Air source heat pumps are electrical appliances that can be used to transfer thermal energy from one place to another. They absorb heat stored naturally within the outside air and transfer it inside a home for use as heating and/or hot water.
However, air source pumps have a typical outdoor air temperature range in which heat can be extracted effectively, and if temperatures are outside of this range, then they may not work as efficiently.
Furthermore, air source heat pumps will also have a maximum outlet temperature (for heating) and minimum outlet temperature (for cooling).
These maximum and minimum temperature ranges for both operating temperatures and outlet temperatures can vary between manufacturers and models of heat pump, but typically can work within a general temperature range.
Air source heat pumps can typically operate down to around -4°F (-20°C) and up to around 68°F (20°C) for outside air temperatures. Output temperatures for air to water heat pumps can be up to around 140°F (60°C) for heating and down to around 41°F (5°C) for cooling.
Our air source heat pump (ASHP) has an outdoor temperature range in which it is guaranteed to work and extract heat for our home for use in central heating and domestic hot water.
As it’s the air to water heat pump type (rather than air to air), it also has a maximum outlet water temperature that can be reached, as well as a minimum outlet temperature if the heat pump was to be used for cooling purposes.
We discuss in more detail below exactly what the maximum and minimum operating temperatures are for our air source heat pump, how output temperatures can be affected by outdoor air temperatures, and what temperatures you can expect your heat pump to work in and produce.
Air source heat pumps typically work in temperatures of between -4°F (-20°C) and 68°F (20°C). At temperatures below this range, they must work harder to extract heat, reducing efficiency, and the required heat output may not be guaranteed.
The actual operating temperature range for outdoor air of an air source heat pump will differ between models of heat pump.
However, as all ASHP’s work on the same principle, there can be a typical outdoor air temperature range in which they will work most effectively.
This range can be on average between -4°F (-20°C) and 68°F (20°C) but may be less of more depending on what the manufacturer of a heat pump guarantees.
It’s usually around this range that a manufacturer will guarantee that the heat pump can extract heat energy sufficiently from the air to use in a home.
For example, the manufacturer of our own air source heat pump provides a guaranteed operating temperature range for both heating and cooling for outside air.
As we live in a moderate climate, we don’t experience such extremes in temperature that would cause our heat pump to stop working effectively.
If a local climate experiences temperatures below the typical operating range of an air source heat pump, then it may need to be coupled with another form of heating appliance such as boiler or furnace to provide a hybrid system.
See our article explaining hybrid heat pumps for more information.
While air source heat pumps have a general temperature range for outdoor air in which they operate at their best, they can also have a maximum output temperature for heating and a minimum output temperature for cooling.
For example, the image below shows the maximum and minimum outlet water temperatures that can be produced by our air source heat pump.
As air source heat pumps can only extract and deliver so much heat, the maximum output temperature can be dictated by the outdoor temperatures.
The graph below is taken from the manual for our heat pump and shows the relationship between the outdoor air temperatures and the maximum outlet water temperature that can be produced.
As outdoor temperatures reach the minimum operating temperatures for our heat pump, the maximum output water temperature reduces (for our particular model of heat pump).
The manual to our ASHP also provides information on the relationship between two scenarios where outdoor air temperatures are different but the output water temperatures remain constant.
The Coefficient of Performance (COP) is reduced and the electrical power input is increased at a lower outdoor air temperature to generate the same temperature water.
See our article explaining air source heat pump COP for more information.
Understanding how air source heat pumps work is important in comprehending how they extract and provide heat to a home.
Air source heat pumps are similar to refrigerators. Typically, a fridge extracts heat inside the unit and releases it to the outside. An air source heat pump does the opposite– it absorbs heat from the outside and releases it inside your home.
A typical air source heat pump uses electricity to move heat from one place to another through the compression and expansion of refrigerants.
Refrigerant in liquid form absorbs heat from the air flowing through an air source heat pump unit and turns into a gas. This heat can be captured effectively within the outdoor temperature range.
A heat pump system contains a compressor, an expander, and an evaporator coil. The compressor compresses refrigerant to increase its temperature and maximize temperature gains. This heat then passes through the evaporator coil and used within a house.
The gas refrigerant then travels to the expander and loses heat by expanding and returns to liquid form. Cooler, liquid refrigerant returns to an air source heat pump for the cycle to begin again.
Here’s a great video on how air source heat pumps work:
As air source heat pumps extract heat energy stored naturally within the outside air, they can only operate down to certain temperatures before there isn’t enough heat within the air to effectively use for heating or hot water.
Air Source Heat Pump Minimum Temperature
On average, the lowest temperature a heat pump can work at efficiently is around 4°F (-20°C), but the exact temperature can depend on the model of heat pump. Below these temperatures, the desired output heating temperatures may not be guaranteed.
The low temperature limit for heat pumps can depend on the type of compressor installed.
According to a 2019 study by Slipstream, a nonprofit organization that develops renewable energy programs, there are two types of heat pump compressors– fixed speed and variable speed compressors.
In general, variable speed compressors are more efficient than fixed-speed compressors. Most notably, the former continually adjusts heat pump operation to meet the demand of the house, while the latter regulates heat pump operation based on an average temperature.
As a result, variable speed compressors tend to produce higher cooling output than fixed-speed ones, assuming the power consumption is the same in both cases.
That implies that for a fixed-speed compressor to match the output temperature of a variable speed one, the fixed-speed must use more electricity.
It can be too hot for a heat pump to work, especially in very humid climates where it would have to extract more moisture from the cool outdoor air. When it’s too hot, the system must work harder to remove or add heat, and it will thus use more energy than usual.
The main issue with extreme heat is that there’s a more negligible thermal difference between indoor and outdoor temperatures, which means it’ll take much longer for the house to warm up or cool down.
Due to this reduced energy source, some systems may not sufficiently raise air temperature during extremely hot periods.
Instead, the unit might consume more energy in the process, which may not be cost-effective.
At this point, it may be more cost-effective to use an alternate heating system such as an electric baseboard heater or gas furnace rather than force the compressor to work harder than necessary at higher than ideal temperatures.
What Should The Output Temperature From A Heat Pump Be?
The average output temperature of air source heat pumps is 85-92°F (29-33°C). However, the model, age, condition, and unit type determine the actual temperature. A newer heat pump with an efficient compressor and evaporator coil may produce hotter output than an older, less efficient one.
Typically, air source heat pumps are designed to meet the heating requirements of most homes. As such, they create just enough heat to offset the difference between indoor and outdoor temperatures without overdoing it.
Our heat pump is the air to water type and as mentioned previously has a maximum output temperature of 60°C (140°F), but this lowers as outdoor temperatures lower.
Going for lower output temperatures from a heat pump rather than maximizing the output temperature is more efficient and will use less electricity, which is why air source heat pumps are typically paired with efficient means of transferring heat inside a home such as underfloor heating and radiators.
Here are a few ways to help keep your heat pump from freezing in the winter:
- Keep your outdoor unit clean and clear of debris.
- Create a wind barrier around the heat pump.
- Insulate the heat pump drain.
- Drain the water that accumulates in the drain pan.
Let’s go over each of these tips in more detail below.
A dirty unit can make it harder for the system to extract heat from the outdoors. As a result, your house can take longer to warm up, and you may end up spending more money on electricity than necessary.
Check that there aren’t any leaves or snow resting near or on top of your outdoor unit. If need be, switch your heat pump off and use a broom or blower to clear the area.
See our article on air source heat pump maintenance for more information.
A strong wind can carry away warm air, making it more difficult and costly for the compressor to maintain adequate heating. In icy temperatures, this may even cause your heat pump to freeze.
To prevent the unit from freezing, you can erect an enclosure around your outdoor unit to protect it from the wind, while still ensuring that airflow through the heat pump isn’t compromised.
Different heat pumps utilize various types of drain pans. Some are insulated, and some aren’t. If yours isn’t, cover it with Styrofoam or foil-backed bubble wrap to help prevent freezing in the winter.
Just be sure to place a layer of water underneath if you use bubble insulation so that if it does happen to freeze, you won’t end up with a significant ice build-up.
This step is crucial if your heat pump drain pan isn’t insulated since any water that accumulates in the pan may freeze and cause damage to your system. Periodically check the pan and remove any accumulated water, giving it enough time to drain outside.
The actual minimum and maximum outdoor air operating temperatures can differ between models of heat pump but can be expected to be between around 4°F (-20°C) and 68°F (20°C).
At lower minimum outdoor air operating temperatures, maximum output temperatures, whether that be indoor air or water, may reduce.