Short-cycling, running all day, cold-but-clammy rooms — most AC symptoms have a cheap maintenance cause before they have a sizing cause. Tick what you're seeing; we'll show each symptom's honest differential, then how to verify the sizing theory with real numbers instead of a salesperson's guess.
Tick symptoms above to see their differentials.
1. Find your current unit's real tonnage. It's coded in the outdoor unit's model number — paste it into the model number decoder (22 brands, instant).
2. Calculate what the space actually needs. Run your real room data — area, climate, sun, ceiling, occupants — through the tonnage calculator.
3. Compare. On the calculator's result panel, set "Compare with your current AC" to the decoded size. Half a ton or more above the calculated load → oversized (short-cycling, humidity); half a ton under → undersized (marathon runtimes). Within the band → your symptoms are maintenance, not sizing.
If the verdict is oversized and the unit otherwise works, the economical move is usually to ride it to natural end-of-life with lower fan speeds, then downsize at replacement — the oversizing guide covers living with one in the meantime.
The telltale combination is short cycles (5–10 minutes) plus a cold-but-clammy feel — the unit satisfies the thermostat before removing moisture. Confirm by decoding your unit's tonnage from its model number and comparing it against a proper load calculation for the space; more than about half a ton above the calculated load is meaningfully oversized.
Not on symptoms alone, and rarely as an immediate purchase: first rule out the cheap causes (filter, coil, fan settings), then verify the mismatch with real numbers. If a working unit is oversized, the practical moment to downsize is at natural replacement time — meanwhile, lower fan speeds and longer gentle cycles recover much of the comfort.
In order of likelihood: dirty filter or coils, a refrigerant leak, duct leakage into unconditioned space, an extreme-heat day, and only then genuine undersizing. Verify the sizing theory by calculating the space's actual load and decoding the unit's tonnage before buying anything.
No — a clogged filter, an icing coil, a weak capacitor or a badly placed thermostat each cause it too, and they're all cheaper to fix. Oversizing is the structural cause that remains when the maintenance causes are ruled out.
correct sizing is defined by ACCA Manual J (load) and Manual S (equipment selection), ANSI/ACCA 2 & 3 — ACCA. See our full methodology.