Ducted and Ceiling Cassette Air Conditioning in Homes

Ducted air conditioning is the concealed form of home cooling and heating: a single indoor unit, roughly the size of a boiler, sits hidden in a loft, cupboard or ceiling void and pushes conditioned air through slim ducts to grilles in the ceilings of several rooms. Energy Saving Trust describes the arrangement plainly: "The heat pump warms the air in an air handling unit, which is roughly the size of a boiler."¹ A ceiling cassette is the other concealed option, and it works differently: the unit sits flush in the ceiling with only the vents visible, connected to an outdoor unit by slim piping, with no ductwork at all².

Both are reverse-cycle systems, so they heat as well as cool, and both run on electricity rather than gas. Capacities in the published UK guidance run from 2.0 kW units suited to a single room up to 10.0 kW models for whole-floor coverage³, and the building regulations set a minimum seasonal efficiency in cooling of 4.0 for existing dwellings and 4.6 for new ones⁴. The choice between them comes down to the fabric of the house: whether there is a ceiling void deep enough to hide a unit and its ducts, and whether the household wants one grille per room or one flush panel in one ceiling.

Where a hidden unit fits compared with a ceiling cassette

The two systems look similar from below, a grille or panel in the ceiling, but they are built differently behind it.

A ducted system is a central one. One air handling unit conditions all the air, and ducts carry that air to each room. The ducting runs concealed above ceilings and inside cavities: one UK whole-house system routes "fully insulated and sound-attenuated ducting" above ceilings and inside cavity walls to deliver conditioned air through multiple branches to room outlets⁷. The same concealment principle appears in ventilation systems, where mechanical ventilation with heat recovery uses "concealed ducting (piping) in ceiling cavities that lead to the heat exchanger unit in a cupboard, utility room or sometimes a loft"⁸. For exhaust air heat pumps, a ducting system "needs to be installed above the ceiling, unless you don't mind exposed duct work", and homes with metal web floor joist systems make multi-storey runs easier, with the unit ideally close to an exterior wall⁹.

A cassette is a local unit. Hitachi's buying guide describes it: "Instead of being mounted on the wall, like the split-system, cassette type air conditioners are placed in the ceiling, with just the vents visible, connected to an outdoor unit via slim piping and no ductwork required."² Each cassette conditions the room it sits in, and a multi-split arrangement can serve several rooms from one outdoor unit, with a cassette or other indoor unit in each.

The practical difference for a householder is the void. A cassette needs a shallow pocket in one ceiling and a pipe route to outside. A ducted system needs a continuous route for ducts from the air handling unit to every grille, which is why ducted systems suit new builds and homes that already have ducts, and why retrofitting them into an existing UK house is disruptive.

Matching ducted, cassette and wall-mounted types to your property

Each of the three forms has a natural home, and the published guidance is consistent about where each belongs.

Ducted air conditioning, in the maker's own words, "is ideal if your home already has ducts fitted, new builds or for homeowners that want to control the climate of the entire house", and is "not appropriate for small apartments, registered buildings or smaller budgets"². Energy Saving Trust adds a caution about the building itself: "Ducted systems may not be suitable for older or larger houses with high heating needs, though."¹ Which? makes the retrofit point directly: a ducted air-to-air system "can be disruptive to install unless you already have ducted warm air heating, which is uncommon in the UK"¹⁰. Ducted systems are "typically seen in energy efficient houses such as Passivhauses", where the heating demand is low enough for air distribution to carry it¹.

Wall-mounted units are the default for room-by-room cooling. Fixed wall-mounted air conditioning "is usually better for regular use, especially in bedrooms, living rooms, offices and garden rooms", and many models provide heating as well as cooling³. A wall-mounted internal unit is compact, typically "80cm wide, 30cm tall, and 20cm deep"¹¹, which is why it fits where nothing concealed can.

The cassette sits between the two. It needs a ceiling but no ducts, so it suits rooms where a wall unit would be unwanted: above a kitchen island, in a hallway, in a room with high or glass walls. Where a household wants several rooms cooled but the building cannot take ducts, the multi-split is the recognised answer: Hitachi describes it as "the right choice for people that live in buildings where ducted systems are not suitable", appropriate for both houses and apartments².

One further variant blurs the categories. Some ducted systems are also known as exhaust air heat pumps, and "can also supply hot water"¹, combining ventilation, heating and water heating in one ducted package. These are covered in more detail in air conditioning that heats.

Unit sizes ranging from 2.0 kW to 10.0 kW

Capacity is matched to floor area, and the published sizing guidance gives a direct mapping between the two.

The table comes from a UK air conditioning sizing guide³, and the same guide is the basis of our fuller page on air conditioning sizing. For a ducted system serving several rooms, the requirement is the sum of the rooms, which is what pushes whole-house ducted systems toward the upper end of the range.

Two published figures show what a multi-room outdoor unit delivers. A Daikin multi-split outdoor unit is listed with a cooling capacity of 5.2 to 9.0 kW and a heating capacity of 6.8 to 10.0 kW¹², which spans most of the table above. Uswitch's guide to whether air conditioning is worth it in the UK gives a narrower band for the typical domestic case, listing "Multi-split or ducted system | 1-5 kW"¹³.

There is also a regulatory ceiling worth knowing about. A consultation on the F-gas rules in Great Britain considered a proposal to "limit the size of air-to-air systems using flammable refrigerant to around 6.5-7kw"¹⁴. If brought in, that would cap a single flammable-refrigerant system near the middle of the range, which matters for a household planning a large ducted installation. The rules around refrigerants are covered in F-gas rules for home air conditioning.

How efficient they must be: SEER floors of 4.0 and 4.6

Efficiency in cooling mode is measured as SEER, the seasonal efficiency across a whole summer, and the building regulations set the floor.

Approved Document L, Volume 1 states the requirement for England: "For new dwellings, a minimum of 4.6." and "For existing dwellings, a minimum of 4.0."⁴ The Welsh consultation version of the same document carries identical figures, 4.6 for new dwellings and 4.0 for existing ones¹⁵. Those are legal minimums, not good practice: any system legally installed in a UK home meets them.

Actual products run well above. A Carrier ducted system certified by Eurovent carries a SEER of 6.20¹⁶. Uswitch's guidance goes further on what to look for: "look for ratings of 7+ to keep costs down"¹³. The gap between the regulatory floor of 4.0 and a certified 6.20 is the distance between the worst a home may legally get and what a mid-range certified product delivers, and it translates directly into electricity used for the same cooling.

For a household weighing running costs, the SEER and its heating counterpart SCOP are the numbers to compare between models, and both appear on the energy label. How the ratings are calculated, and how to read them, is set out in air conditioning efficiency ratings, and the electricity they translate into is covered in air conditioning running costs.

Noise: 35 to 46 dB(A) indoors, and what the outdoor unit adds

Because a concealed system lives in a ceiling, noise is often the reason a household chooses it, and the published figures bear that out.

The Fujitsu AUXG45KRLB ceiling cassette, a model sold in the UK, is specified at 46 dB(A) on high, 42 dB(A) on middle, 39 dB(A) on low and 35 dB(A) on quiet, in cooling⁵. The quiet setting is the one that matters for a bedroom ceiling: 35 dB(A) is around the level of a quiet library, and the drop from high to quiet comes from slowing the fan.

That mechanism is explicit in the multi-split ranges. The Daikin multi-split indoor units have a quiet operation mode, which "reduces the sound pressure by reducing the airflow rate"¹². The same ranges give the outdoor unit its own setting: it "has a 'night quiet' mode setting to reduce the sound pressure during the night"¹⁷. So both halves of the system can be quietened, at the cost of airflow indoors and, presumably, some capacity outdoors.

Two things follow for a householder. First, the quiet setting is a real, specified mode, not a marketing phrase, and its figure appears on the product datasheet. Second, the trade-off is airflow: less air moved means less cooling delivered at that moment, so a room may take longer to reach temperature on a hot evening. The outdoor unit is the part a neighbour hears, and its noise is a planning matter in each nation, covered in air conditioning planning permission in England, Scotland, Wales and Northern Ireland, and in how noisy air conditioning is.

Pipe lengths, refrigerant and fitting constraints

Every concealed system is a split system underneath: an indoor unit, an outdoor unit, and refrigerant piping between them. The rules around that piping shape what can be installed.

The refrigerant in current ranges is R32. Daikin's Altherma 3 H HT heat pump range is typical: "the system uses R32 refrigerant"⁶. R32 is a fluorinated gas, so installation, leak checks and end-of-life recovery must be carried out by a certified engineer under the F-gas rules. The consultation on those rules in Great Britain noted the proposal to "limit the size of air-to-air systems using flammable refrigerant to around 6.5-7kw"¹⁴, which would constrain single-system size for homes.

The inspection regime applies only above a threshold. The Energy Performance of Buildings Regulations 2012 state: "This Part applies to air-conditioning systems with an effective rated output of more than 12kW."¹⁸ A closed consultation on changing those regulations records that the threshold "was changed from systems with an effective rated output of over 12kW to an effective rated output of over 70kW"¹⁹. Either way, a domestic ducted or cassette system at 2.0 to 10.0 kW sits far below the line, so no regular inspection is triggered.

Installation brings its own limits. The electrical load matters in homes with electric heating: official research in Scotland notes that "use of multiple heaters simultaneously can trip a domestic property ring circuit by exceeding the maximum designed electrical loading" of 23kW²⁰, and a ducted heat pump adds to that load. On the drainage side, where a condensate pipe runs externally, industry guidance states the pipe "needs to be increased to at least a 30mm inside diameter and insulated" to resist freezing²¹. Within the ductwork itself, the SAP guidance permits only "small sections of flexible ducting for vibration isolation, maximum length 200mm per section", with otherwise rigid ducting²².

There is a VAT position worth knowing on the heating side. HMRC's manual on energy saving materials and grant-funded heating covers a reduced rate for "installing, maintaining and repairing central heating systems and similar, and renewable source heating systems"²³. How installation works in practice, from survey to commissioning, is covered in air conditioning installation.

Controls and running features: timers, fresh air ducts and restart

Concealed systems are controlled from a wall panel or a remote, not from the unit itself, and the published feature lists are consistent across the market.

Air-to-air heat pumps "are programmable, just like other heating systems and often come with a remote and/or smart wi-fi" option, and many offer dehumidification and air purification functions¹⁰. The same pattern appears across modern electric heating: programmable timers, thermostats, remote wi-fi controls and open window detectors are standard features of current-generation equipment²⁴. The practical point for a ducted system is that the controls live wherever the household wants them, since the unit itself is hidden.

Fresh air is a genuine option on some models, not a universal one. The Fujitsu AUXG45KRLB cassette states: "Outside air can be introduced by attaching a locally purchased duct to the fresh air knockout and an optional part."⁵ That is a modest intake, not a ventilation system. It matters because cooling a sealed house raises the fresh air question: ventilation is deliberate planned air movement, and is "different to air infiltration (draughts), which is unplanned air movement through gaps in the building fabric"²⁵. A fresh air duct on the air conditioner is one way to admit some planned air; a full mechanical ventilation system with heat recovery is another, and the two are compared in air conditioning, dry mode and humidity.

Power cuts are handled automatically on models with auto restart. The same cassette states: "In the event of a temporary power failure, the air conditioner will automatically restart in the same operating mode as before, once the power supply is restored."⁵ For a system hidden in a void, that matters more than it would for a wall unit: there is no visible unit to notice is off, so silent recovery is the behaviour a household wants. The wider control options, including wi-fi, are covered in air conditioning controls.

Filter upkeep, drain pumps and reaching parts for servicing

A concealed unit cannot be glanced at, so maintenance has to be planned rather than reactive.

The homeowner tasks are simple and are the same as for any split system: Hitachi's guide lists "washing the filter, removing dust, and checking for mould and other build-ups"². On a cassette the filter lifts out through the panel in the ceiling; on a ducted system the access is at the air handling unit or at the return grille, depending on the design. A filter reminder on the controller is common, and a clogged filter restricts airflow before anything else fails.

The annual service is an engineer's job. Energy Saving Trust lists the tasks: "cleaning coils, cleaning fan blades, inspecting filters, checking refrigerant level and pressure, checking that all electrical connections are safe"²⁶. Refrigerant checks in particular are not a homeowner task, since the F-gas rules require certified engineers for refrigerant work. Some heat pump products add remote monitoring "for diagnostics, servicing, and maintenance"²⁷, which lets a fault be seen before it stops the system.

Drainage deserves attention in a ceiling. Every cooling indoor unit produces condensate, and in a concealed system that water is carried away by a pump and a small pipe run through the void. Where any condensate pipe runs externally, the industry guidance on freezing weather applies: the pipe "needs to be increased to at least a 30mm inside diameter and insulated"²¹. A blocked or frozen condensate route in a ceiling is a water damage risk, not just a comfort one, which is why the drain pump is checked at service. The full routine is set out in air conditioning maintenance, and the risks in air conditioning safety.

The contribution a hidden air conditioning setup makes to self-sufficiency

A ducted or cassette system is an electric appliance that heats as well as cools, and that changes what a household depends on.

The independence gain is real. A reverse-cycle system running on electricity can take over space heating from gas, and some ducted types, the exhaust air heat pumps, "can also supply hot water"¹, cutting the gas connection out of the house entirely. Because the system is ducted or ceiling-mounted, it also conditions the whole home rather than one room, and a fresh air duct on a suitable model admits planned outside air without throwing open windows and losing the cooled or heated air⁵. In a well-insulated home, that combination of electric heating, controlled cooling and planned ventilation is close to a self-contained indoor climate.

The dependencies that remain should be stated just as plainly. The system runs on grid electricity, so a power cut stops it, and auto restart only restores it once the supply returns⁵. It depends on a certified engineer for refrigerant work under the F-gas rules, and on the maker for spare parts and the controller's software. A ducted system additionally depends on the fabric of the house: the voids that carry the ducts are fixed at installation, and a future change of heart about where air is delivered means opening ceilings again. The refrigerant itself is a managed substance, with the phasedown of fluorinated gases under active reform in Great Britain¹⁴, so what a system contains in twenty years is a policy question as well as a technical one.

For a household weighing the whole picture, the comparison pages gather the alternatives: ducted vs multi-split air conditioning, types of air conditioning, and the pillar guide to home cooling. The honest summary is that a concealed system buys comfort and quiet with electricity, and trades away the simplicity of a unit on a wall.