Why sizing off the old equipment is wrong
The most common way AC systems get sized in residential is by matching the tonnage of the unit being removed. The problem: if that unit was wrong, the new one inherits the mistake. A lot of older homes were installed with oversized equipment because the rule of thumb was 'a ton per 600 square feet,' which doesn't account for any actual property of the house. Drop in another oversized box and you get short-cycling, poor dehumidification, and a 12-year warranty that runs out two years early.
What goes into the calculation
Square footage by room. Ceiling heights. Window area, orientation, and glazing type. Wall, ceiling, and floor insulation values. Air infiltration. Duct location (conditioned vs. unconditioned space) and estimated leakage. Local climate data — design dry-bulb and wet-bulb temperatures for Chattanooga. Internal gains from people, appliances, and lighting. The software runs the load through ACCA's calculation method and produces a heating load and a cooling load, broken down by room.
What we do with the result
The cooling load tells us tonnage. The heating load tells us BTU/hr. The room-by-room breakdown drives the Manual D duct design — so the new system doesn't just have the right total capacity, it can actually deliver that capacity to the rooms that need it. Equipment selection (Manual S) follows the load, not the other way around.
Why doesn't everyone do this?
Honest answer: it takes a couple of hours and most contractors aren't paid for those hours. The pricing model in residential AC rewards speed of quote, not accuracy of quote. The result is a market full of oversized systems and homeowners who don't know that's why their houses are humid. For a deeper read on what this actually catches, see Why we run Manual J before quoting a replacement.