How your air conditioner works
Your AC does not make cold air. It moves heat. It picks heat up inside your home and carries it outside, over and over, using a fluid that boils and condenses on command. Here is the whole loop, one step at a time.
The big idea: moving heat, not making cold
Inside your system is a sealed loop of refrigerant. Two simple facts make it work. When a liquid evaporates, it soaks up heat. When a gas condenses back into a liquid, it gives that heat off. And pressure decides the temperature where that happens: squeeze refrigerant and it condenses while still hot, let the pressure drop and it boils while very cold.
So the system keeps the refrigerant at low pressure inside, where it boils and absorbs heat, and at high pressure outside, where it condenses and releases it. The compressor and the expansion device are the two doors between those sides.
The cycle, step by step
Outdoor unit
Step 1: The compressor squeezes the refrigerant
Cool, low-pressure refrigerant gas comes back from inside your home. The compressor squeezes it into a small space, which makes it much hotter and puts it under high pressure. The gas leaves the compressor hotter than the outdoor air, even on a Florida afternoon. That matters, because heat only flows from hot to cooler. The compressor is the heart of the system and uses most of its electricity.
Outdoor unit
Step 2: The condenser coil dumps the heat outside
The hot gas runs through the long, finned coil that wraps around the outdoor unit. The big fan on top pulls outdoor air across those fins and blows it out the top. That warm air is heat that came out of your house. As the refrigerant gives up its heat, it condenses into a warm liquid, still under high pressure.
At the indoor coil
Step 3: The expansion device drops the pressure
The warm liquid travels back inside through the smaller copper line. Right before the indoor coil it is forced through a tiny opening, either a thermostatic expansion valve or a fixed metering device. On the other side the pressure falls sharply, some of the liquid flashes into vapor, and the mix gets very cold, often around 40 degrees.
Indoor air handler
Step 4: The evaporator coil soaks up heat from your home
The blower pushes your home's warm air across the cold evaporator coil. The refrigerant absorbs that heat and boils back into a gas, the same way sweat cools your skin as it evaporates. The air comes off the coil cooler and drier, and the ducts carry it back to your rooms. The cool gas then flows outside through the larger insulated line, and the loop starts again.
The parts of a split system
Most central AC systems in homes are split systems: one box outside, one inside, tied together by copper lines and wires.
Outdoor unit
Often called the condenser. It holds the compressor, the condenser coil and the outdoor fan. Its job is to get rid of heat.
Indoor unit (air handler)
Usually in a closet, garage or attic. It holds the evaporator coil and the blower. Many Florida air handlers also hold electric heat strips for backup heat.
Refrigerant lines
Two copper lines connect the two units: a larger insulated suction line that carries cool gas outside, and a smaller liquid line that brings refrigerant back in.
Thermostat and controls
The thermostat senses the room and calls for cooling over low-voltage wires. A contactor in the outdoor unit switches the compressor and fan on, and capacitors give the motors a push to start.
Ductwork, filters and airflow
Air comes into the air handler through the return grille, passes through the filter, crosses the cold coil and gets pushed out through the supply ducts to each room. The filter protects the blower and the coil from dust. When it clogs, too little air crosses the coil, the coil can get cold enough to freeze, and cooling drops off.
Ducts matter as much as the equipment. A leak or a crushed run in a hot attic can dump cooled air where you never feel it, which is a common reason one room stays warm. See our ductwork service page for more.
Florida humidity and the condensate drain
Your AC is also a dehumidifier. Moisture in the air condenses on the cold evaporator coil, just like water beads up on a glass of iced tea. That water drips into a drain pan and runs out through the condensate drain line, usually to the outside. On a humid day a home system can pull gallons of water out of the air.
In our climate that drain line can grow algae and sludge and clog. Many systems have a float switch that shuts cooling off when water backs up, which protects your ceiling but leaves you without AC. That is why the drain gets checked at every tune-up.
Humidity is also why bigger is not better. A system that is too large for the house cools the air fast and shuts off before it has pulled much water out, so the house feels cold and clammy. The EPA suggests keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, to help prevent mold.
Heat pumps: the same loop, run in reverse
Many Southwest Florida homes heat with a heat pump. It is the same equipment, plus a reversing valve. In heating mode the valve sends the refrigerant the other way: the outdoor coil becomes the cold side and picks up heat from the outdoor air (even cool air holds plenty of heat), and the indoor coil becomes the hot side that warms your home.
On chilly, damp mornings frost can build up on the outdoor coil. The system then runs a short defrost cycle, briefly switching back to cooling to warm the coil. You may see steam rising from the outdoor unit. That is normal. Electric heat strips in the air handler can help during defrost and on the coldest days. Learn more on our heat pump service page.
Signs something is wrong
- Air from the vents is warm, or barely cool
- Weak airflow, or some rooms are much warmer than others
- Ice on the copper lines or the indoor coil
- Water around the air handler, or a ceiling stain under an attic unit
- The system turns on and off every few minutes
- The outdoor fan is not spinning while the system is cooling, or the unit hums or buzzes
- Grinding, squealing or banging noises
- A musty or burning smell, or a power bill that jumps for no clear reason
What you can safely check yourself
Thermostat
Make sure it is set to Cool and to a temperature below the room. If it runs on batteries, try fresh ones.
Air filter
Pull it and hold it up to a light. If you cannot see through it, replace it. In Florida, check it every month during the summer.
Breakers
AC systems usually have breakers for the outdoor unit and the air handler. If one has tripped, you can reset it once. If it trips again, leave it off and call.
Outdoor unit clearance
Keep shrubs, grass clippings, mulch and stored items back from the unit on all sides, and nothing on top. Your unit's manual lists the exact spacing.
Outdoor coil
Turn the system off at the thermostat first. Then rinse dust and grass off the fins with a garden hose on gentle spray. Never use a pressure washer, and do not bend the fins.
Drain line and ice
If you see water backing up, or ice on the lines or coil, turn cooling off at the thermostat. To thaw ice, set the fan to On and let it melt. Do not chip at it. Then call for service.
Leave these to a pro
- Anything involving refrigerant. Federal law (EPA Section 608) requires a certified technician to handle it, and do-it-yourself recharge kits can hide a leak and damage the system.
- Opening electrical panels on the equipment. Capacitors can hold a dangerous charge even with the power off.
- Repeatedly restarting a system that will not cool, or running it with ice on the coil or water backing up. That can turn a small repair into a big one.
More reading: all HVAC resources and HVAC maintenance tips for Southwest Florida.
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