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

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

2.5 tons central AC or heat pump, about 26,400 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 59% 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 908 square feet in this climate. For a single room, size by the room: 13.2 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Marseilles Lock/Dam (4 mi away). Jul is the hottest month, averaging a high of 84°F and a low of 61.9°F, with about 14 days a year at 90°F or more. That gives a design temperature of 88°F against 75°F inside.

What it costs to run

About 1,044 kWh a year at SEER2 14.3, or roughly $201 at Illinois's average residential rate of 19.23¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Ottawa

Of the 26,400 BTU/hr, 9,360 comes through walls, roof and windows on a 88°F afternoon (35%), 13,000 is sun on the house (49%), 2,120 is four people and their appliances, and 2,000 is moisture the coil has to wring out of Ottawa's summer air (8%).

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 24,480 BTU/hr before the moisture allowance.

Other house sizes in Ottawa

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,000 BTU/hr; a 350 sq ft living room needs about 4,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 7,900 BTU/hr.

Month by month in Ottawa

Jul alone uses 31% of the year's cooling energy. The unit runs the equivalent of 565 hours a year at full output.

Ottawa against nearby cities

Of 225 Illinois cities here, Ottawa has the 201st highest cooling load. The range runs from Fox Lake at 24,900 BTU/hr to Granite City at 36,000.

Where Ottawa's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Marseilles Lock/Dam, 4 miles from the center of Ottawa at 487 ft above sea level. That is close enough that the station's summers are Ottawa's summers.

No other city in this set uses Marseilles Lock/Dam, so this sizing is Ottawa's alone.

The sun over Ottawa in late July

At 41.35°N, the noon sun on 21 July stands 69.0° above the horizon over Ottawa and the day runs 14 h 33 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.

18,775 people live in Ottawa, the 132nd largest of the 225 Illinois cities sized on this site. Coordinates used: 41.3540, -88.8234.

Ottawa AC sizing questions