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Air-to-air heat pumps are often dismissed as “just air conditioning”, but modern systems are reverse-cycle heat pumps capable of both heating and cooling a home very efficiently.
For many smaller properties and retrofits, they can be quicker and cheaper to install than traditional air-to-water heat pumps while still delivering impressive efficiency.
In this discussion with installers Sam and Myuran from The Contractors Group, we look at how air-to-air systems actually work, how heat spreads through a home, and why some installers see them as a practical option for terraces, bungalows and smaller houses.
We also explore emerging systems that combine air-to-air heating and cooling with hot water cylinders using heat recovery, capturing waste heat from cooling and turning it into hot water.
Air-to-Air Heat Pumps Are More Than Air Conditioning
One of the biggest barriers facing air-to-air heat pumps in the UK is perception.
Mention an air conditioning unit and most people immediately imagine cooling a bedroom during a summer heatwave. What is often overlooked is that many modern air conditioning systems are actually reversible heat pumps.
Instead of only moving heat from inside a building to the outside, the refrigeration cycle can be reversed.
During winter, the system extracts heat from the outside air and transfers it indoors. During warmer weather, it operates in the opposite direction, removing heat from inside the property and releasing it outdoors.
That ability to provide both heating and cooling makes air-to-air heat pumps particularly interesting as UK homes experience hotter summers while pressure continues to grow to reduce reliance on fossil-fuel heating.
The UK Government provides further information about heat pumps and low-carbon heating, including how heat pumps work and the wider transition towards lower-carbon domestic heating.
How Does an Air-to-Air Heat Pump Work?
At its simplest, an air-to-air heat pump consists of an outdoor unit connected to one or more indoor units.
The outdoor unit contains components including a compressor and heat exchanger. Refrigerant circulates between the outdoor and indoor equipment, carrying thermal energy through the system.
When heating is required, the heat pump extracts energy from the outside air.
Even when outdoor temperatures feel cold, there is still thermal energy available. The refrigerant circuit and compressor allow this low-temperature energy to be collected and raised to a temperature that can be useful inside the home.
The indoor unit then transfers this heat directly into the room air.
This differs significantly from an air-to-water heat pump.
An air-to-water system normally heats water that is then circulated through radiators or underfloor heating. An air-to-air system removes that additional stage and transfers heat directly into the air inside the building.
That simplicity can offer advantages, particularly during retrofit projects.
Why Installation Can Be Simpler
Retrofitting an air-to-water heat pump into an existing home can involve substantial work.
Depending on the property, radiators may need upgrading, pipework could require modification and a suitable hot water cylinder may need to be installed.
The system also needs to be designed around the heat loss of the building.
Air-to-air systems can avoid some of these requirements because they do not rely on a traditional wet central heating system to distribute heat.
Instead, indoor fan units deliver conditioned air directly into rooms.
This can make installation considerably faster in the right property.
A smaller terrace, bungalow, flat or park home may not require the same scale of alterations that could be associated with a complete air-to-water conversion.
However, simpler does not mean that design can be ignored.
The location of indoor units, capacity of the system and heat loss of individual rooms still need careful consideration.
Understanding Heat Pump Efficiency
One of the main attractions of heat pump technology is the amount of heat that can potentially be delivered compared with the electrical energy consumed.
This is often expressed using the Coefficient of Performance, or COP.
A COP of 4 means that under the measured operating conditions, the heat pump delivers around four units of heat for every unit of electricity consumed by the system.
That does not mean the machine is somehow creating free energy.
Instead, electricity is being used to operate the compressor and other components that move existing thermal energy from one location to another.
Performance also changes depending on conditions.
Outdoor temperature, indoor temperature, system design, equipment selection and the amount of heating required can all affect efficiency.
For a better picture of performance across an entire heating season, installers and homeowners may also consider Seasonal Coefficient of Performance figures rather than relying on a single headline COP.
The Energy Saving Trust’s heat pump guidance provides useful independent information about air-source heat pumps, efficiency and factors that can influence performance.
Can Warm Air Really Heat an Entire House?
A common concern with air-to-air heat pumps is heat distribution.
Traditional central heating places radiators throughout a property. With an air-to-air system, homeowners may wonder whether one indoor unit can realistically heat several rooms.
The answer depends heavily on the layout and thermal characteristics of the building.
Warm air naturally moves towards cooler areas, and an open-plan property may allow heat to spread surprisingly effectively.
However, closed doors, long corridors and heavily divided layouts can restrict airflow.
This is why multiple indoor units can be useful.
A multi-split system can connect several indoor units to an outdoor unit, allowing heating or cooling to be delivered more directly to different areas of the property.
Rather than expecting a single unit in the hallway to heat every bedroom, correctly positioned indoor units can provide much more consistent comfort.
Air-to-Air Heat Pumps in Terraced Houses
Terraced properties could be an interesting application for air-to-air heating.
Many terraces have relatively compact floor areas, and neighbouring properties can reduce the number of exposed external walls compared with a detached home.
That does not automatically make every terrace suitable, but it can influence the overall heat-loss calculation.
Installation space can also be a consideration.
Traditional air-to-water conversions may require space for additional equipment and potentially a hot water cylinder. In smaller houses, finding that space can sometimes be difficult.
An air-to-air arrangement may provide another route, particularly where domestic hot water is handled separately.
Every property still needs to be assessed individually. Insulation, glazing, airtightness, layout and occupant behaviour all influence how effectively a heating system will perform.
Bungalows and Park Homes
Bungalows can also suit air-to-air heat pumps because their single-storey layout may simplify heat distribution.
With careful positioning of indoor units, conditioned air can potentially reach a significant proportion of the property without needing to travel between floors.
Park homes present another interesting possibility.
These properties are often relatively compact and may have limited space for traditional heating equipment.
A suitably designed air-to-air system can potentially provide heating and cooling from relatively compact equipment.
Again, sizing is critical.
Installing equipment that is too small may leave the system struggling during colder weather. Oversizing equipment can introduce different performance and comfort problems.
Professional heat-loss calculations remain important regardless of the type of heat pump being considered.
Heating and Cooling From One System
Cooling is arguably one of the strongest additional benefits of air-to-air heat pumps.
British homes have historically been designed primarily around keeping warm during winter.
However, overheating is becoming an increasingly important consideration, particularly in highly insulated homes, properties with large areas of glazing and bedrooms exposed to strong summer sunshine.
An air-to-air heat pump provides active cooling without requiring a completely separate system.
During winter, it heats the property.
During summer, the refrigeration cycle reverses and removes unwanted heat from the rooms.
That dual function can make the investment more attractive to homeowners who are considering both winter heating costs and summer comfort.
Using Waste Heat for Hot Water
One particularly interesting development is the integration of air-to-air systems with domestic hot water production.
Traditionally, one limitation of air-to-air heating has been that it does not automatically provide hot water.
An air-to-water heat pump can heat a cylinder as part of the same overall system, while an air-to-air installation usually needs another solution for showers, baths and hot taps.
Heat recovery technology could change this.
Some systems can recover heat from the refrigerant circuit and use it to contribute towards heating domestic hot water.
This becomes particularly interesting during cooling operation.
When a house is being cooled, the system is actively removing heat from indoors.
Instead of simply rejecting all that energy outdoors, heat recovery can potentially redirect some of it towards a hot water cylinder.
In effect, unwanted heat from the building becomes useful energy elsewhere in the home.
Manufacturers including Daikin and Mitsubishi Electric provide information on modern heat pump and air-conditioning technologies, including systems designed for domestic and commercial applications.
What Happens During Very Cold Weather?
Another frequent question is whether air-source systems can continue heating when outdoor temperatures fall.
Modern heat pumps are designed to operate at low external temperatures, although efficiency and available output can change as conditions become more demanding.
During cold weather, moisture can also freeze on the outdoor heat exchanger.
The heat pump therefore periodically enters a defrost cycle to remove this ice and maintain operation.
Correct sizing is particularly important here.
A system that performs comfortably during mild weather may struggle if it has not been designed for the property’s heating requirement at lower design temperatures.
This is one reason installers need to understand the building rather than simply selecting equipment based on floor area.
Keeping an Existing Boiler as Backup
For retrofit projects, replacing every part of the existing heating system immediately is not always necessary.
Where practical and safe, some installers may recommend retaining an existing boiler initially.
This can provide redundancy.
If temperatures become particularly low, or if the heat pump is temporarily unavailable, the existing heating system can potentially provide backup.
It can also allow homeowners to transition towards heat pump heating without immediately removing equipment that is still functional.
Whether this approach makes sense will depend on the property, existing system and long-term plans.
The important point is that heating design does not always have to be an all-or-nothing decision.
Why Proper Installation Matters
Air-to-air heat pumps may look straightforward compared with major wet heating installations, but they still require specialist knowledge.
Refrigerant systems operate under pressure and need to be installed, tested and commissioned correctly.
Connections must be properly made and pressure tested before the system is put into operation.
Poor workmanship can lead to refrigerant leaks, reduced efficiency, unreliable operation and premature equipment failure.
Engineers working with fluorinated greenhouse gases must also comply with the relevant requirements.
The UK Government provides guidance on qualifications and certification for working with F-gases, including requirements affecting technicians and businesses working on relevant refrigeration, air-conditioning and heat pump equipment.
This is not an area where installation quality should be compromised simply because the equipment appears compact.
System Design Matters as Much as the Equipment
Choosing a reputable heat pump manufacturer is only one part of achieving good performance.
The design of the installation is equally important.
Installers need to consider heat loss, airflow, indoor-unit positioning, outdoor-unit location, noise, condensate drainage, refrigerant pipe runs and how occupants actually use the property.
A technically excellent heat pump installed in the wrong position may provide disappointing comfort.
Likewise, a correctly designed system using appropriately sized equipment can often outperform a larger system that has simply been installed without proper assessment.
This is why experienced installers play such an important role in the move towards heat pump technology.
Are Air-to-Air Heat Pumps the Answer for Every Home?
No heating technology is perfect for every property.
Large houses with many individual rooms may require multiple indoor units, potentially making an air-to-air installation more complicated.
Some homeowners may also prefer traditional radiators rather than fan-driven heating.
Domestic hot water needs must also be addressed.
But dismissing air-to-air heat pumps simply because they resemble air-conditioning systems overlooks what modern equipment can actually do.
For the right building, they can offer efficient heating, summer cooling, relatively straightforward installation and potentially lower retrofit costs.
As heat recovery and hot water integration develop further, the distinction between traditional air conditioning and whole-home heat pump systems may become increasingly blurred.
For terraces, bungalows, park homes and other smaller properties, air-to-air heat pumps deserve serious consideration alongside conventional air-to-water systems.
Key Takeaways
• Modern air conditioning systems are reverse-cycle heat pumps capable of heating in winter and cooling in summer.
• Air-to-air systems can be significantly quicker and cheaper to install than air-to-water heat pumps in many retrofit situations.
• Efficient systems can achieve COP figures above 4 under suitable conditions, meaning several units of heat output for every unit of electricity consumed.
• Heat recovery technology can potentially use energy from the refrigerant circuit to contribute towards domestic hot water production.
• During cooling, recovered heat that would otherwise be rejected outdoors may potentially be redirected towards hot water.
• Multiple indoor units can improve heat distribution, particularly in properties with separate rooms or more complex layouts.
• Terraces, bungalows and park homes can be particularly interesting applications because of their relatively compact size and layouts.
• Keeping an existing boiler can provide backup and redundancy where the overall system has been appropriately designed.
• Proper heat-loss calculations and equipment sizing remain essential, even when installation appears simpler than an air-to-water system.
• Refrigerant systems must be installed and pressure tested correctly by appropriately qualified professionals.
• Air-to-air heat pumps should not automatically be dismissed as “just air conditioning”. For the right home, they can provide a practical combination of heating, cooling and high efficiency.
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