How does central air work?

Central air works by pulling warm air from your home through return ducts, passing it through an air filter and over a cold evaporator coil, and blowing the cooled air back out through supply ducts. Refrigerant carries the heat it picked up to an outdoor unit, which releases it outside, and the cycle repeats.

What is central air?

Central air is a whole-home cooling system that cools air at one central unit and sends it to every room through ducts. The U.S. Department of Energy describes central air conditioning as a system that circulates cool air through supply ducts and pulls warmer air back through return ducts.

Central air conditioners come in two types: a split system with an indoor and outdoor unit, and a packaged unit with everything in one outdoor cabinet. Both are controlled by one thermostat, and they often share ducts with the home's heating system.

Image source: U.S. Department of Energy

How does central air work, step by step?

Central air works in two halves at once: the indoor side pulls heat out of your air, and the outdoor side dumps that heat outside. Refrigerant flows in a closed loop between the two, changing from liquid to gas and back to carry the heat.

Inside your home: the cold side

Warm indoor air blows across the evaporator coil, which is cold because it holds low-pressure liquid refrigerant. The refrigerant absorbs heat from the air and turns into a gas, and the cooled air goes back into your rooms through the supply ducts.

Moisture in the air also condenses on the cold coil and drains away, so central air lowers humidity as it cools.

Outside your home: the hot side

The outdoor unit's compressor squeezes the refrigerant gas, which raises its pressure and temperature. The gas then flows through the condenser coil, where the outdoor fan blows air across it, and the refrigerant releases its heat outside and turns back into a liquid.

Here's the full cycle:

1. The thermostat senses the room is warmer than its setting and turns the system on.
2. The indoor blower pulls warm air from your rooms into the return ducts.
3. The air passes through the air filter, which traps dust, lint, pollen, and other particles.
4. The filtered air blows across the cold evaporator coil, where refrigerant absorbs its heat and moisture.
5. The blower pushes the cooled air through the supply ducts and out of the vents in each room.
6. The warm refrigerant gas travels through copper lines to the outdoor compressor.
7. The compressor raises the refrigerant's pressure and sends it to the outdoor condenser coil.
8. The outdoor fan blows air across the condenser coil, and the refrigerant releases its heat outside and turns back into a liquid.
9. The liquid refrigerant flows back inside, and an expansion valve lowers its pressure before it enters the evaporator coil again.
10. The cycle repeats until the thermostat setting is reached, then the system shuts off.

For a look at the full heating and cooling system, read our guide on how an HVAC system works.

What are the parts of a central air system?

A central air system has seven main parts that work together.

• Thermostat: turns the system on and off to hold the temperature you set. Our guide on how thermostats work covers the types.
• Outdoor unit (condenser): holds the compressor, condenser coil, and fan that release heat outside.
• Indoor unit (air handler or furnace): holds the evaporator coil and the blower fan that moves air through the ducts.
• Refrigerant lines: copper tubing that carries refrigerant between the indoor and outdoor units.
• Expansion valve: controls how much refrigerant flows into the evaporator coil.
• Ducts and vents: supply ducts deliver cooled air to each room, and return ducts carry warm air back to the indoor unit.
• Air filter: cleans the air in the return path before it reaches the blower and coil.

What are the types of central air systems?

Central air comes in two types: split systems and packaged units. The Department of Energy groups all central air conditioners into these two types, and both move cooled air through ducts.

Split-system central air

A split system has an outdoor unit with the compressor and condenser coil, and an indoor unit with the evaporator coil and blower. The indoor unit is usually a furnace or air handler in a basement, attic, or closet, and it holds the air filter.

A split system is the most common type of central air, according to Carrier, and the usual choice for a home that already has a furnace, since it can share the furnace's blower and ducts. Single-stage models run at full speed whenever they're on, while two-stage and variable-speed models run at lower speeds most of the time, so they tend to be quieter and use less energy.

Packaged central air

A packaged unit holds the compressor, both coils, and the blower in one cabinet, usually on a concrete pad beside the house or on the roof. Its supply and return ducts connect through an exterior wall or the roof.

Packaged units suit homes without room for an indoor unit, such as homes with no basement or attic space.

How is central air different from ductless and window units?

Central air cools the whole home through ducts from one system, while ductless mini-splits and window units cool individual rooms without ducts. Ductless and window units aren't central air.

A ductless mini-split has an outdoor unit connected to one or more indoor units mounted on a wall or ceiling, and each room is controlled on its own. Our guide to ductless air filters covers how their filters work.

How efficient is central air?

Central air efficiency is rated by SEER2, and a higher SEER2 number means the system uses less electricity for the same cooling. SEER2 (Seasonal Energy Efficiency Ratio 2) is the US efficiency rating for central air conditioners, based on a test that took effect in 2023.

Since 2023, new split-system central air conditioners must meet at least 13.4 SEER2 in northern states and 14.3 SEER2 in southern and southwestern states for most home-sized units, according to manufacturers Trane and Carrier, citing the Department of Energy standard. Existing systems don't have to be replaced to meet the newer minimums.

Refrigerant changed too. Under the EPA's Technology Transitions rule, new central air conditioners made since January 1, 2025, use refrigerants with a lower global warming potential, such as R-454B or R-32, instead of R-410A, and existing R-410A systems can still be serviced. ENERGY STAR suggests considering a replacement if your air conditioner is more than 10 years old.

Ducts affect efficiency too, since leaks let cooled air escape into attics and walls before it reaches your rooms.

Where is the air filter in a central air system?

The air filter in a central air system sits in the return air path, either in a slot at the indoor unit or behind a return vent in a wall or ceiling. The Department of Energy notes that central air filters are generally located somewhere along the return duct.

The filter keeps dust off the blower and evaporator coil, and a clean filter keeps air moving freely. Our guide on where your air filters are located shows the common spots, and the size is printed on the frame of your current filter, so you can match it in our air filter sizes.

Change a 1-inch filter every 30–90 days, and sooner during heavy summer use or with pets or allergies. Our guides on how to change a furnace filter and how often to change your air filter walk you through it, and our auto-delivery saves 5% and ships filters on your schedule.

A MERV 13 air filter suits homes with allergies or asthma, though older or low-powered systems may lose airflow with one. MERV (Minimum Efficiency Reporting Value) is a 1–16 score for how well a filter traps particles, and our guide to which MERV rating to use helps you choose.

If your central air runs but doesn't cool well, a clogged filter is one of the first things to check. Our guide on what to do when your AC isn't blowing cold air covers the other common causes.