The Cooling Cycle, Step by Step
1. The thermostat calls for cooling
The thermostat compares room temperature to your setpoint. When the room is warmer, it closes a low-voltage circuit that starts the indoor blower and the outdoor unit. A thermostat in direct sun, near a lamp or above a supply register reads the wrong temperature and produces odd cycling.
2. Return air enters the system
Warm indoor air is pulled back through return grilles and ducts, passing through the filter. This is where airflow problems begin: a loaded filter, undersized returns or blocked grilles starve everything downstream.
3. The evaporator coil absorbs heat
Inside the air handler, air crosses the cold evaporator coil. Refrigerant inside the coil evaporates as it absorbs heat, and moisture condenses out of the airstream onto the coil surface and drains away.
4. Cooled air is distributed
The blower pushes the cooled, dehumidified air through supply ducts to registers in each room. Leaky or poorly balanced ductwork shows up here as rooms that never match the thermostat.
5. Refrigerant carries heat outdoors
The insulated suction line carries cool refrigerant vapor to the outdoor unit, where the compressor raises its pressure and temperature so the condenser coil can release that heat to outdoor air.
6. The cycle repeats
High-pressure liquid returns through the smaller line to the metering device, pressure drops, and the refrigerant is cold again as it enters the evaporator. The loop continues until the thermostat is satisfied.