Multi-split air conditioning installation

The NUMIQ price list has two items for multi-split systems. The first is installation of a system with two indoor units and one outdoor unit, the second is installation and setting up of a system with three indoor units and one outdoor unit. Both cover the engineer's labour including small consumables, without the units themselves. The work typically includes:

• mounting the outdoor unit on wall brackets, a ground stand or a flat roof frame, including anti-vibration pads,
• fixing the mounting plates and hanging the indoor units in each room,
• core drilling through walls for the pipework, cable and condensate drain,
• running and connecting the copper refrigerant pipework to each indoor unit separately,
• the interconnecting communication cable between the outdoor and indoor units,
• connecting the condensate drain from each indoor unit,
• pressure testing each circuit, evacuating it and topping up the refrigerant where the pipe length requires it,
• commissioning, checking operation and showing the owner how to use the controls.

What installation does not usually include: the units themselves, pipe runs longer than the length stated in the quote, trunking and its colour finish, chasing and making good walls, access equipment or scaffolding, the electrical supply from the consumer unit and its certification. When comparing quotes, it is worth asking how many metres of pipe run are included and how each additional metre is charged.

A multi-split is not automatically better than two or three separate systems, known as single splits. Each solution has its place.

When a multi-split pays off

• There is only room for one outdoor unit. In blocks of flats the freeholder or management company often allows only one outdoor unit, sometimes only on a balcony.
• The rooms are close together. Neighbouring bedrooms in a loft conversion, or a flat with the rooms on one side of the hallway. The pipe runs are short and can be concealed.
• The appearance of the house matters. One outdoor unit at the back of the house is far less intrusive than three.

When separate systems are more suitable

• The rooms are far apart or on opposite sides of the house. Manufacturers limit both the pipe length to each indoor unit and the total length of all circuits. For example, Daikin's specification tables for the MXM range give a total piping length of 30 to 50 m for two-room outdoor units depending on capacity, 50 m for the three-room 3MXM-A units, and a maximum height difference of 15 m between the outdoor and indoor units. In a large, spread-out house these limits may not be achievable.
• Usually only one room is in use. A large outdoor unit running at low load works less efficiently than a small unit designed for a single room.
• Reliability matters. If the outdoor unit of a multi-split fails, no room works. With separate systems, a fault in one does not affect the others.
• Each room wants a different mode. Standard domestic multi-split systems run in one mode at a time. If one unit is cooling, another cannot heat at the same moment. In practice this can be a problem in spring and autumn, when the north side of the house needs heat and the south side needs cooling.

When comparing costs, bear in mind that a multi-split saves on the number of outdoor units and on wall space, but the more demanding installation and longer pipe runs often take back part of that saving. The decision is therefore worth a site survey by an engineer who measures the distances and proposes a specific combination.

Ideally, air conditioning is planned at the shell stage of a new build or during a renovation before plastering. Pipework, cable and condensate drain can then be buried in walls or above a suspended ceiling, and after installation only the outlets are visible. This preparation is known as first fix or pre-piping. In a finished interior the runs go in trunking, which works but remains visible.

Demand for installation rises sharply with the first summer heatwave. Engineers' diaries then fill up for weeks ahead. Ordering the work in winter or spring gives a wider choice of dates and equipment.

Before the engineer arrives, the following should be clear:

• Capacity for each room. Based on floor area, window orientation, insulation and the number of occupants. A south-facing living room with large windows needs more than a north-facing bedroom.
• Position of the outdoor unit. With access for servicing, free airflow, away from neighbours' windows and away from bedrooms.
• Position of the indoor units. So that the airflow does not blow straight onto a bed, desk or sofa.
• Pipe and condensate routes. The condensate needs a fall. If it cannot drain by gravity, a condensate pump is required.
• Electrical supply. A dedicated protected circuit for the outdoor unit according to the manufacturer's requirements. The supply is installed by a registered electrician.
• Consents. In a leasehold flat, written consent from the freeholder or management company, as the external walls are usually not part of the demise.

Survey and system design

The engineer measures the distances from the outdoor unit to each room, the height differences and the possible routes. Based on this, an outdoor unit and a combination of indoor units are proposed. Manufacturers state which capacity combinations may be connected to a given outdoor unit. The total capacity of the indoor units may exceed that of the outdoor unit, because it is assumed that not all of them will run at full output at once. The design should reflect how the household actually uses the rooms.

Mounting the outdoor unit

A multi-split outdoor unit is larger and heavier than a single-room unit. Brackets must be fixed into sound masonry. On cavity walls, fixings suitable for the wall construction are used, and on houses with external wall insulation, stand-off brackets designed for insulated facades are used so that no thermal bridge forms and the render does not crack. The unit is levelled, anti-vibration pads are placed under the feet and clearance is left for air intake and discharge according to the manual. The condensate produced in heating mode is dealt with under the unit, so that it does not freeze on the path in winter.

Mounting the indoor units

The mounting plate is fixed to the wall, exactly level or with a slight fall towards the condensate outlet as specified by the manufacturer. A gap is left between the unit and the ceiling so that the unit can draw in air.

Wall penetrations and pipe runs

The hole through the wall is drilled with a slight fall to the outside so that condensate cannot run back. Each indoor unit has its own two copper pipes, liquid and gas, each in its own thermal insulation. In addition there is the communication cable and the condensate hose. With a multi-split, several bundles therefore converge on the outdoor unit, and their route along the facade needs a tidy solution, for example a shared trunking run.

Connecting, pressure testing and evacuation

The pipe ends are flared and connected to the service valves of the outdoor unit and to the indoor unit connections. Each joint is tightened with a torque wrench, because a joint that is too loose leaks and one that is overtightened cracks. Each circuit is then pressurised with nitrogen and monitored for any drop in pressure. This is followed by evacuation with a vacuum pump, which removes air and moisture from the pipework. Moisture in the circuit shortens the life of the compressor. If the pipework is longer than the factory charge allows for, refrigerant is added according to the manufacturer's table.

Commissioning and setting up

The valves are opened, the electrician completes the connection and the engineer runs each unit on its own and then all of them together. Discharge air temperature, condensate drainage, noise and communication with the remote control or app are checked. The owner is shown how to clean the filters.

Types of indoor unit

• Wall-mounted (high wall). The most common and cheapest option. Suitable for most rooms.
• Floor-standing and console units. Mounted low on the wall. Practical in loft rooms and under windows.
• Ceiling cassettes and ducted units. Concealed in a suspended ceiling. They need ceiling void depth and are usually planned at the build stage or with a new ceiling.

Equipment prices

The price of the equipment varies considerably by brand, capacity and number of indoor units. In the UK, multi-split ranges from Daikin, Mitsubishi Electric, Fujitsu, Toshiba, Panasonic, LG and Samsung are commonly quoted. An exact figure only comes with a quote for a specific combination of units.

• Capacity by guesswork. It happens that the same unit goes into every room regardless of floor area and orientation. The south-facing room then cannot keep up, while the north-facing one cycles on and off needlessly.
• Pipe lengths exceeded. If a distant room is connected despite the manufacturer's limits, output drops and the warranty may be lost.
• Evacuation skipped. The circuit is only purged with refrigerant. Air and moisture in the system show up a few seasons later as a compressor failure.
• Condensate without a fall. Water sits in the hose, the unit drips down the wall, and slime and odours develop in the hose.
• Circuits mixed up. With a multi-split, several indoor units are connected to the outdoor unit. If pipework and communication cables are swapped between circuits, the units show error codes or the wrong room gets cooled.
• Brackets on insulated walls without proper fixings. The render cracks around the fixings and the unit sags over time.

Only an engineer holding a valid F-gas qualification may open, connect, charge and leak check the refrigerant circuit, and the business must hold company F-gas certification from an approved body such as REFCOM or FGAS Register. Most air conditioning installers hold Category I, which covers all work on systems of any charge size. With a multi-split the circuit is always connected on site, so a certified engineer is essential, not a bonus. Manufacturers also make the warranty conditional on installation by a qualified company, often with registration of the installation. Before placing an order, ask for the engineer's and the company's certificate numbers and check them on the certification body's online register.

The supply to the outdoor unit is normally a new dedicated circuit, which is notifiable electrical work in England and Wales. It should be carried out by an electrician registered with a competent person scheme such as NICEIC or NAPIT, who issues an Electrical Installation Certificate and notifies building control.

Check with the local planning authority whether the outdoor unit needs planning permission, particularly for flats, listed buildings and conservation areas.

A multi-split that works as an air-to-air heat pump and replaces fossil fuel heating may qualify for a grant under the Boiler Upgrade Scheme in England and Wales, applied for by an MCS certified installer. Check current conditions on the official Ofgem portal.

At handover, ask for the warranty documentation, the refrigerant charge, the pressure test and evacuation record, and the Electrical Installation Certificate for the new circuit. Then go through these points:

• switch on each indoor unit on its own and confirm that the right room is being cooled,
• switch on all units together and run them for at least half an hour to see whether any shows an error,
• check that water flows from the condensate hose outside or into the waste trap and does not drip down the wall,
• inspect the pipe insulation for tears and, on external runs, check that it is protected from sunlight,
• check that the outdoor unit is level, does not vibrate and has clear space around it,
• look at the wall penetrations to make sure they are sealed outside and no water can get in,
• test the remote control and the app and set the timer programmes.

The NUMIQ price list gives the price of multi-split installation for a set of two indoor units with one outdoor unit and for a set of three indoor units with one outdoor unit. It covers the engineer's labour including small consumables, without the units themselves. The current price is in the heating work section of the price list. Prices vary by region and city.

The final cost is mostly affected by:

• the length of the run to each indoor unit and whether it goes in the wall, above a ceiling or in trunking,
• the number of penetrations through walls and floors and the wall material, for example reinforced concrete,
• the mounting height of the outdoor unit and the need for access equipment or scaffolding,
• the condensate solution, especially if a condensate pump is needed,
• the type of indoor units, as cassettes and ducted units take more work than wall-mounted ones,
• making good chases and redecorating after installation.

When comparing with several separate systems, add up the full cost, meaning equipment, installation, trunking and pipe runs. For a single room, the price in the article on air conditioning installation applies. If heating is also being planned, see the article on air-to-water heat pump installation, which heats water rather than air. If cable chases are needed for the air conditioning, the article on cutting cable chases in brick, aircrete and concrete will help.