What is a central air conditioner?
Updated August 2026
A central air conditioner is a two-piece split system that cools the whole house through the same ducts the heating system uses: an outdoor condensing unit holding the compressor, and a matched evaporator coil indoors, tied together by a pair of copper refrigerant lines. Capacity is measured in tons, each ton equal to 12,000 BTU per hour. Most houses need 2 to 5 tons, very roughly one ton per 500 to 700 square feet, though a real load calculation beats that rule of thumb every time.
Where it is
Two locations again. The condensing unit is the boxy fan on a pad beside the house or on a flat roof. The indoor coil hides in the plenum above the furnace or inside the air handler, draining into a condensate drain line. The nameplate on the outdoor unit repays a close read: the model number usually encodes tonnage in its digits (024 for 2 tons, 036 for 3, 048 for 4), and the manufacture date tells you how far along the lifespan you already are. Homes in the South and older mobile homes often use a packaged unit instead, with everything in one outdoor cabinet.
What it does
Refrigerant hauls heat out of the house on a loop: it evaporates in the indoor coil, gets compressed and squeezed hot outside, sheds that heat through the condenser coil, and returns cold to start again. Failures arrive on a fairly reliable schedule. Years 5 to 12 bring electrical parts, capacitors and contactors at $150–400 each. Years 8 to 15 bring refrigerant leaks and corroded coils at $900–2,500. At the end comes compressor failure, $1,800–3,500, which is normally where the conversation turns to replacement. The rules moved too: systems sold since 2023 must meet SEER2 minimums of 13.4 in northern states and 14.3 across the South and Southwest, and new equipment now ships with R-454B in place of R-410A.
Maintenance
Two habits do most of the work. Change filters every 30 to 90 days, and run the system for 15 minutes on a mild spring day so you discover a dead capacitor in April instead of during the first 95-degree week. Add one professional tune-up a year at $100–250, which cleans the condenser coil, verifies the charge, and meters the electrical parts. One rule holds regardless: never top off refrigerant season after season, because a sealed system that needs a recharge has a leak. See how often to test the AC before summer and how often to recharge AC refrigerant.
What it costs to replace
A replacement central air conditioner runs $4,500–9,000 installed for a matched condenser and coil, or $8,000–14,000 when the furnace or air handler is replaced at the same time.
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Frequently asked questions
Should I replace the furnace and AC at the same time?
If both are past 12 years, usually yes. The coil and condenser have to be a matched pair to reach their rated efficiency, and pairing a new high-efficiency condenser with a 20-year-old blower gives away much of what you paid for. Doing both at once also saves a second round of labor, commonly $1,000–2,000. If the furnace is under 10 years old and running well, replace the air conditioner alone and keep the paperwork for the eventual match.
How long does central air last?
12 to 17 years for most systems, shorter along the coast where salt air eats condenser fins, longer in a dry climate with light use. Annual service and clean coils genuinely move that number. Once yearly repairs pass roughly $1,500, or the compressor fails on a unit older than 12, replacement is the better spend. The full curve: how long a central air conditioner lasts.
Why is my AC running but not cooling?
Work from cheap to expensive: a filter loaded until airflow collapsed, a thermostat set to ON instead of AUTO (which blows unconditioned air between cycles), a tripped breaker feeding only the outdoor unit, an iced indoor coil, or a low charge from a leak. A condenser fan that sits still while the unit hums points at the capacitor. If air at the register is only 3 to 5 degrees cooler than room air instead of 15 to 20, the system has a real fault, not a settings problem.
Does the R-410A phase-out affect my current system?
Only at repair time. Installed systems keep running on R-410A, and it stays available for service, but production is winding down on the same kind of schedule that retired R-22, so expect the per-pound price to climb over the next several years. New equipment sold since 2025 uses R-454B, which is not a drop-in substitute. Practically, that math pushes an older leaking system toward replacement rather than another recharge.