The four stages
1. Compression — Compressor
The compressor pulls in the cool, low-pressure gas returning from the evaporator and compresses it. Squeezing the gas forces its molecules together, which raises both its pressure and its temperature dramatically — it leaves as a hot, high-pressure gas. Think of it as the heart of the system: nothing moves around the loop without it.
2. Condensation — Condenser coil
The hot gas flows through the condenser coil while a fan blows air across it. The refrigerant gives up its heat and, as it cools, condenses from a gas into a warm, high-pressure liquid. This is the step that actually delivers heat to wherever you want it.
3. Expansion — Expansion valve
The warm liquid is forced through a very small opening — the expansion valve or metering device. On the far side the pressure suddenly drops, and that drop makes the refrigerant expand and its temperature plummet. It emerges as a cold, low-pressure mix of liquid and vapor, ready to soak up heat.
4. Evaporation — Evaporator coil
A blower pushes air across the cold evaporator coil. The refrigerant absorbs heat from that air, which makes it boil and evaporate back into a low-pressure gas. Whatever air passes the coil comes out colder. The gas then heads back to the compressor and the cycle repeats.
The fans that move the air
Condenser fan — Outdoor unit
It's the big fan you see spinning on top of the outdoor unit, blowing warm air straight up and out. The more clean air it moves, the better the refrigerant condenses back into a liquid. If it slows or stops — a worn motor or a dead capacitor — the high-side pressure spikes, the compressor overheats, and the system can trip on a safety switch.
Indoor blower — Indoor unit / air handler
This is the fan you actually hear inside and feel at the vents. It draws warm room air in through the return, forces it over the cold evaporator where it's chilled and dehumidified, and sends it back to your rooms. Choke its airflow — a dirty filter, closed vents, or a tired motor — and the coil gets so cold it freezes over.
What could go wrong
Dirty air filter
Cause: The filter clogs with dust and chokes the airflow across the indoor coil. Effect: Too little warm air reaches the cold evaporator, so the refrigerant can't pick up enough heat to fully boil. The coil keeps getting colder until moisture on it freezes into ice — which blocks the airflow even more. Fix: Check the filter monthly and replace or wash it every 1–3 months. This is the single most common AC problem and the easiest to prevent.
Low refrigerant (a leak)
Cause: A leak somewhere in the sealed loop. Refrigerant is never 'used up' — if you're low, it escaped. Effect: Less refrigerant means the low-side pressure drops, so the evaporator runs colder than designed and can freeze. Meanwhile the compressor runs hot with too little gas flowing through to cool it, and can eventually burn out. Fix: A pro has to find and seal the leak, then recharge to spec. Just 'topping it off' wastes refrigerant and the leak comes back.
Dirty condenser coil
Cause: The outdoor coil gets caked with dirt, grass clippings, cottonwood, and leaves. Effect: The coil can't dump its heat to the outside air, so the high-side pressure and temperature climb. The compressor strains against that high pressure, wastes energy, overheats, and may trip its high-pressure safety switch. Fix: Gently rinse the outdoor coil with a hose (power off) and keep about 2 ft of clearance around the unit.
Dirty evaporator coil
Cause: Dust slips past a cheap or missing filter over months and coats the indoor coil. Effect: The grime acts like a blanket, insulating the coil so it can't absorb heat from your air. Cooling capacity drops and, like a dirty filter, the coil can ice over. Fix: Have a technician clean the coil — it's behind the unit's panels and easy to damage. Then use a good filter to keep it clean.
Frozen evaporator coil
Cause: Almost always a symptom of something else: low airflow (dirty filter or weak blower) or low refrigerant. Effect: Ice encases the coil and blocks the airflow completely, so cooling stops and the melt water can overflow the drain pan and leak. Fix: Switch cooling off and run just the fan to thaw it (a few hours), then fix the root cause — change the filter or call a pro for the leak.
Failing compressor
Cause: Age, an electrical fault, chronic overheating, or liquid refrigerant slugging back and damaging it. Effect: The pump that drives the entire loop weakens or stops. With no pressure difference, no heat moves anywhere. This is the most expensive part to fail. Fix: A pro must replace it — often costly enough that a whole new unit makes more sense. Prevent it by fixing leaks and dirty coils early, since those kill compressors.
Failed capacitor
Cause: Capacitors store and release the electrical jolt that starts and runs the compressor and the fan motors. Heat and age make them weaken, bulge, and fail — it's one of the single most common AC breakdowns, especially in a heat wave. Effect: Without the capacitor's boost, a motor can't get going or keeps stalling. The outdoor fan may sit dead still while the unit just hums, or the compressor strains to start, pulls huge current, and trips the breaker. Fix: It's a cheap part, but it holds a dangerous charge even with the power off. A pro discharges it safely and installs one with the exact microfarad (µF) and voltage rating.
Stuck metering device
Cause: Debris, moisture freezing inside it, or a failed sensing bulb on the expansion valve. Effect: Stuck closed/restricted, it starves the evaporator (which then freezes, just like low charge). Stuck open, it floods too much liquid through, which slugs back and can wreck the compressor. Fix: Needs a technician to diagnose and replace the valve and clear whatever fouled it.
Too much refrigerant
Cause: Someone added more refrigerant than the system is rated for — more isn't better. Effect: Excess liquid raises the high-side pressure and can flood back to the compressor. Counter-intuitively, capacity and efficiency actually drop. Fix: A pro reclaims the excess to bring the charge back to the exact spec on the unit's label.