Glossary · Air conditioning
What is a split system air conditioner?
A split system air conditioner has two parts: an indoor unit in the room and an outdoor unit containing the compressor, joined by refrigerant pipes and cabling. It cools the room in summer and, on most models, can reverse to heat it. It suits a single room or one open-plan space.
Key points
- A split system is one indoor unit plus one outdoor unit. The compressor sits outside, which keeps the room quieter.
- In heating mode the unit takes heat from the outside air and releases it indoors. In cooling mode the process runs in reverse.
- Under the F gas rules, installing a system that holds refrigerant needs a qualified person, and working without the right certificate is against the law.
- Capacity is chosen in kW. Daikin says undersized units struggle and oversized units cycle on and off, raising running costs.
- BASI lists a fitted one-room Bosch Climate air conditioning package from £1,495, confirmed after a free survey.
How a split system works
A split system has two units. The indoor unit sits in the room and delivers the cool or warm air. The outdoor unit holds the compressor, a propeller fan, the circuit board and a heat exchanger, and can usually be mounted on an exterior wall or placed in the garden or on a roof.[1]Daikin UK, Myth busting: Air conditioners Refrigerant pipes and cabling link the two.
The system works like a refrigerator. In heating mode it absorbs energy from the outdoor air and releases it as heat through the indoor unit. In cooling mode the process is reversed: heat is taken from inside the home and released outside.[1]Daikin UK, Myth busting: Air conditioners Daikin calls an air conditioner an air-to-air heat pump for this reason, and the point is covered in air conditioning vs heat pump.
Mitsubishi Electric says a split system suits a single room or an open-plan space. It gives focused temperature control, and because the compressor sits outside, it is quieter indoors.[2]Mitsubishi Electric, How to choose the right home air conditioning It adds that a split system has lower upfront costs and simpler installation than the alternatives.[2]Mitsubishi Electric, How to choose the right home air conditioning
Fitting and refrigerant
The refrigerant is a regulated F gas. GOV.UK says you must hold your own qualification to install new air conditioning and heat pump systems, to service and maintain them, to check for leaks and to recover the gas, even if you work for someone else.[3]GOV.UK, Qualifications to work with F gas Working with F gas without the right qualification is against the law and can bring a civil penalty.[3]GOV.UK, Qualifications to work with F gas This is why a split system is not a DIY job. Read more under refrigerant.
Daikin says a wall-mounted unit needs only a couple of small holes through the wall to connect the indoor and outdoor units, and that the outdoor unit needs clearance for airflow, usually at least half a metre on the sides where air enters and exits.[5]Daikin UK, Air conditioning types: best AC systems explained It also lists sound pressure levels as low as 19 dB(A) for some indoor units.[5]Daikin UK, Air conditioning types: best AC systems explained
Example
Aisha has a home office above her garage in Leeds that is too hot in summer and too cold in winter. One room needs treating, so a single split unit is the match. She books a survey. The engineer measures the room, checks the ceiling height, insulation and window direction, and recommends a capacity in kW. Daikin says undersized units struggle to reach comfort levels, while oversized ones cycle on and off, increasing running costs and wear.[4]Daikin UK, Air Conditioner Size Guide: Find the Right Unit BASI lists a fixed-price one-room package from £1,495, supplied and fitted, with the exact quote confirmed after a free survey.[6]BASI Heating, Air conditioning, supplied and fitted See air conditioning for the range.
What people get wrong
A frequent mistake is buying by the look of the unit. Daikin says a bigger-looking indoor or outdoor unit does not mean more power. The output rating in kW decides how much heat the system can remove from the space.
Daikin makes that point in its sizing guide.[4]Daikin UK, Air Conditioner Size Guide: Find the Right Unit
Another is assuming a split system suits every property. If you need several rooms treated, a multi-split with one outdoor unit may fit better. A portable unit is a different product again, covered in portable vs split air conditioner. Annual checks also matter, as set out under air conditioning servicing.
The bottom line
A split system is one indoor unit and one outdoor unit joined by refrigerant pipes, used to cool or heat a single room or open space. It must be fitted by someone with the right F gas qualification, and its size is chosen in kW for the room, not by how large the unit looks. A survey is the sensible first step.
Common questions
How does a split system differ from a multi-split?
A split system connects one indoor unit to one outdoor unit. A multi-split connects several indoor units to a single outdoor unit, so each room can be controlled separately. Mitsubishi Electric describes it in these terms.
Can a split system heat as well as cool?
Yes, on reversible models. Daikin says that in heating mode the unit absorbs energy from the outdoor air and releases it as heat through the indoor unit, and in cooling mode the process is reversed.
Can I fit a split air conditioner myself?
No. GOV.UK says you must hold your own F gas qualification to install new air conditioning or heat pump systems, and it is against the law to work with F gas without the correct qualification.
How much does a split system cost to have fitted?
BASI lists a fixed-price one-room Bosch Climate package from £1,495, supplied and fitted, with the exact quote confirmed after a free home survey. The price depends on the room and the installation.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Daikin UK. Myth busting: Air conditioners. Checked 11 November 2026. ↩
- Mitsubishi Electric. How to choose the right home air conditioning. Checked 11 November 2026. ↩
- GOV.UK. Qualifications to work with F gas. Checked 11 November 2026. ↩
- Daikin UK. Air Conditioner Size Guide: Find the Right Unit. Checked 11 November 2026. ↩
- Daikin UK. Air conditioning types: best AC systems explained. Checked 11 November 2026. ↩
- BASI Heating. Air conditioning, supplied and fitted. Checked 11 November 2026. ↩