A mini split works by circulating refrigerant between an outdoor compressor and an indoor air handler through copper tubing, absorbing heat from indoor air and releasing it outside (cooling mode) or absorbing heat from outdoor air and releasing it inside (heating mode). The inverter-driven compressor modulates its speed continuously to match the exact cooling or heating load, maintaining ±0.5°F temperature accuracy while using 30–50% less electricity than fixed-speed systems.
Think of a mini split as a heat ferry. It doesn't create cold air or hot air — it moves heat energy from one location to another. In summer, it ferries heat from inside your house to outside. In winter, it reverses direction and ferries heat from outside air into your home, even when it's below freezing.
The 7 Steps of the Cooling Cycle
Here's exactly what happens inside your mini split during cooling mode, step by step:
Step 1: Warm Air Enters the Indoor Unit
The indoor unit's fan draws warm room air (let's say 78°F) through the return air grille and across a set of washable filters. The filters catch dust, pet hair, and other particles before the air reaches the evaporator coil. Clogged filters are the #1 cause of reduced performance — clean them every 2–4 weeks.
Step 2: Refrigerant Absorbs Heat in the Evaporator
The filtered air passes over the evaporator coil, which contains cold, low-pressure liquid refrigerant (typically R-410A, at about 40–45°F). Heat energy transfers from the warm air to the cold refrigerant. The air is cooled by 15–22°F (the "temperature differential" or "delta-T"). So your 78°F room air exits the coil at roughly 56–63°F.
As the refrigerant absorbs heat, it changes from a liquid to a gas (evaporates). This phase change is critical — it allows the refrigerant to absorb a large amount of heat energy (called "latent heat of vaporization").
Step 3: Moisture Condenses on the Coil
As the warm, humid air hits the cold evaporator coil, water vapor in the air condenses into liquid water on the coil surface — the same way water droplets form on a cold glass in summer. This is how your mini split dehumidifies. The condensed water drips into a drain pan and flows out through the condensate drain line. A well-functioning 12,000 BTU mini split removes 1–3 pints of moisture per hour.
Step 4: Cooled Air Returns to the Room
The fan blows the now-cooled, dehumidified air back into the room through adjustable louvers that direct airflow horizontally, vertically, or in a sweeping pattern. The air exits the indoor unit at 56–63°F, mixes with room air, and gradually lowers the overall room temperature toward your thermostat setpoint.
Step 5: Refrigerant Travels to the Outdoor Compressor
The low-pressure refrigerant gas travels through the suction line (the larger of the two copper tubes) to the outdoor unit's compressor. The suction line is insulated to prevent heat gain during this journey.
Step 6: The Compressor Pressurizes the Refrigerant
This is where the magic happens. The inverter-driven compressor squeezes the low-pressure gas into a high-pressure, high-temperature gas (approximately 120–170°F). Compressing the gas concentrates the heat energy it absorbed from your room, raising its temperature well above outdoor air temperature.
The compressor is the most energy-intensive component, accounting for 80–90% of the mini split's electricity consumption. Inverter technology allows the compressor to vary its speed from about 10% to 100% of maximum capacity, matching the exact cooling load at any moment.
Step 7: Heat Releases Through the Outdoor Condenser
The hot, high-pressure refrigerant gas flows through the outdoor condenser coil while a fan pushes outdoor air across it. Because the refrigerant (120–170°F) is much hotter than the outdoor air (even on a 100°F day), heat transfers from the refrigerant to the outdoor air and dissipates.
As the refrigerant releases heat, it condenses back into a high-pressure liquid. This liquid flows through the liquid line (the smaller copper tube) back to the indoor unit, passes through an expansion valve that drops its pressure and temperature, and enters the evaporator coil again at 40–45°F. The cycle repeats continuously.
The Complete Cycle at a Glance
| Step | Location | Refrigerant State | Temperature | What Happens |
|---|---|---|---|---|
| 1 | Indoor unit | — | Room temp (78°F) | Warm air drawn across coil |
| 2 | Evaporator coil | Liquid → Gas | 40–45°F | Absorbs heat from air |
| 3 | Evaporator coil | — | — | Moisture condenses (dehumidification) |
| 4 | Indoor unit | — | 56–63°F | Cooled air returns to room |
| 5 | Suction line | Low-pressure gas | ~55°F | Gas travels to compressor |
| 6 | Compressor | Low-P gas → High-P gas | 120–170°F | Gas compressed, temperature rises |
| 7 | Condenser coil | High-P gas → Liquid | Outdoor temp | Heat released outside |
How Heating Mode Works (Reverse Cycle)
In heating mode, a component called the reversing valve (also called a 4-way valve) switches the direction of refrigerant flow. This reversal turns the outdoor coil into the evaporator (it absorbs heat from outside air) and the indoor coil into the condenser (it releases heat into your room).
| Component | Cooling Mode Function | Heating Mode Function |
|---|---|---|
| Indoor coil | Evaporator (absorbs heat from room) | Condenser (releases heat into room) |
| Outdoor coil | Condenser (releases heat outside) | Evaporator (absorbs heat from outside air) |
| Expansion valve | Before indoor coil | Before outdoor coil |
| Airflow direction | Hot air out at outdoor unit | Hot air out at indoor unit |
The fundamental question: How can a heat pump extract heat from cold outdoor air?
Even at 0°F, outdoor air contains significant heat energy. Remember that absolute zero (the complete absence of heat) is -459.67°F. Air at 0°F still has substantial thermal energy relative to absolute zero. The refrigerant in the outdoor coil is colder than the outdoor air (thanks to the expansion valve dropping its temperature to -20°F to -30°F), so heat naturally flows from the "warmer" outdoor air into the "colder" refrigerant.
As outdoor temperature drops, the temperature differential between the refrigerant and outdoor air shrinks, making heat transfer less efficient. That's why standard mini splits lose heating capacity in cold weather, and why cold-climate models use Enhanced Vapor Injection (EVI) and oversized coils to maintain performance.
Why heat pumps are so efficient: A 1,500-watt electric space heater converts 1,500 watts of electricity into 1,500 watts (5,118 BTU) of heat — 100% efficient but nothing more. A mini split heat pump uses 500 watts of electricity to power the compressor, which moves 1,500–2,000 watts of heat energy from outdoor air into your room. Total heat delivered: 2,000–2,500 watts. That's 400–500% effective efficiency — you get 4–5x more heat energy than you pay for in electricity.
How Inverter Technology Controls the System
The inverter drive is the brain of the compressor motor. Here's how it manages the system:
Initial cooldown (100% power): You set the thermostat to 72°F. The room is currently 82°F. The inverter ramps the compressor to maximum speed, drawing peak watts (800–1,200W for a 12K unit). The room cools rapidly.
Approaching setpoint (50–70% power): As the room reaches 74–75°F, the inverter reduces compressor speed. Power draw drops to 400–700W. Cooling output matches the room's heat gain.
At setpoint (10–30% power): Once the room hits 72°F, the inverter throttles the compressor to minimum speed. Power draw drops to 100–300W. The compressor maintains a gentle, continuous cooling that exactly offsets heat entering the room through walls, windows, and occupants.
Temperature maintained (modulating): The compressor continuously adjusts between 10% and 100% capacity based on changing conditions — a cloud passes and sun exposure drops, someone opens a door, the oven turns on. Each change triggers a smooth compressor adjustment within seconds.
| Operating Phase | Compressor Load | Power Draw (12K) | Duration |
|---|---|---|---|
| Initial cooldown | 80–100% | 800–1,200W | 15–45 min |
| Approaching setpoint | 50–70% | 400–700W | 15–30 min |
| Maintaining temperature | 10–30% | 100–300W | Continuous |
| Responding to load change | 30–80% | 200–900W | 5–15 min |
Defrost Mode: What Happens in Winter
During heating operation at outdoor temperatures between 20–40°F (especially with high humidity), frost accumulates on the outdoor coil. This frost insulates the coil and reduces its ability to absorb heat. The mini split handles this automatically through defrost cycles.
Defrost process:
- Sensors detect frost buildup (coil temperature, timing, or both)
- The reversing valve temporarily switches to cooling mode
- Hot refrigerant flows through the outdoor coil, melting the frost
- The indoor fan may slow or stop (to avoid blowing cold air into the room)
- Water drains from the base pan (heated by the base pan heater in cold-climate models)
- Defrost completes in 2–10 minutes
- The reversing valve switches back to heating mode
You may notice warm water vapor or steam rising from the outdoor unit during defrost — this is normal. You may also notice a brief pause in heating indoors. Cold-climate mini splits minimize defrost interruptions through smarter defrost algorithms and larger coils that resist frost buildup.
Dry Mode: Dehumidification Without Overcooling
Most mini splits include a "dry mode" that prioritizes moisture removal over cooling. In dry mode, the compressor runs at minimum speed and the fan runs at its lowest setting. This keeps the evaporator coil cold enough to condense moisture, but limits the volume of air processed so the room doesn't overcool.
Dry mode is ideal for humid days when the temperature is comfortable but the air feels sticky — common in spring and fall in humid climates. It uses 40–60% less electricity than cooling mode.
| Mode | Compressor Speed | Fan Speed | Primary Purpose | Power Draw |
|---|---|---|---|---|
| Cool | Variable (10–100%) | Variable | Temperature reduction | 100% |
| Dry | Minimum (10–20%) | Minimum | Moisture removal | 40–60% |
| Fan only | Off | Variable | Air circulation | 5–10% |
| Heat | Variable (10–100%) | Variable | Temperature increase | 110–130% |
| Auto | Variable | Variable | Maintain setpoint | Varies |
Key Takeaways
- Mini splits move heat using the refrigeration cycle — they don't create cold or hot air
- Cooling: refrigerant absorbs heat from indoor air and releases it outdoors through the compressor/condenser cycle
- Heating: the cycle reverses — refrigerant absorbs heat from outdoor air and releases it indoors
- Inverter technology modulates compressor speed continuously (10–100%), maintaining ±0.5°F accuracy while using 30–50% less electricity than fixed-speed systems
- Dehumidification happens automatically during cooling as moisture condenses on the cold evaporator coil
- Defrost cycles are normal during winter operation — the system briefly reverses to melt frost from the outdoor coil
- Heat pumps are 250–400% efficient because they move heat rather than creating it