What size AC for San Marcos, TX? 4 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in San Marcos 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 San Antonio (12.9 mi away). Aug is the hottest month, averaging a high of 97.5°F and a low of 72.7°F, with about 128 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 4,940 kWh a year at SEER2 14.3, or roughly $792 at Texas's average residential rate of 16.03¢ per kWh. Cooling runs 9 months of the year here.

What drives the load in San Marcos

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 San Marcos'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 San Marcos

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 San Marcos

Aug alone uses 20% of the year's cooling energy. The unit runs the equivalent of 1,451 hours a year at full output.

San Marcos against nearby cities

Of 252 Texas cities here, San Marcos has the 21st highest cooling load. The range runs from Levelland at 37,300 BTU/hr to Laredo at 51,500.

Where San Marcos's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Austin San Antonio, 12.9 miles from the center of San Marcos at 767 ft above sea level. That is close enough that the station's summers are San Marcos's summers.

2 other cities use the same station and get the same climate inputs. The closest to San Marcos: New Braunfels (16 mi away, 5.2 mi from the station), Seguin (20 mi away, 8.7 mi from the station). San Marcos is farther from the station than 2 of them.

The sun over San Marcos in late July

At 29.87°N, the noon sun on 21 July stands 80.5° above the horizon over San Marcos and the day runs 13 h 39 min from sunrise to sunset. With the sun that close to overhead, the roof takes most of the heat at midday and the west wall takes the rest after 3 pm. Shorter days give the house a longer night to shed the day's heat.

74,316 people live in San Marcos, the 56th largest of the 252 Texas cities sized on this site. Coordinates used: 29.8736, -97.9367.

San Marcos AC sizing questions