What size AC for Rolla, MO? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 32,300 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 30% 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 743 square feet in this climate. For a single room, size by the room: 16.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Vichy Rolla Natl Airport (12.8 mi away). Jul is the hottest month, averaging a high of 87.9°F and a low of 67°F, with about 33 days a year at 90°F or more. That gives a design temperature of 93°F against 75°F inside.

What it costs to run

About 1,817 kWh a year at SEER2 14.3, or roughly $259 at Missouri's average residential rate of 14.28¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Rolla

Of the 32,300 BTU/hr, 12,960 comes through walls, roof and windows on a 93°F afternoon (40%), 13,000 is sun on the house (40%), 2,120 is four people and their appliances, and 4,200 is moisture the coil has to wring out of Rolla'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 28,080 BTU/hr before the moisture allowance.

Other house sizes in Rolla

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 5,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 9,700 BTU/hr.

Month by month in Rolla

Jul alone uses 30% of the year's cooling energy. The unit runs the equivalent of 804 hours a year at full output.

Rolla against nearby cities

Of 80 Missouri cities here, Rolla has the 72nd highest cooling load. The range runs from Hannibal at 30,600 BTU/hr to Sikeston at 37,800.

Where Rolla's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Vichy Rolla Natl Airport, 12.8 miles from the center of Rolla at 1,110 ft above sea level. That is close enough that the station's summers are Rolla's summers.

No other city in this set uses Vichy Rolla Natl Airport, so this sizing is Rolla's alone.

The sun over Rolla in late July

At 37.95°N, the noon sun on 21 July stands 72.5° above the horizon over Rolla and the day runs 14 h 15 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.

20,660 people live in Rolla, the 36th largest of the 80 Missouri cities sized on this site. Coordinates used: 37.9459, -91.7612.

Rolla AC sizing questions