The short answer
Ducted air conditioning hides the indoor unit in a loft, ceiling void or cupboard, and ducts carry the air to grilles in the rooms. Only the grilles are visible, but it needs space and usually more building work than a wall-mounted system.
On this page
Ducted systems are common in offices and in homes in some hotter countries, but less so in UK houses, few of which have space between floors for ductwork. They're most practical when a home is being built, extended or substantially renovated, because the ducts can be planned in before the ceilings are finished.
How ducted air conditioning works
A ducted system is a split system with the indoor unit hidden away. The unit sits out of sight in a loft, above a lowered ceiling, in a bulkhead or in a cupboard. Its fan pushes air across a coil and into ducts that carry it to grilles in the ceiling or high on the walls. Air returns to the unit through a return grille, often in a hallway or ceiling, passing through a filter on the way.
As with any split, refrigerant pipes connect the indoor unit to an outdoor unit outside. The system is often run from a wall controller rather than a handheld remote.
Ducted air conditioning isn't the same as mechanical ventilation with heat recovery (MVHR), which also uses ducts and ceiling grilles. MVHR swaps stale indoor air for fresh outdoor air while recovering heat, and on its own it can't cool your home the way air conditioning can. Most ducted air conditioning, like other split systems, recirculates the air already in your home.
Controlling individual rooms
A single ducted unit has one temperature setting, so every room it serves gets the same treatment. A bedroom that gets the afternoon sun and a shaded study can end up at different temperatures, with no way to adjust them separately.
Zoning is the usual answer. Motorised dampers in the ducts open and close the supply to each room or area, with a controller for each zone, at the price of more parts and more design work. The alternatives are a separate ducted unit for each area, or a multi-split with ducted units on some of its connections.
Where it fits in a UK home
Ducted air conditioning tends to work best:
- in new builds and extensions, where the ducts can be designed in from the start
- on top floors and in loft conversions, where the unit and ducts can sit in the loft or eaves (see loft rooms and top floors)
- in rooms with a lowered ceiling or a bulkhead that can hide the ducts
- in basements and other spaces with a void above the ceiling
It's harder in an existing house where ceilings are fixed straight to the joists. Ducts are usually too big to pass through joists, and cutting or notching joists weakens them, so ducts end up boxed in, which means lower ceilings in places. If there's space above a ceiling but no practical route for ducts, a ceiling cassette is another way to keep most of the unit out of sight.
Ducted or wall-mounted
| Ducted | Wall-mounted split | |
|---|---|---|
| Seen in the room | Grilles and a controller | The indoor unit |
| Rooms per indoor unit | One or several | One |
| Building work | Ducts, grilles, access hatch | Usually one hole |
| Room-by-room control | Needs zoning | Each unit has its own |
| Filter access | At the unit or a return grille | On the unit |
What it does well
- Almost invisible. Only the grilles and a controller show, which suits rooms where you don't want equipment on the walls.
- Quiet in the room when the ducts are designed well, because the fan and coil are elsewhere and the compressor is outside.
- Air from several grilles, which can cover a large room, or several rooms, from one indoor unit.
- No pipework or condensate drain in the rooms themselves.
The drawbacks
- Space. The unit, the ducts and access to both all need room, and ceilings may have to be opened up or lowered.
- Design matters. Ducts and grilles have to be sized and routed properly. Undersized ducts or badly chosen grilles can cause air noise and leave rooms short of air.
- Insulation. Ducts running through an unheated loft must be well insulated, or they lose heat in winter and can gather condensation when carrying cold air in summer.
- Maintenance access. Filters still need cleaning every few weeks during regular use, and the unit needs servicing, so there must be a hatch or panel you can reach safely.
- Hidden water. The unit produces water when cooling, which needs a drain and often a condensate pump. A blocked drain in a loft or ceiling void can go unnoticed until it marks the ceiling below, as condensate drainage explains.
- Heating from above. Warm air released at ceiling level tends to stay high, so the grilles and airflow need choosing with heating in mind.
Related guides
- Ceiling cassettesA ceiling cassette is a split system's indoor unit set into the ceiling, with only a flat panel showing. It suits larger and open-plan rooms with space above the ceiling, but in most houses fitting one means building work.
- Multi-split systemsA multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.
- Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.
- Loft rooms and top floorsLoft rooms and top floors run hotter than the rest of the house because the roof heats up in the sun, warm air rises, and roof windows catch sunshine for much of the day. Shading, ventilation, insulation and well-planned cooling help.