Multi-Split Air Conditioning: One Outdoor Unit, Several Rooms

A multi-split air conditioning system is one outdoor unit connected to several indoor units, typically two to five of them, each mounted in a different room¹. It is the arrangement a household reaches for when more than one room needs cooling or heating but the outside of the house, or the space around it, cannot take a row of separate condensers. One outdoor unit, one set of refrigerant pipes branching to each room, and a high-wall, ceiling cassette or ducted unit in each space.

The system is a heat pump, not just a cooler. Almost all modern systems produce heating for the colder months and cooling for the warmer months from the same indoor units², and each indoor unit can be controlled individually, so the temperature in each room is set separately³. Capacities in one maker's multi-split range run from 4 kW to 12 kW³, against 1 to 5 kW for multi-split or ducted systems generally⁴.

A single external unit powering multiple indoor units

A multi-split reverse cycle air conditioner consists of several indoor units, all connected to a single outdoor unit⁶. That single outdoor unit delivers the energy-efficient heating and cooling for the whole system³, with refrigerant piped from it to an indoor unit in each room the household wants to condition. Independent guidance describes the arrangement plainly: multi-split systems use a single outdoor unit to connect to multiple indoor units, typically two to five, allowing several rooms to be served at once¹.

The indoor units themselves come in more than one form, and a multi-split's appeal is that the forms can be mixed to suit each room. A high-wall unit suits a bedroom or living room; a ceiling cassette suits a space where wall mounting is awkward; a ducted or floor-mounted unit suits other layouts. The maker Midea describes its multi-split range as providing individual room control, flexible installation and smart climate management for residential, retail, office and light commercial applications³.

One structural point matters before anything else: a multi-split is designed for multiple rooms, running one outdoor unit with multiple indoor units, and all the indoor units must be operating in the same mode, cooling or heating, at the same time². The rooms do not all have to be running at the same output, but none of them can cool while another heats. That single constraint shapes everything else on this page, and it is the main thing that separates a multi-split from a row of independent single splits.

Heating and cooling for each room, controlled separately

What a multi-split does well is give each room its own temperature. Each indoor unit can be controlled individually, which makes it possible to cool or heat several areas of the house at once while setting each one differently⁶. A bedroom can run at 18 degrees overnight while the living room unit stays off; a south-facing room can work harder than a north-facing one. Midea's own description of its multi-split systems leads with exactly this: individual room control³.

The heating side is not an add-on. Most modern systems, almost all of them, are heat pumps that produce heating in the colder months and cooling in the warmer months². MCS, the certification body, notes that an air-to-air system can also offer cooling alongside its heating function⁸. At product level, Daikin's Multi+ range is described as a heat pump system that looks after heating, cooling and hot water, all from one outdoor unit, with the Class 90 model connecting up to four indoor units⁹ and the 5 Port Class 68 offering the same heating, cooling and hot water combination¹⁰.

For a household's energy independence this cuts two ways. On the one hand, a reverse-cycle multi-split can take over a large share of the home's heating from the boiler, using electricity rather than gas, and electricity can be generated at home with solar panels. On the other hand, the system remains dependent on the grid for its electricity, on the manufacturer for spare parts and app-based controls, and on an F-Gas registered engineer for the refrigerant circuit. It reduces dependence on gas; it does not remove dependence on electricity.

The drawbacks of multi-split: installation, noise and cost

The limitations are worth stating firmly, because they are not fixable by buying a bigger model.

• All units share one mode. Every indoor unit must be cooling or every one must be heating; the rooms cannot be split between the two².
• Capacity is shared. All the indoor units draw from the one outdoor unit's output, so running every room at once on a hot evening means sharing whatever the outdoor unit can deliver¹.
• More fitting work than a single split. Refrigerant pipe runs must be made from the one outdoor unit to every room, which means longer pipe routes, more wall penetrations and more trunking than a single-room system, and by law all split type systems require an F-Gas registered engineer to carry out the installation or at minimum the commissioning, except DIY-category monobloc products².
• Price. Multi-splits are generally more expensive than monobloc reverse cycle air conditioners, especially since the outdoor unit must be sized for the whole system⁶.
• The outdoor unit is a fixed neighbour-facing object. It has a fan and a compressor that run whenever the system works, and its siting is constrained by planning rules (covered below).

The noise comparison with the monobloc alternative runs in the multi-split's favour, but only relatively. Monobloc reverse cycle air conditioners are rather noisy because their compact size and integrated motor put everything in one box, often inside or venting through a window⁶. A multi-split moves the compressor noise outside, onto the wall, where it becomes a matter of the outdoor unit's specification and siting rather than the room. That trade, more cost and fitting work for less indoor noise and more rooms served, is the whole decision in one sentence. Our page on air conditioning noise covers the detail.

About 4 kW of heat per 1 kW of electricity used

The headline figure is a coefficient of performance of roughly 4: for every kilowatt of energy used, most modern systems produce roughly 4 kW of output². The same guidance puts it another way: air conditioning is roughly 4 times more efficient than plug-in heaters². An independent model of summer cooling, by the Centre for Sustainable Energy, uses a more conservative CoP of 3.5, meaning 1 kWh of electricity gives 3.5 kWh of cooling¹¹. The two figures differ because one is a maker's typical-system claim and the other a deliberately cautious modelling assumption; both describe the same physics, which is that a heat pump moves heat rather than generating it.

Power ratings give a sense of scale. Uswitch lists single-split systems at 0.8 to 2.5 kW and multi-split or ducted systems at 1 to 5 kW⁴, while Midea's multi-split range runs from 4 kW to 12 kW³. The majority of modern systems are a minimum of A rated, with many being A+ or higher².

For the household, the efficiency point is the strongest argument for a multi-split over any resistance heating: the same electricity delivers several times more heat. It is also the strongest argument over portable units, which use around 1 kW of power each¹² while cooling one room only. The running cost of a multi-split is covered in detail on our air conditioning running costs page, and the rating labels on air conditioning efficiency explain what SEER and SCOP figures mean.

How big, heavy and loud is an outdoor unit? The AOEG30KBTB example

Every multi-split lives or dies on its outdoor unit, so a concrete example helps. The Fujitsu General AOEG30KBTB is a ceiling-mounted split system whose outdoor unit specifications are published in full by the maker⁵:

All figures from the maker's product page⁵.

What those numbers mean in practice: the unit is roughly the size of a large suitcase laid flat, heavy enough to need a proper wall bracket rather than a casual fix, and about as loud in cooling as a quiet conversation at close range, slightly louder in heating. Some multi-split outdoor units go further on sound: Daikin's Multi+ Class 90 and 5 Port Class 68 both have a night quiet mode setting that reduces the sound pressure during the night⁹.

The siting decision is therefore physical as much as acoustic. A 52.0 kg unit needs a bracket fixed to a solid wall, clear airflow around it, and a position that keeps its 53 to 55 dB(A) away from the rooms the household sleeps in and from the boundary the neighbours sit beside.

Planning rules and installation: permitted development in England and Scotland

Adding air conditioning to a home may be considered permitted development, meaning no planning application is needed, provided all the limits and conditions set in legislation are met; those limits cover the function, size, placement and quantity of units¹³. Independent reporting in July 2026 put it simply: most AC units on existing buildings fall under permitted development rights¹⁴.

The rules differ between the nations, and the differences matter.

• England. The UK Government amended the domestic air source heat pump permitted development rights so that up to two ASHPs can be installed on detached dwellinghouses, and the amendments allow units used for heating and cooling but not solely cooling¹⁵. This is what brought air-to-air systems, which are heat pumps that cool, within scope.
• Scotland. The Scottish Government has consulted on matching the English changes: amending the PDR to allow up to two ASHPs on detached dwellinghouses, and permitting ASHPs installed under PDR to be used for heating and cooling but not solely cooling, so that air-to-air heat pumps can benefit¹⁵. Scotland's existing permitted development classes cover ground source and water source heat pumps and biomass flues within the curtilage of a dwellinghouse¹⁷.

The detail for each nation is on our pages for planning permission in England, Scotland and the other nations, and the installation process itself, from survey to commissioning, is covered under air conditioning installation.

What ownership involves: maintenance, working life and guarantees

Ownership has a rhythm to it, and most of it is light work. The system should be serviced annually by an F-Gas registered engineer, which keeps it running efficiently; between services, the indoor unit should be cleaned every one to 3 months and the outdoor unit every 6 to 12 months, depending on usage². That annual cadence matches the general advice for heating systems: get your heating system serviced once a year¹⁹, and Plymouth Energy Community likewise recommends a detailed check by a professional installer, typically every year²⁰. Solid fuel guidance points the same way, with a routine service at least once a year meeting the manufacturer's maintenance instructions²¹.

On lifespan, the figures converge from different directions. A well-maintained, good quality reverse cycle air conditioning system can last between 15 and 20 years⁶. Independent European guidance gives heating systems an average lifetime of 15 to 20 years²². The maker's support guidance is more conservative for heavy use: a minimum of 6 years reliable life for systems used an average of 8 hours per day, cooling in summer and heating in winter, and maintained correctly². Most systems carry a minimum 5 year warranty².

There is also a statutory inspection regime for air-conditioning systems: the law requires that the system be inspected by an energy assessor at regular intervals not exceeding five years²³, and the Scottish building standards guidance describes the resulting range as three years for poorly maintained and inefficient systems up to five years for those that are well maintained²⁴. In practice, an annual service by an F-Gas engineer covers the household's side of this; the F-Gas rules and air conditioning maintenance pages carry the detail.

Dehumidifying, condensate and water

A question that comes up often, and can be answered firmly: a sealed air conditioning system poses no legionella risk. As there is no standing water, Legionnaires' disease does not apply to air conditioning; the idea that it does is described as a complete myth with regard to sealed systems². The contrast is with hot water systems, where stored water is the risk: the Vaillant aroTHERM plus heat pump, for example, is designed to run hot enough to remove the need for an electric immersion heater for legionella protection²⁵. A multi-split has no stored water at all, so the question does not arise.

The only water a multi-split handles is condensate: in cooling mode, the system takes moisture out of the air, so it acts like a dehumidifier as a side effect of cooling². Most systems also have a Dry mode, but it is not designed to be a standalone dehumidifier, and dehumidification does not happen in heating mode². For a damp problem in winter, a dedicated dehumidifier is still the right tool; the interaction between air conditioning and moisture is covered on our air conditioning and humidity page.

The 0% VAT rate and what you pay

The tax position on installation is unusually favourable at the moment. In Great Britain, you pay a rate of 0% VAT on both certain products supplied by your installer and the cost of all work to install those products in your home⁷. In Northern Ireland, installations of energy-saving materials, including solar panels and heat pumps, now benefit from a zero rate of VAT²⁷; where the zero rate does not apply, a 5% rate applies if the cost of the products, excluding VAT, is 60% or less of the total installation cost, excluding VAT⁷.

The official worked example of how the reduced rate used to apply shows the mechanics: on a £1,000 installation where the insulation material is £400 excluding VAT, 40% of the cost of the total supply, the reduced-rate VAT charge was £50, that is 5% of £1,000²⁸. The zero rate removes that charge entirely for qualifying products.

On the price of the system itself, there is no published installed price for multi-split systems, so prices are installer-quoted and depend on the number of indoor units, the pipe runs and the outdoor unit's capacity. What can be said is the relative position: multi-splits are generally more expensive than monobloc reverse cycle air conditioners, especially since the outdoor unit must be sized for the whole system⁶. Our air conditioning cost page covers how quotes are built up.

Choosing between multi-split and individual single-room units

The choice comes down to how many rooms need conditioning and what the outside of the house can take. Single-split systems are designed for one room or location²; multi-split systems are designed for multiple rooms, running one outdoor unit with multiple indoor units². Independent guidance frames the multi-split as the single outdoor unit connected to multiple indoor units, typically two to five¹.

A multi-split fits a home that wants several rooms conditioned but can only reasonably site one outdoor unit, whether because of space, appearance or the planning limit of one air source heat pump per building and curtilage. Separate single splits fit a home that wants one or two rooms done now and the option to add more later, at the cost of another outdoor unit per room and, in England, the possibility of hitting the one-unit limit. Power ratings track the choice: single splits run 0.8 to 2.5 kW, multi-split or ducted systems 1 to 5 kW⁴, with multi-split ranges reaching 12 kW³.

The fuller comparison, including running several single splits against one multi-split, is on our single split vs multi-split page, and the wider choice of system types is on types of air conditioning and the home cooling guide.