What size AC for Chatham, IL? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 31,500 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 33% 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 3 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 763 square feet in this climate. For a single room, size by the room: 15.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Springfield Abraham Lincoln Ca (11.8 mi away). Jul is the hottest month, averaging a high of 86.8°F and a low of 66.2°F, with about 28 days a year at 90°F or more. That gives a design temperature of 92°F against 75°F inside.

What it costs to run

About 1,739 kWh a year at SEER2 14.3, or roughly $334 at Illinois's average residential rate of 19.23¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Chatham

Of the 31,500 BTU/hr, 12,240 comes through walls, roof and windows on a 92°F afternoon (39%), 13,000 is sun on the house (41%), 2,120 is four people and their appliances, and 4,100 is moisture the coil has to wring out of Chatham'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 27,360 BTU/hr before the moisture allowance.

Other house sizes in Chatham

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,400 BTU/hr; a 350 sq ft living room needs about 5,500 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 9,400 BTU/hr.

Month by month in Chatham

Jul alone uses 29% of the year's cooling energy. The unit runs the equivalent of 790 hours a year at full output.

Chatham against nearby cities

Of 225 Illinois cities here, Chatham has the 28th highest cooling load. The range runs from Fox Lake at 24,900 BTU/hr to Granite City at 36,000.

Where Chatham's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Springfield Abraham Lincoln Ca, 11.8 miles from the center of Chatham at 590 ft above sea level. That is close enough that the station's summers are Chatham's summers.

2 other cities use the same station and get the same climate inputs. The closest to Chatham: Springfield (8 mi away, 4.3 mi from the station), Taylorville (22 mi away, 27.9 mi from the station). Chatham is farther from the station than 1 of them.

The sun over Chatham in late July

At 39.67°N, the noon sun on 21 July stands 70.7° above the horizon over Chatham and the day runs 14 h 24 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.

14,639 people live in Chatham, the 167th largest of the 225 Illinois cities sized on this site. Coordinates used: 39.6745, -89.6933.

Chatham AC sizing questions