Ductless mini-split
A heating and cooling system with one or more indoor units connected to an outdoor unit by refrigerant pipes and a control cable, with no duct network in between. Because each indoor unit conditions the space it is mounted in, a fault can affect one room while the rest of the building carries on normally.
Also called mini split, ductless system, split air conditioner
What is a ductless mini-split?
A ductless mini-split is an air conditioner or heat pump that connects an outdoor compressor unit to one or more wall, floor, or ceiling indoor units through refrigerant piping instead of ducts.
Ductless mini-split: how it works
The outdoor unit holds the compressor and one coil, each indoor unit holds the other coil and a fan, and refrigerant carries heat between them through the line set. Nothing but refrigerant and a control signal passes between the two, so the indoor unit blows only the air already in the room it serves.
Where you meet the term
- On a quotation, where the number of indoor heads sets the price far more than the outdoor unit does.
- In a fault description, when one room stops working and the others do not, which points at that head or its branch rather than at the outdoor unit.
- In manufacturer documentation, where single-zone and multi-zone models of the same series carry different code tables.
How to check it
- Count the indoor heads. One indoor unit on one outdoor unit is single-zone. Two or more is multi-zone, and the code table you need is the multi-zone one for that series. Owner check
- Read the model number from both units. The head and the outdoor unit are separate products with separate labels. Photograph both plates before calling anyone, because which one answers a question depends on the question. Owner check
United States and United Kingdom usage
- United States
- Sold as ductless or mini-split, sized in BTU per hour, and quoted with SEER2 and HSPF2 ratings.
- United Kingdom
- Called air conditioning or an air-to-air heat pump, sized in kilowatts, and quoted with SEER and SCOP.
Ductless mini-split: what it is not
- Ductless does not mean the same as portable or window. A mini-split is a fixed installation with a sealed refrigerant circuit made at commissioning.
- A mini-split is not always heating capable. Cooling-only models exist, and the model number rather than the brand is what tells you.
Ductless mini-split questions
Is a ductless mini-split the same as a heat pump?
A mini-split can be a heat pump, and many are, but the two words describe different things. Ductless describes how the air is delivered. Heat pump describes whether the system can run in reverse to heat as well as cool.
Why does only one room stop working?
Each indoor unit has its own fan, coil, sensors, and control board. A fault confined to one room narrows the search to that head, its wiring, and its branch of the pipework, which is the first thing a technician establishes.
Can a mini-split be added to a house that already has ducts?
Yes, and it is a common way to condition an extension, a loft conversion, or a room the existing duct run never reached properly. The two systems operate independently.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Ductless System MaintenanceTrane homeowner and professional maintenance boundaries for single-zone and multi-zone ductless systems.
- AIRSTAGE Mini-Splits TroubleshootingGeneral homeowner checks and normal operating behavior for AIRSTAGE mini-splits.