Air source heat pumps work differently compared to other more conventional forms of heating system and transfer heat from one place to another.
The parts that make up a whole air source heat pump system can therefore differ, and an external air source heat pump unit will have its own specific set of components that helps it extract heat from the outside air.
The main parts of an external air source heat pump unit can include a compressor, outdoor coil, and fans. Internal apparatus for a whole air source heat pump system can include the indoor coil, buffer tank, expansion vessel, hot water tank, air handler, radiators, and underfloor heating.
Our own air source heat pump system is the air to water type which uses an air source heat pump located outside our home to deliver heat for use with a water-based central heating system and domestic hot water system.
The two main parts of our heating system therefore consist of the external heat pump unit and the internal heating apparatus, the latter of which is located in one of our cupboards.
We discuss in more detail below the main parts of an air source heat pump using our own heat pump system as an example.
As an external air source heat pump is only part of an overall heating system, we’ve split this guide up into two parts to explain:
- Parts of an air source heat pump
- Parts of an overall air source heat pump system
Parts Of An Air Source Heat Pump
An air source heat pump (ASHP) works by extracting heat energy stored naturally within the outside air, for use indoors within a heating system.
As a result, the main air source heat pump unit as part of an overall ASHP system needs to be located outside and therefore has a range of components that allows it to extract heat from the air.
The main parts of an external air source heat pump unit can include:
- Main body
- Outdoor coil
- Refrigerant pipe connections
- Power cable
- Base with vibration mounts
The main body of an air source heat pump contains all the major components necessary to extract heat from the outside air, and many air source heat pumps can look like larger commercial air conditioning units.
Our heat pump is rectangular in shape and located up against the side wall of our house.
This particular air source heat pump is made up of two containers, with the left side container housing the fans and refrigerant coils, and is open on the front, back and side to allow for airflow through the unit.
The right-hand side container of our ASHP unit houses (behind an access panel) all the other necessary equipment to help the heat pump to function, such as the power unit, compressor and any other associated valves and pipes.
The fan(s) in an air source heat pump can make up much of the overall body of an air source heat pump.
The aim of a fan inside a heat pump is to force air through the unit to allow heat to captured. The more air that can be moved through a heat pump, the more heat can be extracted.
Depending on the model of heat pump and its heat output, an ASHP may have more than one fan.
Our air source heat pump is a dual fan unit.
As this heat pump needs to provide both heating and hot water for our 5-bedroom house, the installer chose a larger heat output unit. Having two fans means that more air can be forced through the heat pump, allowing more heat to be available to be extracted.
These fans suck in air through the back and side of our heat pump, and out the front.
In order for an air source heat pump to capture heat from the air, it uses refrigerant flowing through the unit to absorb heat energy from the air.
A series of coils inside an ASHP, in which refrigerant flows through, allows heat to be extracted more efficiently.
These coils can be seen on the back of our heat pump.
The fan(s) force air over these coils for heat to be captured as effectively as possible.
The refrigerant used in air source heat pumps has a low boiling point, meaning that when it extracts heat from outside air it turns from a liquid to a gas.
When in heating mode, this outdoor coil of an air source heat pump unit can therefore be considered as the evaporator part of an ASHP system.
An inlet and outlet for the refrigerant pipes of an air source heat pump will typically be found on the back of the unit.
These pipes head straight into our home to and to the internal heating apparatus where the heat extracted from the outdoor air source heat pump unit is delivered.
These bare pipes are insulated to help prevent heat loss.
Air source heat pumps are electrical appliances and unlike other more traditional heating systems don’t rely on any form of power or fuel apart from an electrical connection.
An air source heat pump will therefore have its own power source. Our heat pump takes power from inside our home and connects into the heat pump unit on the side.
See our article on the electrical requirements for air source heat pumps for more information.
Base With Mounts
An air source heat pump will need to be mounted to onto the floor, but it may also be possible to mount an air source heat pump on a wall if in line with the manufacturer’s guidelines.
Our ASHP is installed onto a concrete base with rubber vibration mounts to help prevent excessive vibration and noise during operation.
Heat pumps aren’t able to generate that much heat from simply extracting heat from the outside air alone, especially when temperatures are very low in winter and an ASHP has to work harder to extract that heat.
Air source heat pumps therefore use a compressor to help generate the most amount of heat possible.
After refrigerant flowing through the heat pump coils picks up heat from the air (being forced through the unit from the fans), it travels to the compressor before going indoors.
A compressor increases the pressure of the refrigerant, which in turn increases the temperature of it.
The compressor for our air source heat pump is located behind the access panel inside the appliance. The image below taken from our manual shows what the compressor would look like.
This compressor allows water temperatures of up to 60°C (140°F) to be generated for use in our home, which is much higher than what could be generated without the use of a compressor.
Parts Of An Air Source Heat Pump System
While an external air source heat pump unit has a range of components that helps it to extract heat from the air, it’s only one part of the whole air source heat pump system that can be used to heat or cool a home.
An air source heat pump sits outside a home to extract heat from outside air. Refrigerant pipes from an ASHP will typically take heat to the remaining apparatus located indoors.
The parts used indoors as part of a whole air source heat pump system can depend on the type of heat pump.
There are two main types of air source heat pump:
- Air to water heat pumps
- Air to air heat pumps
Our heat pump is the air to water type, meaning that it works with a water-based central heating system inside our home.
An air to water heat pump will work alongside apparatus such as underfloor heating and radiators to help release heat into a home. They can also be used with hot water tanks for domestic hot water purposes.
Air to air heat pumps deliver heat through space heating, using apparatus such as air handlers. Ducts can also be used to help move heated air around a home and an air to air heat pump can be more suitable for when needing to cool a home, as well as heat.
See our main article explaining the differences between air to air and air to water heat pumps here.
Indoor parts of an air source heat pump system can therefore include:
- Air handler
- Buffer tank
- Expansion vessel
- Hot water tank
- Underfloor heating
- Central control panel
- Internal pipework
- Expansion valve
The indoor coil of an air source heat pump system delivers the heat that’s extracted using the outdoor coil.
As heated refrigerant passes through the indoor coil, heat is released.
The form in which the indoor coil is found can depend on the type of air source heat pump system and therefore the parts used.
Air handlers can be used as part of an air to air heat pump system and use an internal fan to blow air into the room over the indoor coil.
See our main guide to air source heat pump air handling units for more information
As part of an air to water heat pump system, a buffer vessel and expansion tank(s) can be used to help transfer the heat from the refrigerant to the water flowing around the home as part of the central heating system.
When also being used for domestic hot water purposes, heat can be exchanged within a hot water cylinder to heat up water ready for uses in taps, showers etc.
When in heating mode, the indoor coil as part of an ASHP system can be considered as the condenser, as through removal of heat the refrigerant in gas form condenses back into a liquid.
Air source heat pumps can also provide cooling. Heat pumps are able to achieve this by using a reversing valve to alter the direction of flow and therefore change the way in which heat is moved.
In cooling mode, the indoor coil extracts heat from the indoor air and the outdoor coil expels that heat outside, therefore lowering the temperature of the air inside a home.
Air source heat pumps systems work differently compared to other more conventional heating systems in that they extract and deliver heat more slowly, and at lower temperatures.
An ASHP system therefore typically needs to run for longer periods of time in order to achieve the same desired indoor temperatures.
To help a heat pump system deliver heat more effectively to a home, they’re typically paired with heating devices such as underfloor heating, which have a large surface area that helps to release the head more efficiently.
Our home has underfloor heating on the lower floor, and each room can be individually controlled using thermostats.
Also as part of an air to water heat pump system, radiators can be used alongside, or instead of, underfloor heating.
The upper floor of our home uses modern, large surface area radiators to help heat the upstairs rooms.
See our article on using air source heat pumps with radiators for more information.
Central Control Panel
A modern air source heat pump system can typically be controlled from a central control panel.
We’re able to control all the heating and hot water settings for our home from the control panel shown below.
Unless in situations such as using a package air source heat pump system where both coils are located outside, heat will need to be transferred indoors from the outside coil through pipework to the internal heating apparatus.
The image below shows the pipework connecting the outside air source heat pump to our indoor heating apparatus.
After heat is released by refrigerant through the indoor coil when in heating mode, the refrigerant returns to a liquid form.
Refrigerant passes through an expansion valve that reduces the pressure of the refrigerant, and therefore the temperature, allowing the refrigerant to return to the outdoor coil within the external heat pump unit for the process to be started again.
This video provides a great demonstration of the main parts of an air source heat pump system