What size AC a 2,000 sq ft Columbia home needs

The quick estimate

This Old House offers 20 BTUs per square foot of living area as a starting estimate, and recommends a proper load calculation to confirm it. Two thousand square feet times 20 is 40,000 BTUs. A ton of air conditioning moves 12,000 BTUs an hour, so 40,000 is about 3.3 tons, which in the real world means a choice between a 3 ton and a 3.5 ton unit.

What Manual J does that the estimate can't

The DOE says equipment should be sized from the house's calculated heating and cooling loads using ACCA Manual J, and cautions that rules of thumb tend to oversize. A DOE guide for contractors makes the same point: oversizing often traces back to a rule of thumb used in place of a load calculation.

Around Columbia, the inputs that swing the result most:

• Glass facing west or south, and how much of the roof sits under mature oaks or maples.
• Attic insulation, which in an older house is often well short of what a new one gets.
• A basement, which stays cool and takes load off the main floor.
• Drafty original windows and doors that let muggy air in.

So a split-level near Rock Bridge and a ranch in Ashland, both 2,000 square feet, may need different equipment.

In a humid climate, too big is the costlier mistake

An oversized condenser cools the rooms fast and shuts off before much water has condensed on the coil. The DOE says plainly that oversized units remove humidity poorly. In July, with the average low at the airport around 69.2°F, the result is a house that hits the setpoint and still feels damp. City of Columbia Utilities also says it may refuse the rebate for a unit oversized for the home.

Installed prices by size

Tonnage	Fixr installed range
3 ton	$5,000 to $9,150
3.5 ton	$5,450 to $9,700
4 ton	$6,950 to $11,600

Fixr's $8,750 national average is built on a 3 ton, 16 SEER split system with new ducts in a 2,000 square foot house. Installation prices and the rebate.