How An Air Source Heat Pump Works (With Real Example)

An air source heat pump is a simple yet effective way to heat and cool a home and help make a house more energy efficient and less reliant on burning fuels directly to produce heat.

While there are two main different types of air source heat pump, the principles on how they work remain the same.

An air source heat pump however works much differently compared to other more traditional heating systems such as boilers or furnaces.

An air source heat pump works by extracting heat energy found naturally within outside air and moving that heat indoors for use in heating and hot water. In cooling mode, an air source heat pump reverses the process and transfers heat from indoors to outdoors.

Our air source heat pump was installed when our house was built and we’ve been using this heat pump for heating and hot water for over 5 years.

Air Source Heat Pump
Air source heat pumps such as our one shown force air through them using fans to extract heat from the air into refrigerant passing through the unit, which in turn heads into a home for use within heating appliances

In this article we explain in much greater detail exactly how an air source heat pump works, using our own heat pump system for reference.

How Air Source Heat Pumps Work

An air source heat pump works by:

  • Using both indoor and outdoor apparatus to provide a complete air source heat pump system. The main air source heat pump unit will be located outside while all other equipment will typically be located indoors.
  • Extracting heat energy stored naturally within the outside air by using fans to force air over a series of coils, in which refrigerant flows through.
  • With a low boiling point, the refrigerant can absorb heat energy from the outside air and evaporates from a liquid to a gas.
  • The gas refrigerant is compressed using a compressor to increase pressure, which in turn increases temperature and helps maximize the heat gained from extracting heat from outside air.
  • Higher pressure, higher temperature gas refrigerant delivers heat indoors via the indoor unit, which depending on the type of air source heat pump system, can be an air handler unit or a water tank. The refrigerant condenses back to a liquid again as it releases heat.
  • The refrigerant passes through an expansion valve to lower pressure, and therefore lower temperature, and returns to the outdoor heat pump where the cycle begins again.

Although primarily used for heating, an air source heat pump is a versatile appliance that can perform a range of functions including:

  • Space/central heating
  • Cooling
  • Providing domestic hot water

We discuss how an air source heat pump can achieve all of these throughout this article.

Extracting Outdoor Heat

An air source heat pumps work differently compared to more conventional forms of heating system because they move heat from one place to another rather than generating heat through combustion.

For ASHPs, this involves moving heat energy from outside air into an indoor space for use as heating.

Air source heat pumps are able to achieve this because heat energy is stored naturally within outside air, even at low outdoor temperatures that you would otherwise consider cold.

To extract heat from outside air, an air source heat pump system uses an outdoor heat pump unit that typically sits up against the external wall of a building.

Our external air source heat pump unit is located down the side of our house, away from the wall to allow for airflow around the unit.

Air Source Heat Pump
Our air source heat pump unit is placed in an open area down the side of our house, with sufficient space around it on all sides to allow for enough airflow to extract as much heat from the air as possible

Correct placement of an outdoor air source heat pump unit is important as wind, sunlight and lack of sufficient airflow can affect how efficiently a heat pump performs. See our article on where to install an air source heat pump for more information.

Air source heat pumps are electrical appliances and so ours takes power from inside our home. Unlike other forms of heating system, not other power or fuel is required.

Air Source Heat Pump Power Supply
An air source heat pump uses electricity as the sole source of power to run components such as the fan(s) and the compressor

An ASHP contains one or more fans that force air through the unit. Our heat pump is a dual fan unit; required to help the heat pump extract enough heat energy from the air to meet the demands of our larger sized home.

Air Source Heat Pump
Fans force air through the heat pump from the back

As air is pushed through, refrigerant flowing through coils inside the heat pump unit absorbs the heat energy from the air.

Air Source Heat Pump Back
Coils on the back of the unit extract heat from the air as its pulled through the heat pump
Air Source Heat Pump Side Coils
Refrigerant used to absorb the heat flows through the heat pump in pipes and these coils act as a reverse heat sink when in heating mode to aid in capturing heat

This liquid refrigerant has a low boiling point and evaporates to a gas as it picks up heat from the outside air.

This allows the use of a compressor to compress the gas to increase its pressure, which proportionally increases the temperature of the gas as a result.

The compressor for our ASHP is in the right-hand side of the outdoor unit, behind an access panel.

Air Source Heat Pump
A compressor, hidden behind the access panel on the right side of our heat pump, pressurises the refrigerant to generate even more heat

The use of a compressor helps an air source heat pump to generate the most amount of heat from the outside air.

Transferring Heat Indoors

To be able to heat a home, an air source heat pump must transfer the heat energy it has extracted from the outside air indoors.

Refrigerant from a heat pump will flow indoors through pipes.

The inlet and outlet pipes for the refrigerant are on the back of our air source heat pump unit.

Air Source Heat Pump Pipe
Refrigerant flows in and out of a heat pump at the inlet and outlet points

These pipes head through the external wall of our house and into our garage.

Air Source Heat Pump Back
Pipes from our air source heat pump head indoors

The pipes continue through our garage to our utility room cupboard where all of our main internal heating apparatus is located.

Air Source Heat Pump Pipes
Pipes carry refrigerant to and from the internal heating apparatus from an air source heat pump, shown here heading through our garage

Delivering Heat Indoors

Heat being carried by refrigerant inside an air source heat pump system will be transferred indoors via the interior coil heat exchanger.

Depending on the type of air source heat pump, the indoor heat exchange can be different.

The two main types of air source heat pump are:

  • Air to water heat pumps
  • Air to air heat pumps

Air To Water Heat Pumps

Our ASHP system is the air to water type, meaning that it extracts heat from outside air to be transferred indoors to water for use as central heating.

Air Source Heat Pump Plate
Our air source heat pump system is the air to water type, where heat is delivered to a water-based central heating system

At the other end of our air source heat pump system is therefore a water-based heating system.

Within this cupboard is where the heat is delivered to the water-based system through the interior heat exchanger.

Air Source Heat Pump Hot Water Tank
Heat is delivered to our indoor apparatus for heating and hot water purposes

Our internal cupboard consists of a range of apparatus to help deliver this heat around our home via a central heating system, including expansion and buffer tanks, pipes, valves, control systems and more.

For our home, the lower floor utilizes underfloor heating to deliver the heat, while the upper floor uses radiators.

See our main articles on using air source heat pumps with underfloor heating and using air source heat pumps with radiators for more information.

There’s a thermostat control panel in each room to control the underfloor heating in that room.

Air Source Heat Pump Thermostat
Heat can be delivered into a home from an air source heat pump via underfloor heating, of which we can control using these thermostats

Upstairs, there’s at least one large, modern and high surface area radiator in each room to help transfer the heat from the water flowing through the radiators as efficiently as possible.

Air Source Heat Pump Radiator
Large surface area radiators are an effective way to release heat generated by an air source heat pump

The whole air source heat pump heating system can be controlled from the central control panel located outside our cupboard that contains all the main heating apparatus.

Air Source Heat Pump Water Heater Control Panel
This control panel allows us to control both the heating and hot water settings for our air source heat pump system

As the refrigerant loses its heat indoors to heat up the water in the central heating system and the hot water tank, it condenses back into liquid form from a gas.

This refrigerant passes through an expansion valve to decrease pressure again, and therefore lower temperature, before returning to the outside heat pump unit for the cycle to start again.

This video explains more about how an air to water heat pump system works:

Air To Air Heat Pumps

Air to air heat pumps systems differ to air to water systems in that the end process is to heat the air in the home as space heating instead of heating water for use as central heating.

Rather than a water-based system, the indoor equipment of an air to air heat pump system can consist of air handling units/blowers/fan coil units.

These units can look much like standard domestic air conditioning units but used for blowing hot air instead of simply conditioning the air.

An example of an air handler that can be used to blow heated air into a room

These air handling units will be where the heat is exchanged from the refrigerant flowing through, to the air, which is forced out into the room using an internal fan.

See our main guide on air source heat pump air handling units for more information.

Most air to air heat pump systems are split systems, which is where one coil is located inside (the air handling unit etc) and one is located outside (the heat pump unit itself).

Packaged air to air heat pumps systems can have both coils located outside, with air delivered around the internal rooms via ductwork (see our guide to ducted and ductless heat pumps here).

Again, once the heat is delivered internally, the liquid refrigerant returns to the outside heat pump unit for the process to be repeated.

Air to air heat pumps systems are more popular in warmer climates because they can more effectively be used for cooling as well as for heating (we discuss how ASHPs can cool further in this article.

Where we live (in the UK), air to water heat pump systems are more popular because cooling isn’t such a necessity and air to water heat pumps can also be used to provide domestic hot water.

See our article comparing air to air and air to water heat pumps for more information.

How An Air Source Heat Pump Can Heat In Winter

Air source heat pumps work differently compared to other more traditional forms of heating system because they move heat from place to another (outdoors to indoors) rather than generating heat from the combustion of fuels.

One of the issues with the effectiveness of how well an air source heat pump can work is when outside temperatures get extremely low.

Technology has allowed heat pumps to work down to very low temperatures but at extreme temperatures there can be not enough heat energy within the outside air to extract efficiently.

Our air source heat pump can work down to -20°C (-4°F), but below this it’s not guaranteed to generate sufficient warmth to heat out home.

Air Source Heat Pump Operating Temperatures
The outdoor air temperature range in which our air source heat pump can operate effectively for heating

As temperatures don’t reach this low in our climate, we don’t have to worry about our ASHP not being able to perform. However, in areas that are subject to more extreme temperatures then an air source heat pump may need to be coupled with another form of heating, such as with a boiler to provide a hybrid heat pump system.

Down to this minimum operating temperature of -20°C (-4°F) for outside air, our air source heat pump is still able to generate a sufficient level of output heat for our home.

At these low temperatures there’s is still a sufficient amount of heat energy stored within the outside air for it to extract.

The graph below shows how our heat pump is able to provide a sufficient level of heating down to the minimum operating temperature. However, this output temperature drops off significantly as temperatures get very cold.

While air source heat pumps can still work in very cold temperatures, the graph shows how maximum heat output from our heat pump falls as temperatures fall

To work in these low temperatures, an air source heat pump can need to work for extended periods of time, and/or work harder with faster fan speed for increase airflow (which can increase noise levels).

Our article on how an air source heat pump can work in winter provides more detail.

How Air Source Heat Pumps Can Provide Cooling

As mentioned previously, air source heat pumps, in particular air to air heat pumps, can provide cooling as well as heating.

This can be possible when the system is fitted with a reversing valve that allows the indoor unit to extract heat and dispel it outside at the external heat pump when in cooling mode, rather than the opposite process when in heating mode.

Through removing heat energy from the air inside a home, indoor air temperatures lower; therefore providing a cooling effect.

In an air source heat pump system, refrigerant is used to absorb heat from inside your home in order to cool it down.

Heat energy is transferred from the indoor air to the refrigerant by means of the evaporator coil. The heat energy is released outside at the condenser coil inside the external heat pump unit.

While our air source heat pump is the air to water type, it could still be possible to provide cooling with the right setup. The image below shows the outdoor temperature range in which our heat pump could provide cooling.

Air Source Heat Pump Operating Temperatures
Our air source heat pump could provide effective cooling within a wide temperature range (with the right setup)

However, we don’t personally use it for cooling our home, and instead use separate air conditioning units to cool individual rooms.

Air Source Heat Pump & Air Conditioner
We use separate air conditioning units (left) for cooling rather than using our air source heat (right) pump for cooling

See our article comparing air source heat pumps to air conditioning for more information.

How Air Source Heat Pumps Can Provide Domestic Hot Water

While air source heat pumps are primarily used for heating purposes, with the potential to also be used for cooling, they can also be used to provide domestic hot water with the right setup.

Air to air heat pumps can’t be used for providing hot water to taps etc, and so an air to water heat pump system will need to be used. An alternative way of generating hot water for a home will therefore need to be considered when using an air to air heat pump system.

As air source heat pumps extract and deliver heat more slowly over time compared other conventional heating systems (and therefore can’t provide hot water on demand),  an air to water heat pump system will need to be coupled with a hot water tank in order to provide domestic hot water.

We use our ASHP to provide both heating and hot water for our home and so we have a hot water tank located in the same cupboard as all of our other indoor heating apparatus.

Air Source Heat Pump Tank
This tank stores our domestic hot water and allows our heat pump to gradually increase water temperature over time

It’s here where heat extracted from the outside air using the external heat pump unit is transferred to the hot water inside the tank for use across our home in taps, showers etc.

Our ASHP runs until hot water inside this tank reaches temperatures that we would consider ‘hot’.

In times of greater demand where our heat pump wouldn’t quite be able to meet this demand, electric immersion heaters inside this hot water tank can automatically be used to top up the water temperatures when required.

Our hot water settings can also be controlled using the main control panel.

Air Source Heat Pump Water Heater Control Panel
The hot water controls for our air source heat pump system

Our articles on using air source heat pumps to provide hot water and using hot water tanks with air source heat pumps provide more information.

How Air Source Heat Pumps Work

An air source heat pump can perform a range of functions from heating to cooling and to providing domestic hot water.

Air source heat pumps work by extracting heat energy found naturally within outside air, which is transferred indoors for use as heating and or hot water using a range of appliances such as air handling units, radiators and underfloor heating, depending on the type of system used.

In cooling mode, an air source heat pump works in reverse and moves heat outdoor from indoors, effectively cooling the air in a home.

Further Reading

Parts Of An Air Source Heat Pump Explained

Costs To Install And Run An Air Source Heat Pump

Pros & Cons Of Air Source Heat Pumps