Air source heat pumps work by extracting heat energy stored naturally within outside air and using it to help provide heating for a home.
Higher outside temperatures means more heat energy stored within the air and so an air source heat pump can extract the heat more effectively.
At lower outside temperatures, such as those experienced throughout the winter months, an air source heat pump can have a harder time extracting heat to meet demand.
Air source heat pumps can work in winter because there’s still heat energy within outside air, even in minus temperatures. Air source heat pumps use refrigerant with low boiling points and a compressor to help extract and generate the most amount of heat all year round.
Our air source heat pump provides both heating and hot water for our home and is the sole source of heat generation for both.
It’s guaranteed to work down to -20°C (-4°F) and so meets our demands for both heating and hot water throughout the year as temperatures where we live won’t reach lower than this.
Air source heat pumps (ASHP) are able to work in winter at very low cold temperatures for a number of reasons, which we discuss and explain in more detail using our own ASHP as an example throughout this article.
An air source heat pump is able to work in winter through ways of extracting heat from air even when it’s cold because there’s still sufficient heat energy stored within the air even at temperatures below zero.
Air source heat pumps have been developed to work throughout the year in most climates, even in negative temperatures.
Recent advances in technology have allowed air source heat pumps to produce heat more efficiently at these lower temperatures.
Such innovations in their technology have made them efficient and viable options for those with extra chilly seasons. In places with super-low temperatures, like Alaska, air source heat pumps have proven effective.
Manufacturers have developed air source heat pump units to work effectively within certain temperature ranges and this can include down to very low temperatures depending on the model of heat pump.
For example, our air source heat pump has a guaranteed operating range of between -20°C (-4°F) and 21°C (70°F).
This means that our air source heat pump is able to work throughout winter because (in our particular climate) temperatures won’t reach this low.
As a result, we’ve been using this heat pump for over 5 years and have never had any issues with our heating or hot water through the colder months.
While air source heat pumps can work at very low temperatures, it should be noted however that they will typically require more energy to work when it’s colder outside and be less energy efficient at producing the same level of heat for a home.
As the temperature difference between the outdoor and indoor air becomes greater, an air source heat pump can need to work harder to produce the same indoor temperature air or water.
The table below shows how the power input required at lower outdoor air temperatures (A2 [2°C] compared to A7 [7°C]) is greater for our ASHP.
As power input (electricity usage) increases, the Coefficient of Performance (COP) decreases for our air source heat pump. We explain more about COP here.
Air source heat pumps can therefore be less cost effective in winter, but still work efficiently.
An ASHP will still be able to maintain adequate indoor air and/or water temperatures even in very cold conditions through winter. The actual heat output can lower, however, as outside temperatures reach very low.
For example, the graph below shows how the maximum water temperature that can be produced from our air source heat lowers as outside temperatures fall below freezing, but is still sufficient for our needs even at the very minimum operating outdoor temperature of -20°C (-4°F).
An air source heat pump can also be operating for longer periods of time to compensate for the lower amount of heat within the air and to extract the same amount of heat needed to provide the same desired indoor temperature levels.
An ASHP may also become noisier as a result of working harder and for longer in winter.
An air source heat pump is able to work in winter through using refrigerant with a low boiling point that turns from a liquid to a gas when absorbing heat from outside air, even in very cold winter temperatures.
Fans on an ASHP force air through the unit. Heat is then absorbed by the refrigerant flowing through coils located within the heat pump.
As heat energy from the air is captured by the refrigerant, it turns from a liquid to a gas because it has a low boiling point.
This gas is then pressurized using a compressor within the heat pump system, and the temperature of the gas increases with pressure.
This compression helps to maximize the temperature increase from extracting heat from outside air, which can be especially important at lower outside air temperatures during winter.
The compressor in our air source heat pump system is located in the heat pump unit itself.
Refrigerant carries the heat energy indoors to be used for heating and/or hot water.
As heat is released, temperatures of the refrigerant lower and an expansion valve helps to return the gas to a liquid, repeating the cycle.
In winter when outdoor air temperatures are colder, an air source heat pump may have to run for longer and work harder to extract enough heat to meet the demands for the same temperature heating and hot water.
By running for longer and harder, an ASHP can use the fans to force more air through the unit, and for longer periods of time, to capture more heat.
Air source heat pumps can’t deliver the same instantaneous heat as you might expect with a traditional on-demand gas combi boiler, and so need to extract heat over a longer period of time to achieve the same results.
In winter, an ASHP may simply need to operate for longer to generate enough heat for a home, but they will still be able to provide the required heat if outdoor temperatures are within the operating range for that heat pump.
This diagram here by Energy.gov helps explain the process of how an air source heat pump works and how they are able to extract heat even in winter.
This video by Technology Connections goes through the basics of how an air source heat pump operates:
In general, air source heat pumps are more efficient than furnaces and boilers. This doesn’t necessarily mean they’re less expensive, though.
Depending on where you live, electricity may be more expensive than natural gas. And if electricity is more expensive, you may have a larger energy bill. Regardless, you’re still using a more efficient energy system.
As discussed above, an air source heat pump can use more electricity as outdoor temperatures drop because there’s less heat in the air. Though, this may not be true for all systems; newer systems may fare better than older ones.
While outside air is typically much colder compared to desired indoor temperatures in winter, there’s still enough heat energy stored within the air for an air source heat pump to extract.
Though an air source heat pump can work in winter down to a certain temperature, it can need to work harder to generate sufficient heat for a home, and energy usage can increase.
Technological advancements has allowed air source heat pumps to work in winter in temperatures as low as around 20°C (-4°F), while still being able to meet demand for heating and/or hot water.