Air-to-air heat pumps are electric heating and cooling systems. They are more commonly known as air conditioning units, and are exactly the same technology as the units that most people are familiar with and use to cool their homes in hot weather.

Unlike gas/oil boilers or air-to-water heat pumps which heat up water that flows through your radiators, air-to-air heat pumps directly heat up the air in your room. They do this by circulating the room’s air over a hot or cold refrigerant inside the room’s internal unit and blowing the warmed or cooled air back into the room.

Most air-to-air heat pumps consist of an outdoor unit and an indoor unit in each room that’s being heated or cooled. When used for home heating, the outdoor unit uses a refrigerant to absorb heat from the air outside. Compression technology then increases the temperature of this refrigerant. The refrigerant flows from the outdoor unit to each of the indoor units. The indoor unit sucks in air from the room and blows it over the hot refrigerant before expelling it back into the room as warm air. When cooling, they do the opposite.

Our research suggests that homeowners are generally very satisfied with their air-to-air heat pump systems. Most homeowners interviewed in our research described air-to-air heating as equal to or better than their previous systems once they found their preferred operating strategy. Homeowners praised the responsiveness of their air-to-air units, noting how rapidly cold rooms could be warmed. Rooms typically became comfortable in 10–20 minutes, with supply heat arriving in 3–4 minutes.

Although some may be concerned about the feeling of blown air for heating, our research found that airflow and draughts from air-to-air indoor units are unlikely to cause comfort issues, provided the units are positioned correctly, do not operate continuously at high fan speeds, and have their louvres adjusted appropriately.

Yes. Air conditioners that are used for cooling can also usually provide heating. They are the same technology as an air-to-air heat pump. Crucially, they have the huge efficiency bonus that heat pumps bring - because they take heat from the air, rather than turning electricity directly into heat.

Pretty warm! Just like a gas or oil boiler, an air-to-air heat pump will use a thermostat to determine how much heat it needs to put into the room. This thermostat may be placed on a wall, as with a gas or oil boiler, or it may be integrated into the indoor air-to-air unit mounted high on the wall in the room. You can usually set this thermostat up to 30°C. However, as with a gas or oil system, it’s unlikely you’ll need to set it quite that high, and setting such a high temperature will reduce the efficiency of your system.

In our research most homeowners described air-to-air heating as equal to or better than their previous systems, which were usually gas boilers.

The number of units you need for an air-to-air system will not only depend on the number of bedrooms your home has, but also how you use your home, your preferred heating pattern and layout of your home.

For most, the most likely configuration is to have an indoor unit (which is the equivalent of a radiator) in each bedroom, and each living or kitchen space. Bathrooms can be heated with an electric heated towel rail. So this might mean a three bedroom house will require four to five units in total. If your living, dining and kitchen area is relatively open plan, you would only need a single unit to heat all of those spaces.

If the household tends to keep their bedroom doors open throughout the day, has an open plan kitchen, living and dining space, and heats their home with a continuous heating pattern (which is also the most efficient way of using any type of heat pump), it’s possible that only two units could be needed. This could cut installation costs as fewer units would be required. For example, one indoor unit could be used to heat an open plan kitchen/living/dining space downstairs, and another unit could be placed on the landing upstairs to heat all bedrooms (providing the doors were left open for the air to circulate). Using a continuous heating pattern is crucial for this layout to work effectively.

A three bedroom house will usually need one or two outdoor units. Although one outdoor unit could provide ample output to heat or cool the whole home, two lower-output outdoor units can often be used instead to increase the speed of installation, minimise disruption and extra labour when installing the system (e.g. drilling through internal walls and piping refrigerant through the inside of the house), and maintain aesthetic requirements inside the home (e.g. avoiding unsightly trunking across rooms). Minimising disruption and installation time can help to keep installation costs lower, despite an extra outdoor unit being used.

Although some air-to-air heat pumps can also provide hot water, most models available in the UK don’t currently do this. If you install a model that doesn’t provide hot water, you can replace your boiler with a hot water heat pump cylinder, heat battery, an electric immersion heater, or a point-of-use water heater. Hot water tends to be only a relatively small part of the heat we use in our homes (roughly 20% of household gas usage tends to be for hot water), so it may not have a huge impact on your energy bill to use direct electric heating for hot water.

No, you will not need to keep your boiler once you install an air-to-air heat pump. Your air-to-air heat pump will provide all your space heating and some models also provide hot water. If you install a model that doesn’t provide hot water, you can replace your boiler with a green, low-carbon hot water solution. This can be a hot water heat pump cylinder, heat battery, an electric immersion heater or a point-of-use water heater.

You can also install an air-to-air heat pump and keep your boiler for hot water usage, until you're ready to upgrade your hot water to a greener option. This could allow you to quickly reduce your gas emissions by 80% whilst giving you more time to find the right hot water solution for your home. However, to be eligible for government subsidies you will be required to remove your boiler as part of the installation.

An air-to-air heat pump installer must hold a Category 1 f-gas certificate that will allow them to work with the refrigerants used within air-to-air heat pump systems. If you’re looking for an air-to-air heat pump installer, the best way to find one is to search for “air conditioning installers”, as this terminology is more likely to result in relevant results than “air-to-air heat pump”. Some multi-technology renewable installation companies will offer air-to-air heat pumps, as will some air-to-water heat pump installation companies.

An outdoor unit will usually be roughly one metre high, between 50cm-100cm wide, and around 50cm deep. Wall-mounted internal units tend to be 80cm wide, 30cm tall, and 20cm deep. However these sizes can sometimes be a little smaller or larger depending on the power of the unit and the heat pump model.

Our research suggests that households and their neighbours aren’t disturbed by the internal or external noise of their air-to-air units.

The noise from the outdoor unit of an air-to-air heat pump is likely to be the same as an air-to-water heat pump, which have been shown to be quiet and unobtrusive.

Some people may also be concerned about the noise from the indoor units in each room, as they use a fan to blow warmed or cooled air into the room. Although some may find the noise from these units offputting at their highest fan speeds, when used at a standard fan speed households report being unaffected by the noise levels. The best way to keep fan speeds low is to avoid drastic intermittent heating patterns and use a continuous heating pattern instead.

Installing an air-to-air heat pump system that doesn’t provide hot water usually costs from £1,700 to £2,700 per room, depending on how budget or premium the model is. The cost of installing a hot water solution is likely to be between £1,000 to £4,000 depending on whether a basic immersion heater or a more expensive heat battery, smart hot water cylinder or a hot water heat pump cylinder is chosen.

In England and Wales the UK government’s Boiler Upgrade Scheme will soon offer homeowners a £2,500 grant for replacing a boiler or other heating system with an air-to-air heat pump. In England, air-to-air heat pumps will also be covered by the Warm Homes Local Grant for those on a low income, getting certain benefits or living in certain postcode areas.

Running costs will depend on how the system is used. At current energy pricing, the costs are likely to be lower than using an oil boiler and similar to a gas boiler or an air-to-water heat pump with a standard electricity tariff. If you use a special heat pump tariff it could make an air-to-air heat pump cheaper to run than a gas boiler.