The Short Version
A central air conditioner doesn't create cold air — it removes heat from your home and moves it outside. The process uses a refrigerant that cycles between liquid and gas states to absorb heat indoors and release it outdoors. The whole cycle repeats continuously until your thermostat is satisfied.
Here's what happens step by step.
The Four Key Components
Evaporator coil: Located inside your home, usually above the furnace or in the air handler. This is where refrigerant absorbs heat from your indoor air.
Compressor: Located in the outdoor unit. This is the heart of the system — it pressurizes the refrigerant and drives the entire refrigeration cycle.
Condenser coil: Also in the outdoor unit, wrapped around the compressor. This is where refrigerant releases the heat it absorbed from your home.
Expansion valve (TXV or metering device): Controls the flow of refrigerant into the evaporator coil, regulating how much heat the system can absorb.
Step 1 — The Thermostat Calls for Cooling
When your home rises above the set temperature, the thermostat sends a 24-volt signal to the air conditioner's control board. The outdoor unit and indoor blower both receive the signal simultaneously.
Step 2 — The Compressor Starts
The compressor in the outdoor unit starts and begins pressurizing the refrigerant. Low-pressure refrigerant gas enters the compressor and exits as high-pressure, high-temperature gas. This is the most energy-intensive part of the cycle — the compressor accounts for the majority of your AC's electricity use.
Single-stage compressors run at one speed — fully on or fully off. Two-stage compressors have a low and high setting. Variable-capacity (inverter) compressors adjust output continuously from roughly 40% to 100%, which is why they're more efficient and provide more consistent comfort.
Step 3 — Heat Is Released Outdoors
The hot, high-pressure refrigerant gas flows from the compressor into the condenser coil. The outdoor fan pulls ambient air across the coil. Heat transfers from the refrigerant into the outdoor air, and the refrigerant cools and condenses into a high-pressure liquid.
This is why your outdoor unit blows hot air — it's exhausting the heat removed from your home. On a hot Chicago day, the outdoor unit may be exhausting air at 120°F or higher.
Step 4 — Refrigerant Expands and Cools
The high-pressure liquid refrigerant passes through the expansion valve (also called a TXV or metering device). The valve restricts flow, causing the refrigerant to drop rapidly in pressure and temperature. The refrigerant enters the evaporator coil as a cold, low-pressure mixture of liquid and gas — typically around 40°F.
Step 5 — Heat Is Absorbed Indoors
The indoor blower pulls warm return air from your home and pushes it across the evaporator coil. Heat from the air transfers into the cold refrigerant, which absorbs it and evaporates back into a gas. The now-cooled air continues through your supply ducts and into your living spaces.
Humidity is also removed in this step. As warm, humid air contacts the cold coil, moisture condenses on the coil surface and drains away through the condensate drain line. This is why your AC also dehumidifies your home — a critical function in Chicago's humid summers.
Step 6 — The Cycle Repeats
The low-pressure refrigerant gas returns to the compressor, and the cycle begins again. This continues until the thermostat is satisfied, at which point the system shuts down. The blower may continue running briefly to extract remaining cooling from the coil.
What Refrigerant Does Your AC Use?
R-22 (Freon): Phased out as of 2020. If your system uses R-22, it's at least 15 years old. Replacement refrigerant is expensive and increasingly scarce. Replacement is almost always the right call.
R-410A: The standard refrigerant from roughly 2010 through 2025. Still widely available. New equipment using R-410A is no longer manufactured after January 1, 2025 under EPA regulations.
R-454B and R-32: The new lower-GWP refrigerants used in current Ruud Endeavor™, Lennox, Carrier, and Goodman equipment. These meet current EPA and DOE environmental requirements. D's Air technicians are certified to handle both.
What Can Go Wrong
Low refrigerant: Refrigerant doesn't get consumed — if the level is low, there's a leak. Low refrigerant causes the evaporator coil to freeze, reduces cooling capacity, and can damage the compressor. A technician must find and repair the leak before recharging.
Dirty evaporator or condenser coil: Dirt insulates the coil and reduces heat transfer. A dirty evaporator coil can cause freezing. A dirty condenser coil forces the compressor to work harder and can cause overheating and early failure.
Failed capacitor: Capacitors help start and run the compressor and fan motors. A failed capacitor is one of the most common AC repairs — the system may hum but not start, or the outdoor fan may stop spinning.
Blocked condensate drain: Algae and debris can clog the drain line, causing water to back up and overflow the drain pan. This can damage ceilings, walls, and the air handler itself.
Compressor failure: The compressor is the most expensive component. Compressor failure is often caused by low refrigerant, dirty coils, or electrical issues that went unaddressed. When a compressor fails on an older system, replacement is usually more cost-effective than repair.
Why Annual Tune-Ups Matter
A spring AC tune-up catches low refrigerant, dirty coils, weak capacitors, and blocked drains before they become failures in July. D's Air performs a full system inspection on every tune-up — not just a filter swap. Call (773) 940-0965 to schedule before the summer rush.