How Your Central A/C System Works

• 1. Thermostat — you set the temperature, and it tells the system when to cool
• 2. Return vent — pulls warm air from your home back to the air handler
• 3. Air handler / furnace — blows air across the evaporator coil and circulates cooled air through your home
• 4. Evaporator coil — absorbs heat from your home's air; refrigerant inside evaporates and carries that heat outside
• 5. Supply vents — deliver the cooled air back into your rooms
• 6. Refrigerant lines — carry refrigerant between the indoor and outdoor units
• 7. Condenser unit — releases heat outside and cools the refrigerant back into a liquid
• 8. Condensate drain — removes the moisture pulled out of the air as it cools

The thermostat closes a circuit calling for cooling. The blower in the air handler starts, pulling warm indoor air through the return grille and filter, then pushing it across the cold evaporator coil.

Inside that coil, low-pressure liquid refrigerant boils as it absorbs heat from your air. The now-cool, drier air travels through the ducts to your supply vents, while the refrigerant — now a warm vapor — travels through the suction line to the outdoor unit.

In the condenser, the compressor raises the refrigerant's pressure and temperature so it is hotter than the outdoor air. The condenser fan pushes air across the outdoor coil, the refrigerant sheds its heat and turns back into a liquid, and the cycle repeats. All summer, your air conditioner is essentially a heat pump moving indoor heat outdoors.

The coil surface sits below the dew point of your indoor air, so water condenses on it exactly the way it beads on a cold glass. That water drips into a pan and exits through the condensate drain.

In McAllen's humidity, that can mean gallons a day. This is why dehumidification matters as much as temperature here, and why drain problems are one of the most common causes of a system shutting itself off.

• Thermostat: blank screen, readings that disagree with the room, system that will not start or stop
• Return vent or filter: weak airflow at every vent, coil freezing
• Air handler blower: no air movement, squealing, or reduced airflow throughout
• Evaporator coil: ice buildup, musty odors, refrigerant leaks
• Supply vents and ducts: some rooms cool while others stay warm
• Refrigerant lines: oily residue, hissing, frost on the suction line
• Condenser: fan not spinning, humming with no start, high pressures, breaker trips
• Condensate drain: water around the air handler, ceiling stains, system shutting off and restarting later

An air conditioner can only reject heat if the outdoor air is cooler than the refrigerant in the condenser coil. On a 100-degree Valley afternoon, that margin narrows, so anything that reduces heat rejection — a dirty coil, restricted airflow, a struggling fan motor — has an outsized effect right when you need cooling most.

The same is true indoors. Attic ductwork surrounded by very hot summer air makes leaks costly, and high humidity means the coil and drain are working constantly. Understanding the loop makes it obvious why maintenance focuses on airflow, clean coils, correct charge, and clear drains.