What Are Mini-Splits?
Understanding ductless HVAC technology from an expert perspective
What exactly is a mini-split system?
A mini-split is a ductless heating and cooling system consisting of an outdoor condensing unit connected to one or more indoor air handlers via refrigerant lines. Unlike central air systems that rely on ductwork to distribute air, mini-splits deliver conditioned air directly into each zone. They're called "mini" because they're smaller than commercial package units and "split" because the system is divided between indoor and outdoor components.
The Fundamentals
After 15+ years at an HVAC distributor, I've specified thousands of these systems. Let me explain what they actually are without the marketing fluff.
Mini-splits are heat pumps—they move heat from one place to another using refrigeration technology. In cooling mode, they extract heat from inside your home and dump it outside. In heating mode, they do the reverse, extracting heat from outdoor air (yes, even cold air contains heat energy) and moving it inside.
The Two Main Components
Outdoor Condensing Unit
The outdoor unit houses the compressor (the heart of the system), a heat exchanger coil, a fan, and expansion valve. This is where the heavy lifting happens—the compressor pressurizes refrigerant to move heat.
- Compressor: Variable-speed inverter-driven (this is what makes them efficient)
- Outdoor coil: Absorbs heat in winter, rejects heat in summer
- Fan: Moves air across the coil for heat exchange
- Defrost system: Prevents ice buildup during heating mode
- Electronics: Inverter board, sensors, control modules
Indoor Air Handler(s)
Indoor units contain an evaporator coil, a fan, filters, and electronics. They're responsible for distributing conditioned air into your space. Unlike a furnace with ductwork, each indoor unit directly conditions the room it's installed in.
- Evaporator coil: Heat exchanger where refrigerant absorbs or releases heat
- Blower fan: Circulates room air across the coil
- Filtration: Washable filters (some models have advanced filtration)
- Condensate pan & drain: Removes moisture during cooling
- Remote sensor: Monitors room temperature and conditions
What Connects Them?
The indoor and outdoor units are connected by refrigerant lines, electrical wiring, and a condensate drain. This is the "line set" we run through walls:
Refrigerant Lines (Liquid & Suction)
Two copper pipes—one small (liquid line) and one larger (suction line)—carry refrigerant between units. These must be properly sized, insulated, and brazed. The liquid line carries high-pressure liquid refrigerant, while the suction line returns low-pressure vapor to the compressor. Maximum line lengths vary by model (typically 50-165 feet), and total elevation differences are limited (usually 15-50 feet).
Communication Wire
Low-voltage control wiring (typically 18/4 or 18/2) connects the indoor and outdoor units for communication. This is how the indoor unit tells the outdoor unit what to do. It's not a thermostat wire—the thermostat (remote control) communicates wirelessly or via infrared with the indoor unit.
Condensate Drain
Indoor units produce condensation during cooling (just like any AC system). This moisture must be drained away via gravity drain or condensate pump. In heating mode, the outdoor unit produces condensation and must also have proper drainage.
Power Supply
The outdoor unit requires dedicated electrical service (typically 208-230V). Indoor units are powered through the line set communication cable from the outdoor unit—they don't need separate electrical connections in most residential applications.
Single-Zone vs Multi-Zone Systems
Single-Zone (1:1 System)
One outdoor unit connected to one indoor unit. Simpler, more efficient for conditioning a single space. The outdoor unit is sized specifically for that one indoor unit—no compromises.
- Single room additions
- Garages or workshops
- Bonus rooms or in-law suites
- Maximum efficiency in one zone
Multi-Zone (1:2, 1:3, 1:4, 1:5, etc.)
One outdoor unit connected to multiple indoor units (2-8 zones depending on model). Each indoor unit can be controlled independently. The outdoor unit modulates capacity across all connected zones based on demand.
- Whole-home or multiple room applications
- Independent zone control
- Situations where outdoor space is limited
- Keeping equipment consolidated
Important: Total connected indoor capacity should be 100-130% of outdoor unit capacity for proper operation. Going too small or too large affects performance and longevity.
How Mini-Splits Actually Work
Here's the refrigeration cycle explained at a tradesman level:
Compression
The compressor takes low-pressure refrigerant vapor and compresses it into high-pressure, high-temperature vapor. This is where energy is added to the system. Inverter-driven compressors can vary their speed (unlike old on/off compressors), which is why mini-splits are so efficient.
Condensation
High-pressure vapor flows to the condensing coil (outdoor in cooling mode, indoor in heating mode). As air moves across the coil, the refrigerant releases heat and condenses into a high-pressure liquid. This is where heat is rejected.
Expansion
The high-pressure liquid passes through an expansion device (TXV or EEV), which drops the pressure dramatically. This causes the refrigerant temperature to plummet—it's now a cold, low-pressure mixture of liquid and vapor.
Evaporation
The cold refrigerant enters the evaporator coil (indoor in cooling mode, outdoor in heating mode). As air moves across the coil, heat is absorbed from the air, and the refrigerant completely evaporates into a low-pressure vapor. This vapor returns to the compressor, and the cycle repeats.
The Reversing Valve
Mini-splits can heat and cool because of a reversing valve in the outdoor unit. This valve changes the direction of refrigerant flow, swapping which coil acts as the evaporator and which acts as the condenser. It's the same cycle, just reversed—that's why they're called "heat pumps."
Mini-Splits vs Other HVAC Systems
vs Central Air Systems
vs Window Units
vs Portable Units
Key Advantages of Mini-Split Technology
Inverter-Driven Efficiency
This is the game-changer. Old AC systems run at 100% or 0%—they cycle on and off. Inverter compressors vary speed from ~20% to 100%+ capacity. They ramp up to reach temperature quickly, then maintain it at low speed. Less cycling = better efficiency, better comfort, longer equipment life.
Zone Control
Each indoor unit operates independently. Living room at 68°F, bedroom at 72°F, office at 65°F—all simultaneously. You're not wasting energy conditioning unoccupied spaces. With central air, it's all or nothing.
Installation Flexibility
No ductwork means you can condition spaces that would be prohibitively expensive with ducted systems. Old homes, additions, garages, bonus rooms—mini-splits work where traditional systems can't.
Cold Climate Performance
Modern mini-splits (hyper-heat or cold climate models) maintain full heating capacity down to +5°F and continue operating to -22°F or lower. Compare that to older heat pumps that needed backup heat below 40°F.
Quiet Operation
Indoor units: 19-35 dB (library quiet). Outdoor units: 45-58 dB. Compare that to window units (55+ dB) or central air handlers (40-50 dB) and duct noise. You won't hear a properly installed mini-split.
No Duct Losses
Typical duct systems lose 20-30% of energy through leaks, poor insulation, and conductive losses. Mini-splits deliver conditioned air directly to the space—no losses. This is huge for efficiency.
Real-World Applications
After specifying thousands of these systems, here's where they excel:
Homes Without Ductwork
Older homes, homes with radiators or baseboard heat, homes where adding ducts would be cost-prohibitive. Mini-splits are the obvious choice.
Additions & Renovations
Finished basements, converted garages, bonus rooms, in-law suites. Extending ductwork is expensive and often impossible. A mini-split solves it with minimal construction.
Hot & Cold Spots
That bedroom that's always too hot, the bonus room that's freezing—spots your central system can't handle. Add a mini-split zone and solve it permanently.
Energy-Efficient Retrofits
Replacing old electric baseboard, oil heat, or propane with a heat pump mini-split can cut heating costs by 50-70%. The efficiency gains are massive.
Supplemental Zones
Keep your central air for most of the house, add a mini-split to problem areas. Best of both worlds—especially for master suites or home offices.
Commercial Applications
Server rooms (precise temperature control), retail spaces (independent zone control), offices (tenant control), restaurants (kitchens need different conditions than dining areas).
Important Limitations & Considerations
Let's be honest about where mini-splits aren't perfect:
Visual Impact
Wall-mounted units are visible. Some people don't like the look. Options: ceiling cassettes, floor-mounted units, or concealed ducted units—but they cost more. There's no getting around it: you'll see the equipment.
Upfront Cost
Quality equipment with professional installation isn't cheap. Budget $3,000-$5,000+ for a single-zone system, $6,000-$15,000+ for multi-zone. But operating costs are low, and payback is real—especially if you're replacing expensive heating fuel.
Installation Quality Matters
A bad installation will destroy performance. Refrigerant lines must be properly sized, brazed, evacuated, and charged. Condensate drains must slope correctly. Indoor units must be level. Hire experienced contractors—this isn't DIY unless you're an HVAC tech.
Maintenance Requirements
Indoor filters need monthly cleaning (simple—just vacuum or rinse). Annual professional service recommended. Outdoor coils should be kept clean. Neglect maintenance and efficiency tanks.
Extreme Cold Performance
Even cold-climate models lose capacity below 5°F. If you're in northern Minnesota or Alaska, you might need backup heat for the coldest days. Not a deal-breaker, but plan for it.
Brand & Model Selection
Not all mini-splits are equal. Cheap off-brand units from big box stores aren't the same as Mitsubishi, Fujitsu, or LG systems. Pay attention to SEER/HSPF ratings, cold climate performance specs, and warranty terms. See our brand comparison guide.
Types of Indoor Units
Mini-splits aren't just wall units—there are multiple indoor unit styles for different applications:
Wall-Mounted
Most common. Mounted high on the wall (like a painting). Efficient air distribution, easy installation, lowest cost. Learn more about all indoor unit types.
Ceiling Cassette
Flush-mounted in the ceiling. 4-way airflow for even distribution. Great for commercial or open-concept spaces. Requires ceiling access and plenum space.
Floor-Mounted
Mounted low on the wall (like a radiator). Good for rooms with limited wall space or where high mounting isn't possible. Common in commercial settings.
Concealed Ducted
Hidden in the ceiling or closet with short duct runs to grilles. Invisible installation. Can serve multiple rooms from one indoor unit. Higher cost, requires ceiling/attic space.
Climate Considerations
Not all mini-splits are created equal when it comes to climate performance:
Cold Climates (Northern States, Canada)
Look for "hyper-heat," "cold climate," or "H2i" models. These maintain full capacity down to +5°F and operate to -22°F or lower. Check HSPF ratings (10+ is good, 12+ is excellent). Brands like Fujitsu and Mitsubishi excel here.
Moderate Climates (Mid-Atlantic, Pacific Northwest)
Standard mini-splits work great. Focus on year-round efficiency—balance SEER (cooling) and HSPF (heating). These climates benefit most from mini-split efficiency.
Hot Climates (Southern States, Southwest)
Prioritize high SEER ratings (20+). Cooling performance is critical. Some models are optimized for high ambient temperatures (operating to 115°F+ outdoor temp). Ensure adequate outdoor unit shading without blocking airflow.
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