Air-to-air (A2A) heat pumps – often known as air conditioning – are electric heating and cooling systems. When heating, they extract heat from the air outdoors into a refrigerant and use that to warm up the air inside the home. When cooling, they do the opposite.

There’s been an increasing interest in A2A heat pumps as a home decarbonisation solution in the UK. This is because they can offer affordable, efficient and comfortable heating and cooling in many property types, contexts and occupancy patterns.

We worked with researcher John Ewbank to build a better understanding of how A2A heat pumps could be used for home heating decarbonisation in the UK. This was done through interviews with homeowners, HVAC engineers, and industry professionals, and by reviewing forum discussions, manufacturer datasheets and scientific papers.

• Homeowners were satisfied with running costs and the comfort that A2A systems provided across different property types.
• A2A heat pump installations can be faster and less disruptive than A2W heat pumps, with comparable or lower costs depending on the property type.
• A2A heat pumps are particularly advantageous in contexts where an A2W system would require major pipework upgrades and in smaller properties or those with lots of open plan space, where the number of indoor units required can be kept low. These factors can cause A2A installations to be cheaper, less disruptive and faster than an A2W installation.
• Although A2A heat pumps work best when used with a continuous heating pattern, they can also be used for zoned and intermittent heating, something that often isn’t optimal with A2W heat pumps. This makes them useful for a wider range of heating patterns and lifestyles, for example, those who spend most of their day in one room, those who are away from the house for long periods of time, or those who are managing their energy use closely.
• A2A systems might be more likely to require planning permission than A2W systems. This is because there’s a greater chance that they’ll need two outdoor units to keep routing, length and complexity of pipework simple.
• A2A systems don’t resolve the challenges of installing A2W systems in flats. Although A2A heat pumps are likely to be cheaper to install and don’t require as much internal space for ancillaries, they still require outdoor space for the outdoor unit and are subject to the same freeholder consent issues that A2W installations face.
• A challenge for A2A heat pumps as a decarbonisation method is that many homeowners retain their fossil fuel boilers for hot water, as hot water isn’t provided by most A2A systems. However, A2A systems may allow homeowners who are otherwise unlikely to decarbonise their home in the near future to immediately reduce their direct carbon emissions by approximately 75%, with hot water decarbonised in the future.
• The HVAC sector is already close to capacity, and the number of engineers will need to grow to accommodate increasing homeowner demand for A2A heat pumps. However, it can be challenging for new entrants to gain the necessary skills to install this technology, whether they’re coming from a gas heating background or an unrelated profession.
• It’s hard to predict the success of the new £2,500 grant for A2A heat pumps under the UK government’s Boiler Upgrade Scheme. This is because HVAC engineers may not want to get the MCS certification necessary for grant-funded installations due to the added paperwork and more time-intensive heat loss calculations required. Additionally, some common A2A system designs may not meet MCS standards, leading engineers or customers to avoid grant funding.

Our findings suggest A2A heat pumps can be considered an equally viable decarbonisation option as A2W systems for almost all UK homes, and they’re particularly advantageous in certain contexts. However, there are still key challenges when it comes to installing this technology, ranging from barriers to growing the size of the HVAC workforce to an increased chance of requiring planning permission.