The right size for a heat pump or air conditioner comes from a load calculation for your specific house — a Manual J, which accounts for square footage, insulation, windows, orientation, air leakage, occupants and local design temperatures — not from a rule of thumb like “one ton per 500 square feet” and not from the size of the unit being removed. In North Carolina’s humid climate the common mistake is oversizing, which cools quickly but doesn’t dehumidify, so if you take one thing from this page it’s that a quote without a load calculation is a guess.
What a ton is
A “ton” is 12,000 BTU per hour of cooling capacity. Residential systems run from 1.5 to 5 tons in half-ton steps. Heating capacity is rated in BTU per hour too, and for a heat pump it falls as outdoor temperature falls, which is why heating sizing and auxiliary heat sizing are separate questions.
Why square-footage rules fail
Two 2,000-square-foot houses in Raleigh can need very different systems: one built in 1985 with single-pane windows and leaky ducts in a vented crawlspace, the other built in 2015 with tight construction and sealed attic. Rules of thumb ignore insulation, window area and orientation, air leakage, ceiling height, occupancy, internal loads and, in our climate, the moisture load, which is a large share of the summer job. They also ignore ductwork, which limits how much air a system can actually deliver.
What Manual J accounts for
The calculation totals the heat gain (summer) and loss (winter) of the house at local design conditions, room by room: conduction through walls, roof, floor and windows; solar gain by orientation; infiltration; internal gains from people and appliances; and the latent (moisture) load. The output is a sensible and latent cooling load and a heating load in BTU/hr. Manual S then selects equipment whose rated capacity at those conditions matches the load, and Manual D sizes the ducts to deliver the airflow. What a proper installation includes.
Why oversizing is the humid-climate mistake
An oversized system reaches setpoint fast and shuts off, so it doesn’t run long enough for the coil to pull moisture out of the air; the house is cool and damp, people set the thermostat lower to compensate, and bills rise. It also short-cycles, which is hard on compressors and inefficient. A right-sized system, especially a two-stage or variable-speed one, runs long cycles at lower output and dries the air. Humidity complaints in North Carolina homes trace back to oversizing more often than to anything else. Humidity control in NC homes and variable-speed systems.
A rough table to sanity-check a quote
| Home (conditioned sq ft), NC Piedmont/coast, average construction | Typical cooling load | Typical size quoted |
|---|---|---|
| 900–1,300 | 18,000–30,000 BTU/hr | 1.5–2.5 tons |
| 1,300–1,800 | 24,000–36,000 | 2–3 tons |
| 1,800–2,400 | 30,000–48,000 | 2.5–4 tons |
| 2,400–3,200 | 42,000–60,000 | 3.5–5 tons |
These are ranges, not answers; a tight new house sits at the bottom of its band and a leaky older one at the top. If a quote lands well outside its band without a calculation to justify it, ask.
What to ask the contractor
Did you run a Manual J, and can I see it? What design temperatures did you use? What are the sensible and latent loads? Why this tonnage and this stage/speed type? Can the existing ducts deliver the airflow (what’s the measured static pressure)? For a heat pump: what’s the heating capacity at 17°F and how much auxiliary heat are you installing? A contractor who has done the work will answer in a minute; one who hasn’t will talk about brands. Every contractor on the North Carolina registry shows the license status we found; the sizing questions tell you about the work.