Central air cools the whole house from one system: an outdoor unit, a coil indoors, and ductwork that carries cooled air to every room. Getting the most from it comes down to three decisions, its size, its efficiency and the condition of your ducts.
Size it by tonnage from a load calculation, choose an efficiency (SEER2) that suits your climate's cooling hours, and fix leaky ducts first. A 3-ton system running an assumed 1,500 full-load hours costs about $680 a year at 14.3 SEER2 and 18 cents per kWh, or $486 at 20 SEER2; the DOE puts a central air conditioner's lifespan at 15 to 20 years.
AC Tonnage Calculator
The right central AC size for your home.
How central air works
The outdoor unit holds the compressor and a coil that releases heat outside. Refrigerant carries heat from the indoor coil, mounted on the furnace or air handler, to the outdoor unit. The blower pushes house air across the cold indoor coil and through the supply ducts, and return ducts bring it back.
Sizing
Central systems are sized in tons; one ton is 12,000 BTU per hour. The industry method is ACCA's Manual J, a room-by-room load calculation.
A quick rule of thumb assigns about 20 BTU per hour per square foot of floor area. For a 2,000 sq ft home that is 2,000 × 20 = 40,000 BTU per hour; dividing by 12,000 BTU per ton gives 3.33 tons, which rounds up to the next standard size of 3.5 tons. Standard sizes step in half-tons: 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5 tons.
The square-foot rule runs generous, so a tight, well-insulated home often needs less than it suggests. An oversized system cools quickly but shuts off before removing much humidity; an undersized one runs nonstop on hot days. Our AC tonnage calculator applies that 20 BTU baseline, then adjusts it for climate, insulation, ceiling height, sun and windows.
Efficiency and running cost
SEER2 rates seasonal cooling efficiency. The federal minimum for split systems is 13.4 SEER2 in the North and 14.3 in the South and Southwest for systems under 45,000 BTU per hour. For a 3-ton system at an assumed 1,500 full-load hours and 18 cents per kWh:
| SEER2 | Cost per year |
|---|---|
| 13.4 | $725 |
| 14.3 | $680 |
| 16 | $608 |
| 20 | $486 |
Each step up saves less than the last; our SEER2 comparison calculator works out whether a higher rating pays back for you.
Single-stage, two-stage and variable-speed compressors
The compressor does the cooling work, and how it runs sets the comfort and efficiency you feel. A single-stage compressor has one speed, full on or off, so it cools fast on a hot day but cycles off quickly in milder weather, which cuts its runtime. A two-stage compressor adds a lower speed it uses most of the time, running longer at part load.
Humidity leaves the air only while it keeps moving across the cold coil, so short cycles leave a house cool but clammy. The longer, gentler runs of a two-stage or variable-speed unit pull more moisture out. A variable-speed (inverter) compressor ramps continuously between a low minimum and full output to track the load closely.
That steady part-load running is why variable-speed systems hold a more even temperature, dehumidify better, and can post the highest SEER2 ratings, since a compressor loafing at low speed draws far less power than one slamming on and off. The trade-offs are a higher purchase price and more electronics to service. Mini splits use this same inverter approach, which is part of why they run efficiently; see how a mini split works.
Installation cost
Installed prices depend on size, efficiency, ductwork condition and region. See our central AC cost guide for typical ranges, and get at least three itemized quotes.
Ducts
The DOE puts duct air losses at about 30% of a cooling system's energy, so a new system on leaky ducts won't deliver its rated efficiency. Have ducts checked and sealed when replacing equipment; see duct leakage testing.
Refrigerants
New systems use lower-impact refrigerants, mainly R-454B and R-32, under the EPA's phase-down of R-410A. See the refrigerant phase-out for the dates and what they mean for repairs.
Maintenance and lifespan
The DOE says clean filters can lower an air conditioner's energy use by 5 to 15%; change or clean them on schedule, and keep the outdoor unit clear. A pre-season tune-up catches refrigerant and electrical problems early; see HVAC maintenance cost. The DOE puts a central air conditioner's life at 15 to 20 years.
Central air vs. heat pump vs. mini split
A heat pump is a central air conditioner that also heats, running the same refrigerant cycle in reverse. A ductless mini split is a heat pump with no ducts that conditions one room, or a few, through wall or ceiling heads.
| Central AC | Heat pump (ducted) | Mini split (ductless) | |
|---|---|---|---|
| Cooling | Yes | Yes | Yes |
| Heating | No, pairs with a furnace | Yes | Yes |
| Duct losses | ~30% of cooling energy (DOE) | ~30% of cooling energy (DOE) | None, no ducts |
| Zoning | Whole house, one thermostat | Whole house, one thermostat | Each head runs independently |
| What raises the install | Adding or replacing ductwork | Ductwork, same as central AC | Each added indoor head |
See mini split vs. central air and furnace vs. heat pump for the full comparisons.
Frequently asked questions
How long does central air last?
15 to 20 years, according to the DOE, with regular maintenance.
What SEER2 rating should I buy?
At least the federal minimum for your region; higher ratings pay back faster where summers are long and hot. Compare the price difference with the yearly savings.
What size central air do I need?
Use a Manual J load calculation; our tonnage calculator gives a starting estimate from your home's size, climate and insulation.
Why isn't my central air cooling?
Common causes include a dirty filter, a blocked outdoor unit, low refrigerant and leaky ducts; see AC not cooling.
Can I add central air to a house without ductwork?
Yes, but the ductwork becomes most of the project, since supply and return runs, a return plenum and registers all have to be built into walls, floors or soffits. Where running ducts is impractical, a ductless mini split delivers central-style cooling without them; see mini split vs. central air.
How long does a central AC install take?
It depends mostly on whether the ducts are reused or replaced. A like-for-like swap onto sound existing ducts is the quickest case, while adding or rebuilding ductwork, a new electrical circuit or a new refrigerant line set extends the work. Get the scope in writing so the timeline is clear before work starts.
Is a bigger AC better?
No. An oversized unit cools the air fast, then shuts off before it has run long enough to pull humidity out, leaving the house cool but damp and cycling the compressor harder. Sizing to the actual load with a Manual J gives longer, steadier runs that dehumidify properly; our tonnage calculator is a starting estimate.