What size AC for Charleston, IL? 2.5 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in Charleston needs

2.5 tons central AC or heat pump, about 29,800 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 41% more than this climate needs. An oversized unit cools the air fast, shuts off before it pulls moisture out, and leaves the house cold and clammy.

If a contractor quotes you more than 2.5 tons for an average 2,000 sq ft house here, ask to see the Manual J load calculation behind it before you sign. That works out to one ton per 805 square feet in this climate. For a single room, size by the room: 14.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Mattoon Charleston Coles County Airport (5.1 mi away). Jul is the hottest month, averaging a high of 85.6°F and a low of 65.7°F, with about 20 days a year at 90°F or more. That gives a design temperature of 90°F against 75°F inside.

What it costs to run

About 1,551 kWh a year at SEER2 14.3, or roughly $298 at Illinois's average residential rate of 19.23¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Charleston

Of the 29,800 BTU/hr, 10,800 comes through walls, roof and windows on a 90°F afternoon (36%), 13,000 is sun on the house (44%), 2,120 is four people and their appliances, and 3,900 is moisture the coil has to wring out of Charleston's summer air (13%).

Sun is the biggest share here, more than heat through the walls, so shading west windows and a light roof cut the size you need more than extra insulation would. Sensible heat totals 25,920 BTU/hr before the moisture allowance.

Other house sizes in Charleston

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,200 BTU/hr; a 350 sq ft living room needs about 5,200 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,900 BTU/hr.

Month by month in Charleston

Jul alone uses 30% of the year's cooling energy. The unit runs the equivalent of 744 hours a year at full output.

Charleston against nearby cities

Of 225 Illinois cities here, Charleston has the 75th highest cooling load. The range runs from Fox Lake at 24,900 BTU/hr to Granite City at 36,000.

Where Charleston's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Mattoon Charleston Coles County Airport, 5.1 miles from the center of Charleston at 710 ft above sea level. That is close enough that the station's summers are Charleston's summers.

1 other city uses the same station and get the same climate inputs. The closest to Charleston: Mattoon (10 mi away, 4.8 mi from the station). Charleston is farther from the station than 1 of them.

The sun over Charleston in late July

At 39.48°N, the noon sun on 21 July stands 70.9° above the horizon over Charleston and the day runs 14 h 23 min from sunrise to sunset. A lower noon sun puts more of the day's heat on east and west walls than on the roof, which is why shading west glass pays off here. Shorter days give the house a longer night to shed the day's heat.

17,361 people live in Charleston, the 149th largest of the 225 Illinois cities sized on this site. Coordinates used: 39.4848, -88.1776.

Charleston AC sizing questions