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

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

2 tons central AC or heat pump, about 24,900 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 69% 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 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 965 square feet in this climate. For a single room, size by the room: 12.4 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Shabbona 5 Nne (7.9 mi away). Jul is the hottest month, averaging a high of 81.5°F and a low of 60.5°F, with about 8 days a year at 90°F or more. That gives a design temperature of 86°F against 75°F inside.

What it costs to run

About 776 kWh a year at SEER2 14.3, or roughly $149 at Illinois's average residential rate of 19.23¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in DeKalb

Of the 24,900 BTU/hr, 7,920 comes through walls, roof and windows on a 86°F afternoon (32%), 13,000 is sun on the house (52%), 2,120 is four people and their appliances, and 1,800 is moisture the coil has to wring out of DeKalb's summer air (7%).

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 23,040 BTU/hr before the moisture allowance.

Other house sizes in DeKalb

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,900 BTU/hr; a 350 sq ft living room needs about 4,300 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 7,400 BTU/hr.

Month by month in DeKalb

Jul alone uses 33% of the year's cooling energy. The unit runs the equivalent of 446 hours a year at full output.

DeKalb against nearby cities

Of 225 Illinois cities here, DeKalb has the 214th highest cooling load. The range runs from Fox Lake at 24,900 BTU/hr to Granite City at 36,000.

Where DeKalb's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Shabbona 5 Nne, 7.9 miles from the center of DeKalb at 861 ft above sea level. That is close enough that the station's summers are DeKalb's summers.

1 other city uses the same station and get the same climate inputs. The closest to DeKalb: Sycamore (6 mi away, 13.9 mi from the station). DeKalb sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over DeKalb in late July

At 41.93°N, the noon sun on 21 July stands 68.5° above the horizon over DeKalb and the day runs 14 h 36 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. Long days mean the house keeps gaining heat into the evening, well after the outdoor peak.

40,491 people live in DeKalb, the 41st largest of the 225 Illinois cities sized on this site. Coordinates used: 41.9282, -88.7476.

DeKalb AC sizing questions