A mini split is a ductless air conditioning (and usually heating) system with two main parts: a small outdoor compressor and one or more indoor air handlers connected by refrigerant lines. Unlike central air, it doesn't need ductwork — each indoor unit delivers conditioned air directly into the room it's mounted in. Unlike window units, it's permanently installed, whisper-quiet (19–30 dB), and 50–70% more energy efficient.
Mini splits have been the dominant HVAC technology in Asia, Europe, and South America for decades. They're now the fastest-growing segment of the US HVAC market, driven by superior efficiency, falling prices, federal tax credits of up to $2,000, and the growing popularity of DIY-installable models like MrCool.
Mini Split vs. Other AC Types: Quick Comparison
| Feature | Mini Split | Central Air | Window AC | Portable AC |
|---|---|---|---|---|
| Ductwork required | No | Yes | No | No |
| Installation type | Permanent (wall mount) | Permanent (ducts + closet) | Semi-permanent (window) | Portable (no install) |
| Efficiency (SEER2) | 17–40 | 14–24 | 8–15 (EER) | 6–12 (EER) |
| Noise (indoor) | 19–30 dB | Silent in room | 50–60 dB | 52–65 dB |
| Heating capability | Yes (heat pump) | No (separate furnace) | Some models | Some models |
| Zone control | Yes (per room) | No (whole house) | Per unit | Per unit |
| Typical cost (12K BTU) | $1,500–$4,500 installed | $3,500–$7,500 (with ducts) | $200–$600 | $300–$700 |
| Lifespan | 15–20 years | 15–20 years | 5–10 years | 3–7 years |
| Monthly energy cost | $25–$75 | $40–$100 | $50–$120 | $60–$150 |
The Two Parts of a Mini Split
Outdoor Unit (Compressor/Condenser)
The outdoor unit sits on a concrete pad or wall bracket outside your home. It contains the compressor (the heart of the system that pressurizes refrigerant), the condenser coil (releases heat outdoors during cooling), and a fan. Residential outdoor units are roughly 30–36 inches wide, 22–26 inches tall, and 12–14 inches deep — about the size of a large suitcase.
The outdoor unit connects to the indoor unit through a bundle of lines that pass through a single 3-inch hole in your exterior wall. This bundle includes the refrigerant lines (two copper tubes), a condensate drain, and a communication cable.
Indoor Unit (Air Handler/Evaporator)
The indoor unit mounts on an interior wall, usually 7–8 feet high. It contains the evaporator coil (absorbs heat from indoor air during cooling), a fan/blower, air filters, a thermistor (temperature sensor), louvers for directing airflow, and the electronic control board.
Indoor wall-mount units are typically 30–36 inches wide, 10–13 inches tall, and 7–10 inches deep. They project about 7–10 inches from the wall. Other styles include ceiling cassettes (recessed into the ceiling), floor-standing units, and concealed slim-duct units hidden above the ceiling.
The Connection: What Runs Between Them
| Component | Purpose | Size |
|---|---|---|
| Liquid line | Carries high-pressure liquid refrigerant to indoor unit | 1/4" or 3/8" copper |
| Suction line | Returns low-pressure gas refrigerant to outdoor unit | 3/8" or 1/2" copper |
| Condensate drain | Drains moisture from indoor coil to outdoors | 3/4" PVC or flexible |
| Communication wire | Carries control signals between units | 14/4 or 16/4 shielded |
All four components typically bundle together and pass through a single 3-inch hole drilled through the exterior wall. This small penetration is one of the biggest advantages of mini splits — compared to the extensive ceiling/wall modifications needed for ductwork.
How a Mini Split Cools Your Room
The cooling process is the same basic refrigeration cycle used by every air conditioner, refrigerator, and heat pump:
Step 1: Warm room air is drawn across the cold evaporator coil inside the indoor unit. The refrigerant inside the coil absorbs the heat and evaporates from a liquid into a gas.
Step 2: The gaseous refrigerant travels through the suction line to the outdoor compressor, which compresses it into a hot, high-pressure gas.
Step 3: The hot gas flows through the outdoor condenser coil, where a fan pushes outdoor air across it. The heat transfers from the refrigerant to the outdoor air, and the refrigerant condenses back into a liquid.
Step 4: The liquid refrigerant flows back indoors through the liquid line, passes through an expansion valve that drops its pressure and temperature, and enters the evaporator coil again. The cycle repeats.
In heating mode, a reversing valve flips the direction. The outdoor coil absorbs heat from outside air, and the indoor coil releases that heat into your room. This is why most mini splits are called "heat pumps" — they pump heat from one location to another, rather than creating it through combustion or electric resistance.
Key concept: Mini splits move heat, they don't create it. This is why they're 2.5–4x more efficient than electric heaters. A space heater converts 1 watt of electricity into 1 watt of heat (100% efficient). A mini split uses 1 watt of electricity to move 2.5–4 watts of heat energy from outdoor air into your room (250–400% effective efficiency). The outdoor air is the "free" heat source.
Inverter Technology: Why Mini Splits Are So Efficient
The technology that makes modern mini splits dramatically more efficient than older AC systems is the inverter-driven compressor.
Old technology (fixed-speed): The compressor runs at 100% power or turns completely off. The room temperature swings 3–5°F between cycles. Every startup draws a surge of electricity. Constant on/off cycling wastes energy and stresses components.
Modern technology (inverter): The compressor speed adjusts continuously from about 10% to 100% capacity. Once your room reaches the set temperature, the compressor slows to a gentle cruise — using just enough energy to maintain temperature. The result: ±0.5–1°F temperature accuracy, 30–50% less electricity, quieter operation, and longer component life.
Every reputable mini split sold today uses inverter technology. It's the single biggest reason mini splits are so much more efficient than window ACs and older central air systems.
Single-Zone vs. Multi-Zone: Which Do You Need?
Single-zone: One outdoor unit + one indoor unit. Conditions one room or open area. Best for: adding AC to a specific room, garage, bonus room, addition, or bedroom. Cost: $1,500–$4,500 installed.
Multi-zone: One outdoor unit + 2–8 indoor units, each in a different room. Each zone has independent temperature control. Best for: conditioning multiple rooms, whole-home replacement, or anywhere you want different temperatures in different rooms. Cost: $4,000–$18,000 installed.
| Factor | Single-Zone | Multi-Zone |
|---|---|---|
| Rooms covered | 1 | 2–8 |
| Best efficiency | Higher (1:1 match) | Slightly lower (compressor oversized for partial loads) |
| Cost per zone | $1,500–$4,500 | $2,000–$3,500 |
| Outdoor units | 1 per room | 1 for all rooms |
| Temperature control | Independent | Independent per zone |
| If outdoor unit fails | 1 room loses AC | All rooms lose AC |
The efficiency trade-off: Single-zone units are 5–15% more efficient than multi-zone systems because the compressor is perfectly matched to one indoor unit. In a multi-zone system, the compressor is sized for the total load but often runs at partial capacity when only 1–2 zones are active, which can reduce dehumidification and efficiency. If budget and outdoor space allow, separate single-zone units for each room provide the best performance.
Indoor Unit Styles
| Style | Visibility | Best For | Cost Premium |
|---|---|---|---|
| Wall-mount | Moderate (mounted high on wall) | Most residential rooms | Base price |
| Ceiling cassette | Low (flush grille in ceiling) | Open plans, offices | +30–50% |
| Floor-standing | Moderate (sits on floor like a radiator) | Attic rooms, elderly access | +10–20% |
| Slim duct (concealed) | Minimal (hidden above ceiling) | Whole-home, aesthetics priority | +20–40% |
Wall-mount is the default choice for 80%+ of residential installations. Choose other styles only if you have a specific need: ceiling cassettes for open-plan offices, slim duct for aesthetics-conscious whole-home installations, or floor-standing for rooms with limited wall space.
Who Should Consider a Mini Split?
Mini splits make the most sense for:
You own a home without ductwork and want efficient, permanent cooling and heating. Installing ductwork costs $5,000–$15,000 — a mini split does the job for $1,500–$4,500 per zone without tearing open walls and ceilings.
You have a room that's always too hot or too cold that your central AC can't fix. Bonus rooms over garages, finished basements, sunrooms, and room additions are classic mini split candidates.
You want to eliminate window ACs. Mini splits are quieter, more efficient, more secure (no open window), and don't block your view. They also provide heating, which window units don't.
You're building a new addition, converting a garage, or finishing a basement. Running ductwork to these spaces is expensive and often impractical. A mini split provides a complete HVAC solution with minimal construction.
You want to reduce energy bills. If you currently heat with oil, propane, or electric baseboard, switching to a mini split heat pump can cut heating costs by 40–60%.
You want zone control. If different family members prefer different temperatures, or if you want to avoid heating/cooling empty rooms, multi-zone mini splits give each room its own thermostat.
Key Takeaways
- A mini split is a ductless AC/heating system with an outdoor compressor and one or more indoor air handlers
- No ductwork required — just a 3-inch hole through the wall connects the two units
- Most mini splits are heat pumps that both cool and heat (down to -13°F or lower with cold-climate models)
- Inverter technology makes them 30–50% more efficient than traditional AC systems
- Indoor noise is 19–30 dB — quieter than a whispered conversation
- Typical cost: $1,500–$4,500 for a single zone installed, $4,000–$18,000 for multi-zone
- Best candidates: homes without ducts, single-room additions, rooms your central AC can't handle, replacing window units