A ductless mini-split pairs an outdoor unit with one or more indoor units mounted in the rooms they serve, joined by a slim bundle of refrigerant lines, control cable and a drain. With no duct system to build, there is little demolition. In Menlo Park, Atherton and Portola Valley the usual reasons to install one are a room addition or converted garage the central system never reached, an older house with no ducts at all, and a single room, often over a garage or facing west, that overheats in a hotter spell while the rest of the house is fine. A home office in Sharon Heights, a converted garage near Downtown Menlo Park or a guest suite on a large Atherton lot is the kind of space it suits.
When ductless is the right call, and when it is not
Good fits:
- additions, converted garages, home offices and detached studios;
- the far end of an older ranch home, where stretching old, thinly insulated attic ducts would starve the rooms they already serve;
- older homes heated by a wall or floor furnace, where ducts would mean opening ceilings;
- rooms with vaulted ceilings and no attic above;
- one hot upstairs bedroom in an otherwise comfortable house;
- replacing noisy window units with permanent, quieter equipment.
Weaker fits:
- a large house with many small rooms, where a head in every room adds up and a ducted central air system may do the job with fewer pieces;
- a home that already has sound ducts and a furnace, where adding cooling is usually AC installation.
Single-zone or multi-zone?
| Setup | What it is | Suits |
|---|---|---|
| Single-zone | One outdoor unit, one indoor unit | One room or one open area |
| Multi-zone | One outdoor unit, two or more indoor units | Several rooms when outdoor space is tight |
| Several single-zones | A separate outdoor unit for each area | Rooms far apart, or areas that should not depend on one unit |
Multi-zone saves wall space outside and still gives each room its own setting. The trade-off: every room depends on one outdoor unit, and a small indoor unit running alone on a large outdoor unit can cycle on and off on mild days.
Choosing the indoor units
- Wall-mounted: the most common type, high on a wall.
- Ceiling cassette: set flush into the ceiling; needs framing space above it.
- Floor console: low on the wall, handy under windows or in rooms with sloped ceilings.
- Slim ducted unit: a small hidden air handler feeding short ducts to two or three nearby rooms.
Each indoor unit is sized for its own room. Oversizing is the common mistake: an oversized unit short-cycles, holds temperature unevenly and makes more noise than it needs to.
Where the outdoor unit goes
It needs a level base, room for airflow and service, and a spot away from bedroom windows. Every system has limits on line length and on the height difference between the indoor and outdoor units, which matters on Portola Valley hillside lots where the only flat ground may sit well below the room being served. Under mature shade trees, an outdoor unit picks up leaves and seed pods, so open sky above it keeps the coil clear. Wall brackets lift units off uneven ground but can carry vibration into a bedroom wall; a ground stand or pad is usually quieter.
Installation day
Most installs take 4 to 8 hours. The mounting plate goes up, a hole about three inches across is drilled through the wall, and the lines, cable and drain run outside under a line-set cover. The outdoor unit is set and wired to its own circuit; the lines are pressure-tested and evacuated; and the system is started in both cooling and heating. Where the drain cannot fall to the outside, a small condensate pump is added. You are shown the remote, the programmable schedules and the filters.
What moves the cost
Typical mini-split installs run $3,000 to $10,000. One wall unit on a short line sits at the low end. Each added indoor unit, a ceiling cassette or ducted unit, long line runs, a condensate pump, electrical panel work and hard access push it up. The free estimate lists the model numbers, the size chosen for each room, the line route, and what is included and excluded.
We install ductless systems from makers such as Mitsubishi Electric and Daikin; see all the brands we work on.
Living with a mini-split
Rinse the indoor units’ screens regularly and keep leaves away from the outdoor unit. The screens catch dust and pet hair but are not smoke filters, so on wildfire-smoke days keep windows shut and run a portable air cleaner; our indoor air quality page covers better options. Because a mini-split is a heat pump, the same unit warms the room on winter nights in the 40s, so a converted garage or a studio in Ladera can retire its space heater. For whole-house heat pump systems, see heat pump installation.