An air to air or air to water heat pump can be an effective and environmentally friendly way to heat a home.
While both types are classed as air source heat pumps and extract heat from outside air for use indoors, their main differences lie with the internal components that are paired with each air to air or air to water system and how they release heat into a home.
Air to air heat pumps provide space heating by delivering heated air using air handling units within specific rooms, or around a home through a series of ducts. Air to water heat pumps are paired with indoor central heating systems using equipment such as radiators and underfloor heating.
Air to air heat pumps can also be more effectively used for cooling purposes as a well as heating, while on the other hand air to water heat pumps can also provide domestic hot water without the need for a separate system.
We own and use an air to water heat pump that’s paired with a central heating system and uses underfloor heating on the lower floor of the house and radiators on the upper floor.
We also have a hot water tank that works alongside our air to water heat pump to provide domestic hot water.
Depending on the country and location, the use of either an air to air or air to water heat pump can be favored more towards one type of system. For example, we live in a milder climate (the UK) and so installation of an air to water system can be more appropriate.
These two types of heat pump use the same operating principle but can differ in cost and ease of installation.
Read on to learn more about the similarities and differences between the two systems, including the advantages and disadvantages of each.
Air to air and air to water heat pump systems work on the same principles of extracting heat but differ in the way that they release that heat into a home.
As an overview of both systems, for an air to water heat pump system:
- Heat is extracted from outside air in the same way an air to air system does.
- Heat is transferred indoors through refrigerant in pipes to a buffer tank located indoors where heat is released into the water-based central heating system.
- Heated water circulates around a home through a central heating system.
- Heat is released into a home via water-based apparatus such as radiators and underfloor heating.
- A hot water storage tank can be used within an air to water heat pump system to store and deliver hot water around a home for taps, showers etc.
For an air to air heat pump system:
- Heat is extracted from outside air in the same way an air to water system does.
- Heat is transferred indoors either through refrigerant in pipes (a ductless system) or through heated air passing through a network of ducts (a ducted system).
- With a ductless system, one or more individual indoor air handling units can be used to heat individual rooms. With a ducted system, a central air handling unit will force heated air around a home to all rooms through a series of ducts. This central air handing unit may be located indoors or outdoors.
- Heat is released into a home via air-based heating apparatus such as air handling units where indoor air is blown over refrigerant to be heated up and forced out into the room.
- An air to air heat pump system can also typically be used for cooling purposes by reversing the process and allowing heat to be extracted from the air inside a home, therefore lowering the indoor air temperature and in effect providing cooling.
For both air to water and air to air domestic systems there will be an outdoor heat pump unit.
Our outdoor air source heat pump for our air to water system is located down the side of our house and extracts heat from the outside air.
For both heat pump types, there will typically be a series of pipes going from this outdoor unit to the internal apparatus.
For our air to water heat pump system, these pipes head behind our heat pump, through the external wall of the house, through our garage and to a downstairs cupboard where all of our internal heating and hot water apparatus is located.
This pipe network carries refrigerant to and from an external heat pump unit to the indoor equipment.
A piped network can exist for either air to air or air to water systems.
For air to water heat pump systems, heat will be carried indoors for use within a central heating system, and potentially also for domestic hot water if setup to do so.
For air to air heat pump systems, a single set of pipes may connect from an external heat pump unit to a main internal air handling unit that distributes air as part of a ducted system. For a ductless mini split heat pump system, where one or more air handling units may be located around a home, there can be a series of pipes leading from one external heat pump unit to numerous indoor units.
You can read more about ducted and ductless heat pump systems here.
Furthermore, for air to air systems heat may travel to or from and external heat pump unit via pipes depending on whether the system is in heating or cooling mode.
With air to water heat pumps, internal heating apparatus can consist of items such as:
- Buffer vessel
- Expansion tank
- Hot water cylinder tank
- Underfloor heating
Our air to water heat pump system contains all of the above, with one cupboard housing both the heating apparatus and the hot water tank for domestic hot water.
Radiators are used on the upper floor to distribute heat.
Underfloor heating is used downstairs instead of radiators and each room can be controlled using its own thermostat.
See our main articles on using radiators with air source heat pumps and using underfloor heating with air source heat pumps for more information.
Our whole air to water heat pump system can be controlled from one panel.
With an air to air system you’ll find either a central air handling unit with a series of ducts throughout a home to distribute the air, or air handling units within one or more rooms to serve those areas of the home specifically.
|Air to Air Heat Pump
|Air to Water Heat Pump
|How heat is released from the refrigerant
|By blowing air across the refrigerant pipes within an air handling unit
|By passing the refrigerant tubes through a water tank
|How the room/home is heated
|By heating the air within the room(s)
|Releasing heat from water flowing around a home
|Individual air handling units or ductwork to distribute hot air to different rooms
|Radiators or underfloor heating
|Can also be used to provide cooling by reversing the process and removing heat from a home
|Can be paired with a hot water tank to store and deliver hot water across a home for use in taps, showers etc.
An air to air heat pump system uses heated air to heat a space, while an air to water heat pump uses heated water. While this is the fundamental difference between the two systems, it results in other differences.
For example, an air to air system doesn’t depend on radiators or underfloor heating, which is necessary for air to water heat pumps to work.
In heating mode, both heat pump system types will use an outdoor air source heat pump unit to extract heat from the outside air. This will be done by using internal fans to force air through the heat pump units.
Refrigerant will be flowing through an external heat pump unit and will capture heat energy from the air. This part of the system can be known as the outdoor coil or the evaporator unit.
The use of a compressor after this process occurs allows the heated refrigerant to increase in pressure and temperature, therefore maximizing heat gains.
Pipes will carry heat to the internal apparatus as part of any air source heat pump system.
Once the heat is released inside a home the cooler refrigerant will return to the outdoor heat pump unit and repeat the cycle again, typically passing through an expansion device to reduce pressure and therefore temperature. This setup can be found with either heat pump system type.
However, the differences between air to air and air to water heat pump system lie in how this heat is released inside a home.
Inside a house as part of an air to air system will be one or more air handling units that can resemble domestic air conditioning units.
Much like the external heat pump unit, a fan located inside an air handling unit blows air across tubes containing the refrigerant. Heat is transferred from the refrigerant to the air flowing the unit, therefore providing warmer air to the room.
Indoor air handling units can be referred to as the condenser or indoor coil as part of an overall air source heat pump system.
This process of releasing heat into the circulating indoor air raises the temperature inside a home over time.
Ductwork may be used to distribute the warmer air from the heat pump to different rooms within a house. Ductwork is a piping system typically optimized to minimize heat loss as it transfers air through a house.
See our main article on air source heat pump air handling units for more information.
Air to air heat pump systems can also provide cooling as well as heating.
An air to air system is able to achieve this by working in reverse and extracting heat from inside a home and expelling it outside.
The internal air handling unit(s) fan(s) will force indoor air over the internal coils inside the units, in which refrigerant is flowing through. The refrigerant absorbs the heat (much like the outside heat pump unit would do in heating mode) and delivers that heat to the external heat pump unit to be released to the outside air.
An air to water heat pump system uses refrigerant to absorb heat from outside air in the same way that an air to air heat pump does.
Heat transfers from ambient air outside the house to the refrigerant, raising its temperature. The refrigerant is then further increased in pressure and temperature by compressing it.
In an air to water system, the heated refrigerant passes through one or more water tanks where the heat is transferred to water for use with water-based central heating and/or for use with domestic hot water.
These indoor water tanks can be referred to as the condenser or indoor coil as part of an overall air source heat pump system.
The heated water inside a central heating tank is then used to release heat across a home in the form of radiators and/or underfloor heating where water is pushed around a home as part of a water-based central heating network.
In a radiator, the hot water passes through a metallic conductor exposed to the air in the room. The high conductivity of the metal maximizes the heat flowing from the water to the air surrounding the radiator.
An underfloor heating system passes the hot water through a network of pipes under the floor, heating a room from beneath.
In addition to central heating, an air to water heat pump system can be used for domestic hot water, where heat from a heat pump is also delivered to a larger hot water storage tank for use across a home in taps, showers etc.
Hot water will be stored within the tank and the heat pump used to top up the temperature of this water as an when required. The hot water tank allows hot water to be provided on demand across a home.
The energy efficiency of air to air and air to water heat pumps can be demonstrated using numbers such as the coefficient of performance (COP).
The COP measures the ratio of total thermal energy produced to the total electrical energy used in the production. If 3 units of electricity are used to produce 3 units of thermal energy, the COP is 1.
The higher the COP, the better the performance of a heating system and the more energy-efficient it is.
See our article on air source heat pump COP for more information.
With air source heat pumps, most of the total heat energy produced comes from the ambient air outside the house instead of from electricity, although these systems use electricity to compress the refrigerant to help maximize temperature gains from extracting heat from outside air.
However, much of the total heat is extracted from the air outside and transferred inside.
Suppose a heat pump produces 12 units of heat energy. The electric power the heat pump uses in compression could be 3 units. The other 9 units of energy will come from ambient air.
Thus, the COP of this system will be 12/3, which is 4. For perspective, a purely electrical heater would have a COP of 1 and use 12 units of electric energy to produce 12 units of thermal energy.
The actual performance of either type of air source heat pump can vary throughout the year because a heat pump can be less efficient when outside air temperatures are colder because the temperature difference between the outside air and desired inside air temperature is greater.
While numbers such as the COP can give an indication of the high efficiency of an air source heat pump at moderate temperatures, looking at seasonal efficiency numbers such as SCOP (seasonal COP) or SEER (seasonal energy efficiency ratio) can give a better understanding of efficiency over the year.
At ideal temperatures, our air to water heat pump can have a COP of over 4 but drops when outdoor temperatures drop.
See our article on air source heat pump efficiency for more information.
The best air source heat pump in your situation will depend on the factors we’ve discussed above. However, there are some situations where an air to air system is naturally suited for the job.
An air to air heat pump system can be ideal if you don’t have a central heating system and you can’t or don’t want to install it. It can also be more appropriate if you are heating a small living space like a single room, or if you want the ability to both heat and cool your house using the same system.
Here are some advantages of an air to air heat pump.
Air to air heat pump systems, including both traditional systems and minis-split systems, can be relatively easy to install.
A basic mini-split installation would require an outdoor heat pump unit and an air handling unit. This would work perfectly when heating a single room. For multiple rooms, you could use multiple air handling units, with one in each room.
Pipes will need to be connected from the heat pump unit to each blower unit, but the routing of the pipes can be flexible.
You could opt for ductwork instead of blowers if you have many rooms. The ductwork is a system of pipes that transmit hot air and release it into the different rooms. However, ductwork can be expensive to install.
If you have existing ductwork then a new air source heat pump system can utilize this.
On the other hand, air to water heat pumps require a central heating system to work. These water-based heating systems include radiator systems and underfloor heating. If you don’t already have a central heating system in place, installing it can also be expensive.
Furthermore, if a home has an existing central heating system but doesn’t have sufficiently sized radiators or pipework then this can also add to installation costs as the indoor heating system must be of sufficient heat output to work with a heat pump.
Additionally, an air to air heat pump can effectively heat a small space. This effectiveness and ease of installation can make an air to air heat pump more suited for heating small living spaces.
The way air to air heat pump systems are set up means that they can be used for cooling as well as heating, and are therefore more suited to warmer climates where domestic air conditioning can be required through summer.
When set up for both heating and cooling, an air to air heat pump system is able to provide both functions without the need for another system.
However, if using an air to air heat pump system then another system for providing domestic hot water will be required.
An air to water heat pump can right for you if you have a water-based central heating system or if you need to use the heat pump to provide domestic hot water. It can also be more convenient when heating a large living space, such as a home with multiple rooms.
An air to water system needs underfloor heating or a series of radiators to work.
Underfloor heating typically consists of a network of pipes that run under the floor. Heated water from an air to water heat pump is circulated through the pipes, warming a room from underneath.
Underfloor heating is excellent when you need uniform heating. With an air to air heat pump that uses blowers to distribute the heated air, the wrong positioning of blowers could cause uneven heating. You’ll thus have pockets of air that are warmer or colder, which could be uncomfortable.
If your central heating system uses radiators instead of, or alongside, underfloor heating, an air to water heat pump can be more appropriate.
However, large surface area radiators are required to maximize the potential from an air source heat pump and so any outdated radiators may need to be replaced.
Air source heat pumps work differently to traditional heating systems in that they deliver heat more slowly over time and so need to be used for longer periods of time to achieve the same desired indoor temperatures.
A radiator, therefore, needs a larger surface area to warm a room efficiently when used alongside a heat pump.
Air to water systems typically aren’t set up to provide cooling and you would need to provide other means of cooling a home.
For example, we only use our air to water heat pump for heating and hot water. We have two additional air conditioning units that serve individual rooms.
However, our heat pump could be used for cooling but to use it we may need to install additional apparatus such as hydronic air handlers.
With an air to air system, you don’t typically need any additional installation to use your heat pump for air conditioning.
See our guide on air source heat pumps and air conditioning for more information.
Air to Air vs Air to Water Heat Pumps
Air to air and air to water heat pumps work in the same way for extracting heat from outside air but differ in their means in distributing that heat inside a home.
In an air to air system, space heating will be provided through warm air. This can be done using a central air handling unit used alongside ducts to move heated air around a home, or using one or more air handling units to serve individual rooms or spaces.
An air to air system can typically be used for cooling as well as heating without any additional installation requirements, but a separate system will be required for providing domestic hot water.
In an air to water system, central heating will be provided by moving heated water around a home. Central heating pipework alongside radiators and/or underfloor will deliver the heat.
Air to water heat pumps can also be used for domestic hot water when used alongside an appropriate hot water tank, but can’t typically be used for cooling unless installed alongside additional equipment. Both types of heat pump have their advantages and disadvantages and the best heat pump for you will be determined by the nature of your situation and your unique needs.