What size AC for Fort Atkinson, WI? 2 tons for 2,000 sq ft

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

2 tons central AC or heat pump, about 25,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 64% 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 935 square feet in this climate. For a single room, size by the room: 12.8 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Fort Atkinson Wwtp (1.5 mi away). Jul is the hottest month, averaging a high of 82.5°F and a low of 61.7°F, with about 11 days a year at 90°F or more. That gives a design temperature of 87°F against 75°F inside.

What it costs to run

About 877 kWh a year at SEER2 14.3, or roughly $167 at Wisconsin's average residential rate of 19.05¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Fort Atkinson

Of the 25,700 BTU/hr, 8,640 comes through walls, roof and windows on a 87°F afternoon (34%), 13,000 is sun on the house (51%), 2,120 is four people and their appliances, and 1,900 is moisture the coil has to wring out of Fort Atkinson'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 23,760 BTU/hr before the moisture allowance.

Other house sizes in Fort Atkinson

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 4,500 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 7,700 BTU/hr.

Month by month in Fort Atkinson

Jul alone uses 34% of the year's cooling energy. The unit runs the equivalent of 489 hours a year at full output.

Fort Atkinson against nearby cities

Of 98 Wisconsin cities here, Fort Atkinson has the 38th highest cooling load. The range runs from Two Rivers at 20,200 BTU/hr to La Crosse at 29,800.

Where Fort Atkinson's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Fort Atkinson Wwtp, 1.5 miles from the center of Fort Atkinson at 787 ft above sea level. That is close enough that the station's summers are Fort Atkinson's summers.

No other city in this set uses Fort Atkinson Wwtp, so this sizing is Fort Atkinson's alone.

The sun over Fort Atkinson in late July

At 42.92°N, the noon sun on 21 July stands 67.5° above the horizon over Fort Atkinson and the day runs 14 h 42 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.

12,377 people live in Fort Atkinson, the 80th largest of the 98 Wisconsin cities sized on this site. Coordinates used: 42.9240, -88.8455.

Fort Atkinson AC sizing questions