A mini-split (also called a ductless mini-split) is an air conditioner, or, more commonly now, a heat pump, that conditions individual rooms without using ductwork. It’s the same refrigerant cycle as central AC, but instead of one big indoor coil in an air handler pushing air through ducts, the indoor coil sits in a wall-mounted head in the room you want to cool.
Here’s how the whole system actually works, in installer-level detail without the marketing fluff.
The 60-second version
A mini-split has three main parts:
- One outdoor unit containing the compressor and outdoor coil
- One or more indoor heads mounted on walls (sometimes ceilings or floors), each containing an indoor coil and a fan
- Refrigerant line set, small insulated copper pipes running between outdoor unit and each indoor head
When you turn the system on, the outdoor compressor circulates refrigerant through the line set to whichever indoor head is calling for cooling. The indoor coil gets cold, the fan blows room air across it, and cold air enters the room. Heat absorbed from your room is carried back to the outdoor unit and released to the outside air.
No ductwork. No central air handler. The indoor head IS the air handler for that room.
Why “mini” and why “split”
Mini refers to the size of the indoor unit compared to a traditional central air handler. Most mini-split indoor heads are 30-40 inches wide and 12 inches tall, small enough to mount unobtrusively on a wall.
Split refers to the fact that the system is divided between an outdoor compressor/condenser unit and a separate indoor air-handling unit, connected by refrigerant lines. (Standard central AC is also technically a “split system”, outdoor condenser plus indoor air handler, but “mini-split” specifically refers to the ductless version.)
How the cooling cycle works
Same refrigerant cycle as any other air conditioner:
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Cold liquid refrigerant flows through the indoor coil. The fan in the indoor head blows warm room air over the cold coil. The refrigerant absorbs heat from the air and turns into gas. The cooled air enters the room.
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The gas refrigerant flows through the line set back to the outdoor unit.
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The outdoor compressor squeezes the gas, which makes it very hot.
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The hot gas flows through the outdoor coil. The outdoor fan blows ambient air across the coil. The refrigerant releases its heat to the outdoor air and condenses back into liquid.
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The liquid flows back to the indoor coil through the line set, and the cycle starts again.
The visual difference from central AC: instead of one huge indoor coil in an air handler in a closet or attic that connects to ducts running all over the house, you have one small coil in a wall-mounted head in each room. Same refrigerant, same cycle, different distribution.
How heat pump mini-splits work in heating mode
Most modern mini-splits sold in 2026 are heat pumps, they can run the refrigerant cycle in either direction. In heating mode, a reversing valve flips the flow:
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Cold liquid refrigerant flows through the OUTDOOR coil. Even cold outdoor air has enough heat energy that the refrigerant absorbs it and turns into gas.
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The gas flows to the compressor, which squeezes it hot.
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The hot gas flows through the INDOOR coil. The indoor fan blows room air over the now-hot coil. Heat transfers from the refrigerant to your room air. Your room warms up.
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The refrigerant condenses back to liquid, returns to the outdoor coil, and the cycle continues.
This is the same refrigerant cycle as in cooling, just running backwards. The reversing valve is the single mechanical part that makes a heat pump different from a cooling-only AC.
Single-zone vs multi-zone systems
Single-zone mini-split: one outdoor unit serving one indoor head. Common for cooling a master bedroom, living room, sunroom, or ADU. Cost in San Diego: $4,500-$7,000 installed.
Multi-zone mini-split: one larger outdoor unit serving 2-8 indoor heads. Each indoor head has its own thermostat and runs independently. Common for cooling 2-4 rooms of a home, especially homes without existing ductwork. Cost: $7,500-$22,000 installed depending on zone count.
The multi-zone outdoor unit is bigger and more expensive, but it’s still one outdoor unit instead of multiple smaller ones, which saves on yard space and electrical work.
Inverter compressors, why mini-splits are different
One technical detail that matters: almost every modern mini-split uses an inverter compressor. A traditional AC compressor runs at full speed when it’s on and off when it’s not, like a light switch. An inverter compressor modulates speed continuously from about 25% to 110% of rated capacity.
The practical effect: mini-splits maintain temperature more precisely (they ramp up and down instead of cycling on/off), they run quieter (mostly at low speed), and they’re more efficient because they avoid the energy waste of constant startup cycles.
This is why mini-split efficiency ratings (SEER2 18-25) are typically higher than central AC ratings (SEER2 14-18). The inverter is the technology gap, not the ductless configuration alone.
Indoor head types
Most San Diego mini-split installs use wall-mounted heads, the rectangular boxes you’ve seen on people’s walls. But there are other options:
- Wall-mount. Most common. Easy install, lowest cost, visible.
- Ceiling cassette. Recessed into the ceiling with a flat grille. Cleaner look but harder install. Adds $400-$800 per head.
- Concealed duct (slim-duct). Hidden in a ceiling cavity, connected to a small section of ductwork for the room. Looks like central AC; costs more.
- Floor console. Mounted near the floor, like a baseboard heater. Useful for older homes where high wall mounting doesn’t fit the aesthetic.
Wall-mount is the default for cost reasons. The other options work for specific design situations.
What you can’t do with a mini-split
Three things to know:
1. You can’t cool one zone while heating another with most residential systems. The system is in heating mode OR cooling mode at any moment. Commercial variable refrigerant flow (VRF) systems can do simultaneous heating and cooling but they’re expensive and rare in residential.
2. You can’t run multiple heads on totally independent schedules from one outdoor unit. All heads share the same compressor. They can have different temperatures and turn on/off independently, but they can’t all be heating while different ones are cooling.
3. You can’t easily heat or cool a huge house with one mini-split. Each indoor head covers roughly 400-800 sqft. A 3,000 sqft house needs 4-6 heads or a different system. Past 4 heads, central HVAC usually becomes more cost-effective.
How mini-splits compare to central AC
Covered in more depth in our ductless vs ducted HVAC guide, but the short version:
Mini-split wins when: you don’t have existing ductwork, you only want to cool specific rooms, you have very different room needs, you want maximum efficiency, you have an ADU or addition.
Central AC wins when: you have existing ductwork in decent shape, you want consistent whole-house cooling, you’re cooling more than 2,500 sqft, you want a single thermostat.
San Diego-specific mini-split notes
Three things particular to our market:
1. They’re the obvious answer for pre-1970 homes. A huge percentage of San Diego’s older housing stock (Coronado, North Park, Hillcrest, Old Escondido, Lemon Grove) has no ductwork. Adding ducts is $4,000-$7,000 extra. Mini-splits avoid it entirely.
2. Coastal corrosion still applies. Mini-split outdoor units corrode in salt air just like central AC condensers. Coastal homes see 10-13 year lifespans; inland 14-18 years.
3. They qualify for the same heat pump rebates. Most mini-splits sold today are heat pumps. SDG&E TECH Clean California and federal 25C credits apply the same as for ducted heat pumps. See our 2026 heat pump rebate guide.
FAQs
How does a ductless mini-split work?
Same refrigerant cycle as central AC. An outdoor compressor circulates refrigerant through small copper lines to one or more indoor wall-mounted heads. Each head has its own coil and fan. The indoor coil absorbs heat from room air (cooling) or releases heat into room air (heating).
What is the difference between a mini-split and central AC?
Central AC uses a single large indoor air handler that distributes conditioned air through ductwork. A mini-split puts smaller indoor coils directly in each room, eliminating ductwork. Same refrigerant cycle; different distribution method.
Do mini-splits work without ducts?
Yes, that’s the entire point. Mini-splits are designed for homes without ductwork. The indoor heads serve as the air distribution points themselves.
Can a mini-split heat and cool?
Most modern mini-splits (heat pump versions) can do both. A reversing valve switches the refrigerant flow direction, allowing the same system to cool in summer and heat in winter. Cooling-only mini-splits exist but are increasingly rare in new installs.
How many rooms can a mini-split cool?
One outdoor unit can serve 1-8 indoor heads depending on the model. Single-zone systems are for one room. Multi-zone systems (2-8 zones) can cool an entire small-to-medium house. Past 4 heads, central HVAC often becomes more cost-effective.
How long does a mini-split last?
12-18 years in San Diego with normal maintenance. Coastal homes see shorter lifespans (10-13 years) due to salt-air corrosion. Inland homes see longer (14-18 years).
Are mini-splits efficient?
Yes. Most modern mini-splits run SEER2 18-25, which is higher than typical central AC ratings of 14-18. The efficiency advantage comes from inverter-driven compressors that modulate speed continuously rather than cycling on/off.
Are mini-splits noisy?
Indoor heads are very quiet, typically 25-35 decibels (whisper level). Outdoor units run 50-65 decibels at full load, comparable to central AC condensers. Inverter-driven systems run quieter at partial loads, which is most of the time.
When to call us
If you’re trying to figure out whether a mini-split fits your home and how many zones you’d need, we’ll walk through it with you. Call (442) 777-6440 for a free in-home assessment.