Why Is Everyone Talking About "Air Conditioning" in the UK? An Engineer's Guide to Air-to-Air Heat Pumps
Industry Commentary
Nathan G
August 03, 2026
Helping YOU find good heating engineers. We share case studies from engineers in the Guild of Master Heat Engineers to help people find top installers, help gas and oil engineers increase their knowledge around heat pumps, and provide a solution for third-sector professionals to understand the industry better.
Recent Heatwaves in the UK
Recent UK heatwaves have driven an unprecedented surge in domestic interest around keeping homes cool. However, as conversation turns toward air conditioning, widespread confusion remains regarding what this technology actually is, how it functions, and its massive potential to solve our winter heating needs.
A Deep Heritage in Air-to-Air Engineering
While the UK is often portrayed as lagging behind in low-carbon heating due to our historically dominant domestic gas grid, the truth is quite different in the commercial sector. The UK actually boasts the largest commercial air-to-air market in the whole of Europe. We are the number one market leaders.
My family has been involved in heating, ventilation, and air conditioning (HVAC) for nearly half a century. We were privileged to install some of the earliest landmark systems in the country, including the first VRV system in a European radio station at Classic FM, and the UK's first Mitsubishi R2 heat recovery system for Paul McCartney’s original offices.
Today, members of the Guild of Master Heat Engineers, alongside traditional air conditioning companies and domestic heating installers, are receiving massive volumes of enquiries as households actively seek out these solutions.
Thousands of your favourite music albums have been recorded in the UK studios my family have installed comfort cooling and heating systems into. Here is the studio we originally installed a system for Dave Gilmore in 1986. We replaced the system in 2024 for the new studio owners. The equipment we took out was still working, testament to the fact if installed and maintained well it can last for a long time. Because we install in acoustic sensitive environments such as recording studios like this or radio stations we test for different frequency levels. You would not want to hear our systems on your favourite Led Zep or George Michael Album. A shout out to anyone related to George Michael. My cousin has worked alongside some of the most famous musicians on the planet. He always said George was one of the kindest, most down to earth clients he ever had. An amazing gentleman…RIP George
Clarifying the Terminology: Comfort vs True AC
Defining these systems accurately helps society embrace low-carbon solutions. At a fundamental thermodynamic level, a heat pump is simply any device that moves thermal energy from an evaporator to a condenser. That includes your kitchen fridge, a domestic cooling unit, and complex commercial plant rooms. However, the industry categorises them by application:
Terminology | Technical Definition | Typical Application | Key Characteristics |
True Air Conditioning (Close Control) | Precision environment control | Museum archives, data centres, operating theatres | Looks identical to standard units from the outside, but far rarer, significantly more expensive, and tightly controls temperature and humidity |
Comfort Cooling & Heating | Human thermal comfort systems | Literally everywhere in the UK outside of our homes (shops, offices, gyms, hotels) | Temperature adjustment with passive humidity extraction |
Air-to-Water (A2W) Heat Pump | Hydronic heating system | Traditional radiator/underfloor retrofits | Transfers heat from outdoor air to a wet central heating circuit |
How an Air to Air Heat Pump Reverses: The Magic of the Reversing Valve
Because a heat pump is fundamentally just a machine moving energy between an evaporator (where heat is absorbed) and a condenser (where heat is released), people often wonder how the same system can cool a house in July and heat it in November.
In the summer, the indoor unit acts as the evaporator (absorbing indoor heat), while the outdoor unit acts as the condenser (dumping that heat outside). In winter, the roles flip completely: the outdoor unit becomes the evaporator (extracting heat from cold outside air), and the indoor unit becomes the condenser (releasing that heat into your room).
How does the fluid change direction without damaging the equipment?
It happens via a small component called a four-way reversing valve. When you switch from cooling to heating, an electromagnet shifts a tiny slide inside this valve, rerouting the pathway of the refrigerant. Crucially, the refrigerant fluid does not reverse direction through the compressor itself. The compressor always receives low-pressure vapour in the exact same direction, compresses it into a hot gas, and sends it out. The reversing valve simply changes which coil that hot gas gets sent to first.
System Types Breakdown
Understanding the available equipment prevents homeowners and designers from over-complicating domestic layouts:
Single Split Systems (Mini-Splits): A single outdoor unit connected to one indoor fan unit. Modern single splits use inverter-driven, variable-speed compressors that smoothly adjust their motor speed to match the room's exact heating or cooling demand. Ideal for targeting a specific problem area, such as an overheated conservatory or a top-floor master bedroom.
Multi-Split Systems: A single outdoor unit connected via separate pipe pairs to multiple indoor fan units, allowing independent temperature control across different rooms.
VRV and VRF Systems (Variable Refrigerant Volume or Flow): Pioneer commercial technology invented by Daikin in 1982. While standard domestic mini-splits modulate power for a few rooms, VRV revolutionised large-scale HVAC by networking dozens of indoor fan coils onto a single shared refrigerant main. Electronic expansion valves dynamically meter exact refrigerant flow to every room simultaneously:
Heat Pump Variant: All connected indoor units operate in the same mode, providing either heating or cooling across the entire building simultaneously.
Heat Recovery Variant: Advanced systems capable of heating one room while simultaneously cooling another. Energy extracted from a room requiring cooling is redirected across the refrigerant circuit to warm a room requiring heat, maximising overall operational efficiency.
Overcoming the "Radiator Mindset" Across the Heating Sector
A widespread misconception across the entire heating sector, stretching from homeowners to installers and commentators in wider industry discourse, is the assumption that every single room requires its own indoor unit. People instinctively mirror traditional hydronic radiator layouts.
Standard radiators give off heat through a combination of radiant heat and natural convection. As water temperatures drop, natural air buoyancy decays rapidly, shifting the emitter's output ratio more toward thermal radiation. However, fan-assisted air-to-air units actively circulate warm or cool air around a space through forced convection. Because of this active air movement, a single well-placed indoor unit can comfortably manage multiple connected open-plan areas.
For example, a two-up, two-down rented cottage with an open-plan downstairs layout can be comfortably heated by a single well-positioned 4kW mini-split unit. Similarly, a couple in an off-grid rural property on oil or LPG, facing high upfront costs to install a full hydronic heat pump system despite the £9,000 Boiler Upgrade Scheme (BUS) grant, could install two strategic mini-splits. This keeps them comfortable throughout most of the winter, reduces installation costs, and gets them comfortable with heat pump technology.
At the start of a massive national heating transition, the rigid idea that a home must immediately ditch 100% of its fossil fuel source can be an obstacle. While complete gas or oil replacement makes sense for fully funded schemes, partial electrification via air-to-air is an incredible, accessible entry point.
Bungalow Retrofits: Unmatched Installation Simplicity
Bungalows present an exceptionally straightforward layout for air-to-air installations. The outdoor unit sits quietly at ground level. A short run of pipework travels just a little way up the external wall before entering the roof space. Tucked neatly out of sight in the loft, discreet ductwork then feeds conditioned air directly into bedrooms and living spaces below, requiring minimal disruption to main living areas.
This image shows the small, flush ceiling grills. They come in a variety of colours and can be quite aesthetic. Some of these grills will extract air while others will input cool or warm conditioned air depending on whether you require heating or cooling.
Internal unit up in the roof space. The ducting from these types of units will be insulated and go directly to the flush ceiling grills in the image above.
An Engineering Aside: Why Refrigeration Pipework Is Different
Most people are used to seeing traditional domestic plumbing pipes, which are rigid half-hard copper (R250 temper) sized in metric dimensions like 15mm, 22mm, or 28mm.
Air-to-air systems use soft, fully annealed copper (R220 temper). Because it is fully malleable, engineers can carefully hand-bend it through loft spaces and tight voids without needing machine benders.
Every split system requires two distinct pipe sizes (a smaller liquid line and a larger gas line). In typical domestic settings, you will most commonly see pairs of 1/4 inch and 3/8 inch, or 1/4 inch and 1/2 inch lines. Interestingly, despite the UK metricating decades ago, the air conditioning world still speaks fluid Imperial, measuring pipework in fractions of an inch rather than millimetres.
Cooling as the Gateway to Low-Carbon Heating: The Installer's Duty
Historically, homeowners bought these systems purely to survive summer heatwaves, completely unaware that the unit on their wall was actually a high-efficiency winter heat pump.
This represents a historical failure in consumer messaging. In the past, installer handovers focused entirely on cooling buttons on the remote control, leaving householders in the dark about their system's dual capability.
Moving forward, engineers installing air-to-air systems must explicitly explain to customers that their cooling unit is an efficient heating system. We need to embed this directly into industry practice:
Mandatory Dual-Function Handovers: Commissioning sign-offs should include demonstrating heating mode to the homeowner, demonstrating how fast it warms a cold space.
Clear Remote Control Guidance: Installers must physically walk customers through the mode button, explaining that the sun symbol yields high-efficiency heating in November just as the snowflake symbol provided cooling in July.
Upfront Quotation Framing: Installers should highlight annual heating cost savings right on the initial quote, showing how the system can offset expensive gas, oil, or direct-electric bills in autumn and spring.
When householders buy a unit to stay cool in July and are shown how to use it in reverse, they are pleasantly surprised by how much a single unit contributes to their winter heating, effectively launching their electrified heating journey.
Mini-split comfort cooling/heating units have been installed in domestic buildings for quite sometime. Unfortunately these are often just used for cooling with the occupant often unaware they are a good heating technology also
Domestic Hot Water and Industry Frameworks
As public enthusiasm grows, addressing secondary heating needs keeps the transition straightforward:
Practical Hot Water Integration: Domestic hot water should not be viewed as a barrier to adopting air-to-air. Many major manufacturers (including Mitsubishi Electric, LG, and Hitachi) produce air-to-air setups capable of integrating hot water generation. Alternatively, homes can retain existing water heating systems, such as cylinders with immersion heaters and electric showers. Localised under-sink water heaters are also readily available.
Grant Accessibility: While the MCS Boiler Upgrade Scheme (BUS) framework currently prioritises hydronic air-to-water systems, ongoing policy discussions continue to elevate public awareness around air-to-air efficiency and running costs. There is now a BUS grant for A2A. My personal opinion is that it will not work in its current form and maybe that’s not so much of an issue yet. The heat wave has engendered interest in the technology.
By viewing domestic comfort cooling as a flexible, dual-purpose heating asset, UK homeowners can achieve exceptional year-round comfort while building a resilient, low-carbon future.
Please share this article to people involved in A2A discourses or asking questions about cooling. See you soon. Nathan.
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Nathan
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