A key benefit of air-to-air systems is their ability to be used well for different heating patterns and with a variety of different system configurations. For example, some choose to install an indoor unit in each room as they prefer to keep doors closed or use their heating in a zoned or intermittent manner.

Others may install only two indoor units for the whole home. One might be used to heat the whole of an open plan downstairs kitchen and living space. The other might be installed on the upstairs landing to heat all rooms on the first floor, by leaving doors to rooms open. Some air-to-air homeowners have even reported that a downstairs unit can provide ample heat to the rooms upstairs in most contexts.

So it’s important that the system design requirements for an air-to-air system to be eligible for the BUS grant don’t limit how an air-to-air system can be installed, such as requiring every habitable room to include an indoor unit, as some have speculated may be expected, as this could drive up both the cost and complexity of installations. It may also require systems to have two outdoor units to avoid overcomplex or inefficient piping, something that is currently only possible via permitted development for detached properties.

One limitation of an air-to-air heat pump is that most models don’t usually provide hot water, so need a separate system for hot water. This is not an insurmountable problem - there are various affordable options for providing separate, low-carbon hot water - but the grant design needs to allow flexibility in hot water systems.

Currently, many homeowners installing air-to-air systems choose to retain their boiler for hot water use. Some may not know what the other options are, whilst others may want to wait until their boiler needs to be replaced. However, this leaves homes in a situation where roughly a quarter of their heat demand is still burning fossil fuels.

That’s why it’s positive to see that heat batteries are also eligible for the new £2,500 grant. This could encourage homeowners to fully decarbonise their heat demand in situations where their installations would become unaffordable, or more expensive than a post-grant air-to-water installation, if they were to decarbonise their hot water too.

However, the grant's scope should ideally be wide enough to include other domestic hot water supply options, such as a hot water heat pump, or even immersion heaters if they’re used alongside an air-to-air system. This would provide stronger support for the low-carbon heating transition by enabling homes that are installing air-to-air to more easily take the final step in fully decarbonising their systems.

There is a risk that planning constraints are a major barrier to many homeowners accessing the new air-to-air grant, and governments should aim to avoid this.

In November 2024 the UK government updated planning rules to include air-to-air heat pumps under permitted development in England - in particular dropping the “one metre rule” and allowing units that provide cooling alongside heating.

However, only detached homes are able to have two outdoor air-to-air heat pump units under permitted development, with all other property types requiring planning permission for additional units. This isn’t an issue for air-to-water heat pumps, where it’s relatively unlikely that multiple outdoor units would be required for non-detached homes, but it could pose a challenge for air-to-air heat pumps and buildings with multiple homes, such as flats.

This is because air-to-air systems are more likely to require two outdoor units to avoid complex, efficiency-reducing, impractical or aesthetically undesirable refrigerant pipe runs to reach all indoor fan coil units.

That’s why the UK government should consider expanding permitted development for two outdoor units to other property types, particularly if they’re using air-to-air systems.

There are also permission constraints for some properties where air-to-air heat pumps are particularly well suited, such as flats.

For example, leaseholders in flats will usually require freeholder consent to install a heat pump, due to either structural changes or external additions to the building. Coupled with the permitted development rights of only one outdoor unit per property, and concerns over the peak load capacity for electricity within the building, this can make it extremely difficult for those in flats to install a heat pump. The Leasehold and Freehold Reform Act 2024 provides an opportunity to try and reduce challenges associated with freeholder consent, especially as many aspects of this act will still require secondary legislation and consultation.

Although the £2,500 of funding available for installing an air-to-air heat pump is roughly commensurate with the lower cost of installing such systems, it’s important to consider what the actual savings will end up being for homeowners.

Air-to-air systems aren’t currently installed under the Microgeneration Certification Scheme (MCS). However, installations eligible for BUS will likely require MCS certification. This is important to ensure the government grant has controls around quality, however, it can increase both financial and administrative costs for installers, potentially significantly reducing the actual saving a homeowner would make if they were to undertake a BUS-eligible air-to-air installation.

Given air-to-air systems are usually installed with a less intensive heat loss survey, and given that they are generally more forgiving in terms of performance, efficiency, user operation and comfort than air-to-water heat pumps, it’s worth considering whether the MCS process could be made less intensive or costly for air-to-air systems, without compromising on quality. This could allow homeowners to make more efficient use of the government funding.

This new support for air-to-air heat pumps and heat batteries will hopefully boost homeowner motivation and ability to decarbonise their heating systems. However, we need a workforce with the capacity to deal with this increased demand, especially given the slightly different qualifications and experience required compared to working with hydronic heating systems.

Two existing groups of installers stand out as having the potential to install these systems: f-gas engineers currently working in commercial heating, ventilation and air conditioning (HVAC), and gas plumbing and heating engineers. There are specific opportunities and challenges to using these cohorts to meet an increasing demand for air-to-air heat pumps.

Commercial HVAC engineers

Some f-gas certified commercial HVAC engineers are already seeing increasing demand from the domestic sector and pivoting their business towards this market. However, there is already a growing capacity shortage in the commercial HVAC sector and many are already extremely busy. Many may not want to do BUS-eligible installations if they have high enough demand from both commercial and domestic sectors already, especially if it requires the additional cost and perceived administrative burden of MCS certification.

Gas plumbing and heating engineers

Another cohort that could install these systems is existing gas heating engineers. However, these engineers will likely face similar issues as they would when upskilling to work with air-to-water heat pumps.

For example, like air-to-water heat pumps, the minimum training and qualifications required to install air-to-air systems are relatively quick to achieve. However, a five-day f-gas course clearly isn’t enough to become confident or competent in installing such systems and, like air-to-water heat pumps, practical experience is required on top of this.

Finding experiences to work alongside more experienced engineers is difficult and often requires high motivation and confidence in reaching out to cold contacts. This is especially difficult if a newly-trained engineer is viewed as a potential business competition, or if established businesses would only want to employ somebody with more experience with the technology.

One benefit of air-to-air systems is that the system design is more forgiving than air-to-water heat pumps, which can make them feel like an easier transition. However, it’s not yet clear what these requirements will be for installations under the BUS or MCS. Another benefit is that engineers may be able to more gradually enter the air-to-air sector, compared with air-to-water heat pumps. For example, they can start by installing simple single-split systems before expanding into more complex multi-split designs. They may also view it as a more impactful skill to have for their business, as they can install it for those who want aircon cooling in the summer months.

This means it may be a little easier or compelling for engineers to upskill in air-to-air heat pump installations. However, many of the wider challenges the heating industry faces with upskilling in hydronic heat pumps may also present with upskilling in air-to-air heat pumps.