What size AC for Box Elder, SD? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 28,800 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 46% 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 834 square feet in this climate. For a single room, size by the room: 14.4 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Rapid City Regional Airport (4.7 mi away). Jul is the hottest month, averaging a high of 86.4°F and a low of 58.5°F, with about 32 days a year at 90°F or more. That gives a design temperature of 91°F against 75°F inside.

What it costs to run

About 792 kWh a year at SEER2 14.3, or roughly $115 at South Dakota's average residential rate of 14.58¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Box Elder

Of the 28,800 BTU/hr, 11,520 comes through walls, roof and windows on a 91°F afternoon (40%), 13,000 is sun on the house (45%), 2,120 is four people and their appliances, and 2,100 is moisture the coil has to wring out of Box Elder'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 26,640 BTU/hr before the moisture allowance.

Other house sizes in Box Elder

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 5,000 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,600 BTU/hr.

Month by month in Box Elder

Jul alone uses 40% of the year's cooling energy. The unit runs the equivalent of 394 hours a year at full output.

Box Elder against nearby cities

Of 14 South Dakota cities here, Box Elder has the 5th highest cooling load. The range runs from Watertown at 24,900 BTU/hr to Pierre at 31,100.

Where Box Elder's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Rapid City Regional Airport, 4.7 miles from the center of Box Elder at 3,162 ft above sea level. That is close enough that the station's summers are Box Elder's summers.

1 other city uses the same station and get the same climate inputs. The closest to Box Elder: Rapid City (7 mi away, 8.3 mi from the station). Box Elder sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Box Elder in late July

At 44.11°N, the noon sun on 21 July stands 66.3° above the horizon over Box Elder and the day runs 14 h 49 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.

13,887 people live in Box Elder, the 9th largest of the 14 South Dakota cities sized on this site. Coordinates used: 44.1109, -103.0811.

Box Elder AC sizing questions