What size AC for Round Rock, TX? 4 tons for 2,000 sq ft

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

4 tons central AC or heat pump, about 48,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, which falls short here. On a design day the unit runs flat out and the house still drifts above 75°F by late afternoon.

If a contractor quotes you more than 4 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 493 square feet in this climate. For a single room, size by the room: 24.3 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Austin-Camp Mabry (15.3 mi away). Aug is the hottest month, averaging a high of 97.8°F and a low of 75.1°F, with about 123 days a year at 90°F or more. That gives a design temperature of 106°F against 75°F inside.

What it costs to run

About 5,168 kWh a year at SEER2 14.3, or roughly $828 at Texas's average residential rate of 16.03¢ per kWh. Cooling runs 9 months of the year here.

What drives the load in Round Rock

Of the 48,700 BTU/hr, 22,320 comes through walls, roof and windows on a 106°F afternoon (46%), 13,000 is sun on the house (27%), 2,120 is four people and their appliances, and 11,200 is moisture the coil has to wring out of Round Rock's summer air (23%).

Heat through the shell is the biggest share here, so insulation and air sealing cut the size you need more than shading would. Sensible heat totals 37,440 BTU/hr before the moisture allowance.

Other house sizes in Round Rock

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,600 BTU/hr; a 350 sq ft living room needs about 8,500 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 14,600 BTU/hr.

Month by month in Round Rock

Aug alone uses 20% of the year's cooling energy. The unit runs the equivalent of 1,518 hours a year at full output.

Round Rock against nearby cities

Of 252 Texas cities here, Round Rock has the 18th highest cooling load. The range runs from Levelland at 37,300 BTU/hr to Laredo at 51,500.

Where Round Rock's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Austin-Camp Mabry, 15.3 miles from the center of Round Rock at 670 ft above sea level. That is close enough that the station's summers are Round Rock's summers.

6 other cities use the same station and get the same climate inputs. The closest to Round Rock: Pflugerville (7 mi away, 12.9 mi from the station), Hutto (7 mi away, 20 mi from the station), Cedar Park (9 mi away, 13.5 mi from the station), Manor (13 mi away, 12.3 mi from the station), Austin (17 mi away, 1.6 mi from the station). Round Rock is farther from the station than 5 of them.

The sun over Round Rock in late July

At 30.53°N, the noon sun on 21 July stands 79.9° above the horizon over Round Rock and the day runs 13 h 41 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.

135,359 people live in Round Rock, the 28th largest of the 252 Texas cities sized on this site. Coordinates used: 30.5261, -97.6635.

Round Rock AC sizing questions