How Central Air Conditioning Works

An air conditioner does not manufacture cold air. It moves heat from inside your home to outside it, using a refrigerant that changes state from liquid to vapor and back again. Four components do the work.

Compressor

The pump at the heart of the system. It raises the pressure and temperature of refrigerant vapor so that heat will flow out of it into the summer air.

Condenser

The outdoor coil. Hot vapor gives up its heat to the outside air and condenses into a liquid. A dirty condenser cannot shed heat, which is why cleaning it is the single most effective outdoor maintenance task.

Expansion valve

A metering device that drops the pressure of the liquid refrigerant, cooling it sharply before it enters the indoor coil.

Evaporator

The indoor coil. Cold refrigerant absorbs heat from the air your blower pushes across it, and the refrigerant boils back into vapor to start the loop again. Moisture condenses here too — a working system dehumidifies as it cools.

Every part of the cycle depends on air actually moving across the coils. Restrict that airflow and heat transfer collapses: the evaporator gets too cold, condensation turns to ice, and the system runs endlessly while cooling less.

• A clogged filter is the most common cause of poor airflow in a residential system.
• Closed or blocked registers raise static pressure and hurt the whole house, not one room.
• Leaky return ducts pull hot attic air into the system before it ever reaches the coil.

SEER2 is the current seasonal efficiency rating for cooling equipment, measured under testing conditions that better reflect real duct systems. A higher SEER2 unit removes the same heat using less electricity — but the payback depends on how many hours it runs.

• Restricted filter or blocked returns starving the blower.
• Condenser coil packed with grass clippings, pollen, or dryer lint.
• Frozen evaporator coil — turn the system to fan-only and let it thaw before diagnosing.
• Low refrigerant charge, which always means a leak, never normal consumption.
• Failing capacitor or contactor preventing the compressor from starting reliably.
• Duct leakage delivering conditioned air into an attic or crawlspace.

• Setting the thermostat to 65°F to cool faster — the system has one speed of cooling.
• Pressure-washing condenser fins, which bends them and reduces heat transfer permanently.
• Skipping condensate maintenance until a ceiling stain appears.
• Installing a filter far denser than the system was designed for.
• Ignoring humidity and blaming the temperature.