A mini split is a ductless heat pump: an outdoor compressor unit moves refrigerant to one or more wall-mounted indoor units, carrying heat into your home in winter and out of it in summer - no ducts, no furnace, no resistance strips. The reason it matters is efficiency: it moves heat instead of making it, and a variable-speed compressor throttles to the exact load instead of blasting on and off. That's the whole idea; here is how each piece actually works.
Split, ductless, heat pump - three words that define it
The name packs in the three things that make the category:
- Split - the system is split into two boxes. The compressor and one coil live outside; a second coil and a quiet blower live inside. They are joined by a slim bundle of copper refrigerant lines called a line set, not by sheet-metal ducts.
- Ductless - conditioned air is delivered directly into the room by the indoor unit. A central system loses roughly 20-30% of its energy to leaks and heat transfer in the ductwork; a ductless unit has no ducts to lose it in.
- Heat pump - the same machine heats and cools. A reversing valve flips the direction of the refrigerant so the unit that dumps heat outdoors in July pulls heat indoors in January.
The refrigerant loop, in plain terms
Everything rides on one closed loop of refrigerant cycling between liquid and gas. Four components do the work:
- Compressor (outdoor) - squeezes the refrigerant gas, which raises its pressure and temperature. This is the pump that drives the whole loop and uses most of the electricity.
- Condenser coil - the hot, high-pressure gas releases its heat here and condenses to a liquid. In cooling mode this is the outdoor coil, shedding your room's heat to the outside air.
- Expansion valve - the liquid passes through a tiny restriction, its pressure drops, and it turns bitterly cold.
- Evaporator coil - the cold refrigerant absorbs heat from the air blown across it, boiling back to a gas. In cooling mode this is the indoor coil, and the air leaving it is what cools your room.
Flip the reversing valve and the coils swap jobs: the outdoor coil becomes the evaporator, pulling heat out of cold winter air (there is usable heat in air well below freezing), and the indoor coil becomes the condenser, releasing that heat into your room.
Why it moves heat instead of making it
This is the efficiency story. An electric baseboard or a furnace makes heat and can never return more energy than it consumes. A heat pump only moves heat that already exists, so for every unit of electricity it can deliver two to four units of heat. That ratio is why a mini split costs a fraction of resistance heating to run - see the electricity usage guide and estimate your own bill with the operating-cost calculator.
The inverter compressor - the real breakthrough
An old-style air conditioner has one speed: full blast until the room overshoots, then off, then on again. That short-cycling wastes energy, swings the temperature, and dehumidifies poorly.
A mini split uses an inverter-driven, variable-speed compressor. It ramps up to recover a hot room quickly, then throttles down to a low simmer that exactly matches the trickle of heat leaking through your walls
- running nearly continuously at low power instead of slamming on and off. The payoff is fourfold: lower energy use, temperature held within a degree, steadier dehumidification, and a compressor that idles quietly instead of lurching. It is the single biggest reason a modern mini split outperforms the window unit it replaces.
What's in the box (and on the wall)
- Outdoor unit (condenser): the compressor, one coil, and a fan. It sits on a pad or wall bracket outside.
- Indoor unit (air handler / "head"): a coil, a variable-speed blower, filters, and the electronics. The wall-mounted style is most common, but ceiling-cassette, floor-mounted, and short-duct versions exist.
- Line set: insulated copper lines carrying refrigerant, plus a control wire and a condensate drain, run through a ~3-inch wall penetration.
- No ductwork, no furnace, no chimney. That is what makes a mini split practical for additions, garages, and older homes with no ducts.
One zone or several
Pair one outdoor unit with one indoor head and you have a single-zone system - the simplest, most efficient, and easiest to install. Connect one larger outdoor unit to several indoor heads and you have a multi-zone system that conditions rooms independently from a single outdoor box. Multi-zone saves outdoor space but is pricier and less efficient per zone; we lay out the tradeoff in single-zone vs multi-zone mini splits.
Do they really work in the cold?
Yes - with the right unit. Because a heat pump extracts heat from outside air, its output falls as that air gets colder. Standard models fade near freezing; cold-climate models use enhanced (flash- or vapor-injection) compressors to hold strong, rated heat down to roughly -13F to -22F. If winter heating is the job, that rating is the spec that matters - the full explanation is in the cold-weather performance guide.
Reading the efficiency ratings
Three numbers describe how well a unit does the work above:
- SEER2 - seasonal cooling efficiency (higher is better).
- HSPF2 - seasonal heating efficiency.
- EER2 - cooling efficiency at a peak-heat design point.
Every model page on this site publishes the AHRI-certified version of these numbers - the independently verified figure, which is often lower than the marketing banner. Compare any two systems side by side with the comparison tool, or filter the whole catalog by efficiency and price in the product finder.
Installing one - and sizing it first
A mini split has to be sized to the room before anything else: an oversized unit short-cycles and dehumidifies poorly, an undersized one never keeps up. Start with the sizing calculator to get the BTU target, then decide the install path. Some lines are engineered for DIY installation with pre-charged quick-connect line sets; most are a professional job requiring refrigerant handling, a vacuum pump, and a dedicated circuit. Either way, the buying guide walks through choosing the right model.
FAQ
Is a mini split the same as a heat pump? A ductless mini split is a type of air-source heat pump - one that delivers heat and cooling directly to rooms through wall units instead of through ducts.
Do mini splits use a lot of electricity? No - they are among the most efficient systems sold, because they move heat rather than generate it and throttle to the load. A single zone often runs on an ordinary circuit. See the electricity usage guide.
Do they need ductwork? No. That is the point of "ductless" - each indoor head conditions its room directly, so no ducts are required.
Can one outdoor unit heat and cool different rooms at once? It can serve several rooms, but most multi-zone systems run all zones in the same mode (all heating or all cooling) at a given time. Details in single-zone vs multi-zone.
How long do they last? A well-maintained, properly sized mini split typically runs 15-20 years - filter cleaning and keeping the outdoor coil clear are most of the upkeep.
Sources
No model-specific systems are linked from this guide yet.