What size AC for St. Marys, GA? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 37,400 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.1 tons. This is one of the climates the chart was written for.

If a contractor quotes you more than 3.5 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 641 square feet in this climate. For a single room, size by the room: 18.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Brunswick 23 S (7.9 mi away). Jul is the hottest month, averaging a high of 88.8°F and a low of 73.5°F, with about 40 days a year at 90°F or more. That gives a design temperature of 94°F against 75°F inside.

What it costs to run

About 3,871 kWh a year at SEER2 14.3, or roughly $599 at Georgia's average residential rate of 15.48¢ per kWh. Cooling runs 7 months of the year here.

What drives the load in St. Marys

Of the 37,400 BTU/hr, 13,680 comes through walls, roof and windows on a 94°F afternoon (37%), 13,000 is sun on the house (35%), 2,120 is four people and their appliances, and 8,600 is moisture the coil has to wring out of St. Marys'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 28,800 BTU/hr before the moisture allowance.

Other house sizes in St. Marys

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,800 BTU/hr; a 350 sq ft living room needs about 6,500 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 11,200 BTU/hr.

Month by month in St. Marys

Jul alone uses 20% of the year's cooling energy. The unit runs the equivalent of 1,479 hours a year at full output.

St. Marys against nearby cities

Of 101 Georgia cities here, St. Marys has the 39th highest cooling load. The range runs from Cumming at 33,100 BTU/hr to Albany at 44,000.

Where St. Marys's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Brunswick 23 S, 7.9 miles from the center of St. Marys at 25 ft above sea level. That is close enough that the station's summers are St. Marys's summers.

2 other cities use the same station and get the same climate inputs. The closest to St. Marys: Fernandina Beach (10 mi away, 10.4 mi from the station), Kingsland (14 mi away, 19.8 mi from the station). St. Marys sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over St. Marys in late July

At 30.76°N, the noon sun on 21 July stands 79.6° above the horizon over St. Marys and the day runs 13 h 42 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.

20,119 people live in St. Marys, the 57th largest of the 101 Georgia cities sized on this site. Coordinates used: 30.7567, -81.5784.

St. Marys AC sizing questions