What size AC for St. George, UT? 4 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in St. George needs

4 tons central AC or heat pump, about 49,200 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 488 square feet in this climate. For a single room, size by the room: 24.6 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at St. George (3.3 mi away). Jul is the hottest month, averaging a high of 101.9°F and a low of 71.7°F, with about 128 days a year at 90°F or more. That gives a design temperature of 110°F against 75°F inside.

What it costs to run

About 3,562 kWh a year at SEER2 14.3, or roughly $470 at Utah's average residential rate of 13.19¢ per kWh. Cooling runs 6 months of the year here.

What drives the load in St. George

Of the 49,200 BTU/hr, 25,200 comes through walls, roof and windows on a 110°F afternoon (51%), 13,000 is sun on the house (26%), 2,120 is four people and their appliances, and 8,900 is moisture the coil has to wring out of St. George's summer air (18%).

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 40,320 BTU/hr before the moisture allowance.

Other house sizes in St. George

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,700 BTU/hr; a 350 sq ft living room needs about 8,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 14,800 BTU/hr.

Month by month in St. George

Jul alone uses 28% of the year's cooling energy. The unit runs the equivalent of 1,035 hours a year at full output.

St. George against nearby cities

Of 67 Utah cities here, St. George has the highest cooling load. The range runs from Heber at 30,700 BTU/hr to St. George at 49,200.

Where St. George's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is St. George, 3.3 miles from the center of St. George at 2,857 ft above sea level. That is close enough that the station's summers are St. George's summers.

2 other cities use the same station and get the same climate inputs. The closest to St. George: Washington (6 mi away, 6.6 mi from the station), Ivins (9 mi away, 5.6 mi from the station). St. George sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over St. George in late July

At 37.08°N, the noon sun on 21 July stands 73.3° above the horizon over St. George and the day runs 14 h 11 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.

106,288 people live in St. George, the 5th largest of the 67 Utah cities sized on this site. Coordinates used: 37.0770, -113.5765.

St. George AC sizing questions