What size AC for St. Augustine, FL? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 40,200 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.4 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 596 square feet in this climate. For a single room, size by the room: 20.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at St. Augustine Lighthouse (1.5 mi away). Jul is the hottest month, averaging a high of 91°F and a low of 73.8°F, with about 62 days a year at 90°F or more. That gives a design temperature of 97°F against 75°F inside.

What it costs to run

About 4,837 kWh a year at SEER2 14.3, or roughly $741 at Florida's average residential rate of 15.32¢ per kWh. Cooling runs 9 months of the year here.

What drives the load in St. Augustine

Of the 40,200 BTU/hr, 15,840 comes through walls, roof and windows on a 97°F afternoon (39%), 13,000 is sun on the house (32%), 2,120 is four people and their appliances, and 9,300 is moisture the coil has to wring out of St. Augustine'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 30,960 BTU/hr before the moisture allowance.

Other house sizes in St. Augustine

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,000 BTU/hr; a 350 sq ft living room needs about 7,000 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 12,100 BTU/hr.

Month by month in St. Augustine

Jul alone uses 18% of the year's cooling energy. The unit runs the equivalent of 1,719 hours a year at full output.

St. Augustine against nearby cities

Of 173 Florida cities here, St. Augustine has the 118th highest cooling load. The range runs from Miami Shores at 36,500 BTU/hr to Plant City at 44,900.

Where St. Augustine's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is St. Augustine Lighthouse, 1.5 miles from the center of St. Augustine at 12 ft above sea level. That is close enough that the station's summers are St. Augustine's summers.

No other city in this set uses St. Augustine Lighthouse, so this sizing is St. Augustine's alone.

The sun over St. Augustine in late July

At 29.90°N, the noon sun on 21 July stands 80.5° above the horizon over St. Augustine 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.

16,033 people live in St. Augustine, the 142nd largest of the 173 Florida cities sized on this site. Coordinates used: 29.8962, -81.3111.

St. Augustine AC sizing questions