the model, some can continue operating at outdoor temperatures of around -20°C to -25°C.
However, efficiency usually falls as the weather becomes colder because the heat pump has to work harder to move heat indoors.
For space heating, many UK systems work efficiently with a flow temperature of around 35°C to 45°C. Underfloor heating can often operate at even lower temperatures. Meanwhile, room thermostats are commonly set between 18°C and 21°C.
Therefore, the key question is not simply how cold a heat pump can operate. It is whether the system can provide enough heat for your home at an efficient flow temperature during cold weather.
Source: https://www.edfenergy.com/energywise/ground-source-vs-air-source-heat-pumps
Air Source Heat Pump Temperature: What Temperature Does an Air Source Heat Pump Work At?
Modern air source heat pumps can keep heating your home when the temperature drops below 0°C. In fact, many current models are designed to operate at around -15°C to -25°C, depending on the make and model.
However, the lowest outdoor temperature is only part of the story.
For UK homeowners, three temperatures matter most:
- the outside air temperature;
- the heating flow temperature sent to your radiators or underfloor heating; and
- the indoor temperature set on your thermostat.
These temperatures affect comfort, efficiency and electricity use in different ways. Therefore, knowing the difference can help you understand whether a heat pump will cope with a UK winter and how to run it efficiently.
This guide focuses only on those temperature questions. You will learn how cold an air source heat pump can work in, what happens during freezing weather, what flow temperature to aim for and when higher temperatures may be needed.
Get a Free Quote: Find out what heat pump size and flow temperature could work for your property with a free, no-obligation assessment.
Air Source Heat Pump Temperatures at a Glance
| Temperature | Typical range | What it means |
|---|---|---|
| Outdoor operating temperature | Often around -15°C to -25°C, model dependent | Lowest outside temperature at which the unit is designed to operate |
| Radiator flow temperature | Commonly around 35°C to 50°C | Water temperature supplied to radiators |
| Underfloor heating flow temperature | Often around 30°C to 40°C | Lower temperatures can work because of the large heat-emitting surface |
| Room thermostat | Around 18°C to 21°C for many homes | Target indoor comfort temperature |
Energy Saving Trust currently gives typical installation benchmarks of around £12,000 for an air source heat pump and around £9,200 for an automatically fed pellet biomass boiler. Actual quotations can vary considerably.
What Outside Temperature Can an Air Source Heat Pump Work At?
Air source heat pumps do not stop working simply because the temperature reaches 0°C.
Instead, modern units are designed to collect heat from outside air even when that air feels extremely cold. Depending on the model, many continue operating at temperatures well below freezing.
Some modern systems are rated to around -20°C or -25°C. For example, Daikin publishes air source heat pump models designed for heating operation at outdoor temperatures as low as -25°C. You can see more in Daikin’s cold-weather heat pump guidance.
Minimum temperature is not the only figure to check
A heat pump being able to operate at -20°C does not automatically mean it can heat every property effectively at that temperature.
Instead, you also need to consider its heating capacity.
Heating capacity means how much heat the unit can deliver. As a result, a properly sized heat pump is more important than simply choosing the model with the lowest advertised operating temperature.
For example, a unit may still run at -15°C but struggle to keep up with a home’s heat loss if it is too small.
Therefore, when comparing heat pumps, check how much heating output the system can provide during cold conditions rather than focusing on one headline temperature.
What Happens When the Temperature Falls Below Freezing?
There is no sudden performance drop when the thermometer passes 0°C.
However, the heat pump has to work harder as the outside temperature falls. At the same time, your home usually loses heat more quickly.
As a result, electricity use can increase during very cold weather.
Defrost cycles are normal in winter
Cold and damp weather can also cause frost to form on the outdoor heat exchanger.
When this happens, the heat pump automatically starts a defrost cycle. During this process, it briefly uses heat to melt the frost from the outdoor unit.
Therefore, you may occasionally notice:
- a temporary pause in heating;
- water beneath the outdoor unit;
- visible vapour or steam; or
- a change in the sound of the system.
This does not automatically mean there is a fault.
Baxi also explains that defrost cycles are a normal part of cold-weather heat pump operation. See its air source heat pump cold-weather guidance.
How Does Cold Weather Affect Heat Pump Efficiency?
Air source heat pumps normally become less efficient as outside temperatures fall.
The reason is simple. The colder it becomes outside, the greater the temperature difference the heat pump has to overcome to heat your home.
A common way to measure heat pump efficiency is COP, or coefficient of performance.
For example, a COP of 3 means the system provides about three units of heat for every unit of electricity used at that operating point.
Which? provides useful example figures showing how COP can change with outdoor temperature:
| Outside temperature | Illustrative COP |
|---|---|
| 15°C | 4.3 |
| 10°C | 3.9 |
| 0°C | 2.9 |
| -5°C | 2.5 |
These figures are examples, not a prediction for every home. Your actual performance will depend on the heat pump, weather, heating controls, flow temperature and property heat loss.
Most importantly, lower efficiency does not mean the heat pump has stopped working.
Instead, it means the system may need more electricity to produce the same amount of heat.
What Flow Temperature Should an Air Source Heat Pump Use?
Flow temperature is the temperature of the water leaving the heat pump and travelling to your radiators or underfloor heating.
For many homes with suitable radiators, around 35°C to 45°C can provide a good balance between comfort and efficiency.
MCS heat pump guidance recommends considering heating systems that can operate at low flow temperatures, with around 45°C often used as an important design point. See the MCS Heat Pump Guide.
Why lower flow temperatures help
Generally, a heat pump works more efficiently when it does not have to produce very hot water.
Therefore, the aim is not to make your radiators feel as hot as they did with a gas boiler.
Instead, the aim is to provide enough gentle heat over a longer period to maintain a comfortable room temperature.
For example:
- underfloor heating may work at around 30°C to 40°C;
- well-sized radiators may work at around 35°C to 45°C;
- some homes may require 50°C or more in colder weather.
High-temperature heat pumps can produce hotter water. However, higher flow temperatures will usually reduce efficiency.
For more detail on the heating cycle itself, see our guide to how air source heat pumps work.
If you are comparing lower-temperature heating with a traditional boiler, read our air source heat pump vs gas boiler guide.
What Temperature Should You Set Your Thermostat To?
For many households, an indoor temperature of around 18°C to 21°C is comfortable.
However, there is no single setting that suits everyone.
Older people, young children and people who spend long periods at home may prefer a warmer room. Meanwhile, bedrooms are often comfortable at slightly lower temperatures.
The bigger difference is how the heat pump reaches that temperature.
Unlike a boiler, a heat pump usually works best when maintaining a fairly steady indoor temperature. Therefore, repeatedly allowing the home to become cold and then asking the system to heat it very quickly can be less effective.
For example, if you are comfortable at 20°C, maintaining a steady temperature close to 20°C may work better than frequently moving the thermostat between 16°C and 22°C.
What Temperature Should Heat-Pump Hot Water Be?
Domestic hot water is different from the water circulating through your radiators.
Many heat pump systems store hot water at around 50°C to 55°C during normal use.
However, the cylinder may periodically need to reach a higher temperature to control bacteria such as Legionella.
Energy Saving Trust explains that many heat pump systems use a regular sterilisation cycle that takes the cylinder above 60°C. In some systems, an immersion heater helps reach that temperature. Read the Energy Saving Trust heat pump guidance.
Therefore, do not reduce your hot-water hygiene settings simply to cut electricity use.
Always follow the settings recommended for your system.
Should You Turn Your Heat Pump Up When It Gets Colder?
Usually, you should not need to make large manual adjustments whenever the weather changes.
Most well-controlled heat pump systems use weather compensation.
Weather compensation automatically adjusts the heating flow temperature based on the outdoor temperature.
How weather compensation works
When the weather is mild, your home loses less heat. Therefore, the heat pump can usually send cooler water around your heating system.
When the weather becomes colder, your home loses heat more quickly. As a result, the system gradually increases the flow temperature.
This relationship is known as the weather compensation curve or heating curve.
However, setting the curve too high can increase electricity use.
Energy Saving Trust notes that incorrect heating-curve settings can affect heat pump efficiency. Its heat pump efficiency guide explains the importance of suitable control settings.
Therefore, if your home stays warm with a 45°C flow temperature, increasing it to 55°C without a clear reason may simply make the system work harder.
How to Find the Right Air Source Heat Pump Temperature Settings
You can use the following process to understand whether your settings are suitable.
- Choose a comfortable room temperature.
For many households, this will be somewhere between 18°C and 21°C. - Check your weather compensation settings.
Find out what flow temperature your system targets during mild and cold weather. - Avoid unnecessarily high flow temperatures.
If your home remains comfortable at a lower setting, the heat pump may operate more efficiently. - Check the coldest room.
One room with a small radiator can cause problems even if the rest of the house is warm. - Leave hot-water hygiene settings alone.
Your cylinder temperature serves a different purpose from your space-heating flow temperature. - Check the manufacturer’s cold-weather data.
Look at heating capacity and efficiency during low outdoor temperatures, not only the headline seasonal efficiency figure.
For a broader look at the strengths and limitations of the technology, see our air source heat pump pros and cons guide.
How Does Temperature Affect Heat Pump Running Costs?
Temperature has a direct effect on the amount of electricity a heat pump may need.
First, colder outside air makes heat extraction more demanding.
Second, higher flow temperatures make the compressor work harder.
Therefore, the combination of freezing weather and an unnecessarily high flow temperature can increase electricity use significantly compared with mild-weather operation.
However, there is no universal saving from changing a flow temperature from, for example, 50°C to 45°C.
Every home has a different heat loss, radiator size and heating pattern.
The best target is therefore the lowest practical flow temperature that still keeps the property comfortable.
If you are considering installing a heat pump, current support may also reduce the upfront cost. You can check the official GOV.UK Boiler Upgrade Scheme guidance or read our Boiler Upgrade Scheme 2026 updates.
An accurate quotation still requires a property assessment because heat loss, radiator requirements and system design vary from home to home.
Which Homes May Need Higher Flow Temperatures?
Not every property can comfortably run at 35°C or 40°C.
For example, higher flow temperatures may be needed where:
- the property has relatively high heat loss;
- existing radiators are small;
- there is limited space for larger radiators;
- some rooms need more heat during cold weather; or
- the chosen system is designed as a high-temperature heat pump.
However, needing a higher temperature does not automatically mean a heat pump is unsuitable.
Instead, the important question is what efficiency the system can achieve at the actual flow temperature your home requires.
This is especially important when comparing quotations. A predicted performance figure based on 35°C flow water is not very useful if your home will regularly need 50°C.
By contrast, ground source heat pumps draw heat from ground temperatures that vary less through winter. If you are considering both technologies, see our air source vs ground source heat pump comparison.
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Frequently asked questions
Find clear answers to common questions comparing air source and ground source heat pumps, including efficiency, installation and running costs, winter performance, noise, land requirements and borehole depth.
Yes. Modern air source heat pumps are designed to operate below freezing. Their efficiency usually falls gradually as the outside temperature decreases.
It depends on the model. Many modern units operate down to approximately -15°C to -25°C, although you should always check the manufacturer's technical specification.
No. A correctly selected modern heat pump should normally continue operating at -5°C. Its COP may be lower than it would be during milder weather.
Some models can. Cold-climate heat pumps are available with published operating ranges reaching approximately -20°C to -25°C.
The lowest temperature that can comfortably heat the property is usually best. Many efficient radiator installations aim for roughly 35°C to 45°C.
Yes. Many systems can supply 50°C water, although efficiency will generally be lower than at 40°C or 45°C.
Yes. Many modern systems can provide temperatures of 60°C or above, particularly for stored hot water. High-temperature heat pumps can go higher still.
Around 30°C to 40°C is common, although the correct setting depends on floor construction, heat loss and system design.
Many households choose somewhere around 18°C to 21°C. Comfort preferences and individual rooms will vary.
Heat pumps generally perform well when maintaining relatively steady temperatures for longer periods rather than operating in short, intense heating bursts.
That can be completely normal. Heat pumps commonly supply radiators with lower-temperature water for longer periods than conventional boilers.
Generally, yes. The temperature difference the system has to overcome becomes greater, while the property itself is normally losing more heat.
It is an automatic process that removes frost from the outdoor heat exchanger. Defrost cycles are normal during cold, damp weather.
Visible vapour can occur during a defrost cycle as frost melts from the outdoor coil. It does not necessarily indicate a fault.
It can improve efficiency if the property still remains comfortably warm. The actual financial saving depends on the property and system.
Many systems normally store hot water around 50°C to 55°C, with periodic higher-temperature cycles for hygiene purposes.
Yes. Correctly specified heat pumps can operate at temperatures considerably below those experienced during most UK winter conditions.
Not always. Each radiator should be assessed against the room's heat loss at the proposed flow temperature.
Final Verdict: What Temperature Does an Air Source Heat Pump Work At?
Modern air source heat pumps can work in temperatures well below freezing. Depending on the model, many are designed to operate at around -15°C to -25°C.
However, the minimum outdoor temperature should not be your main buying criterion.
Instead, focus on three figures:
- the heat pump’s heating output in cold weather;
- the flow temperature your home requires; and
- the expected efficiency at that flow temperature.
For many UK homes, a flow temperature of roughly 35°C to 45°C can provide efficient heating when the radiators and heat pump are correctly matched.
Meanwhile, cold weather will normally reduce efficiency, but it does not mean the heat pump suddenly stops working at 0°C.
Therefore, the best system is not simply the heat pump with the lowest operating temperature. It is the one that can meet your home’s heat demand during cold weather without relying on unnecessarily high water temperatures.
Get a Free Quote: Find out what flow temperature, heat pump size and heating setup could work for your property with a free, no-obligation assessment.
