What size AC for Marathon, FL? 4 tons for 2,000 sq ft

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

4 tons central AC or heat pump, about 42,100 BTU/hr of load

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

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 570 square feet in this climate. For a single room, size by the room: 21.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Marathon Airport (1.5 mi away). Aug is the hottest month, averaging a high of 91.6°F and a low of 79.7°F, with about 105 days a year at 90°F or more. That gives a design temperature of 99°F against 75°F inside.

What it costs to run

About 8,055 kWh a year at SEER2 14.3, or roughly $1,234 at Florida's average residential rate of 15.32¢ per kWh. Cooling runs 12 months of the year here.

What drives the load in Marathon

Of the 42,100 BTU/hr, 17,280 comes through walls, roof and windows on a 99°F afternoon (41%), 13,000 is sun on the house (31%), 2,120 is four people and their appliances, and 9,700 is moisture the coil has to wring out of Marathon'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 32,400 BTU/hr before the moisture allowance.

Other house sizes in Marathon

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,200 BTU/hr; a 350 sq ft living room needs about 7,400 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 12,700 BTU/hr.

Month by month in Marathon

Aug alone uses 12% of the year's cooling energy. The unit runs the equivalent of 2,735 hours a year at full output.

Marathon against nearby cities

Of 173 Florida cities here, Marathon has the 73rd highest cooling load. The range runs from Miami Shores at 36,500 BTU/hr to Plant City at 44,900.

Where Marathon's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Marathon Airport, 1.5 miles from the center of Marathon at 3 ft above sea level. That is close enough that the station's summers are Marathon's summers.

No other city in this set uses Marathon Airport, so this sizing is Marathon's alone.

The sun over Marathon in late July

At 24.73°N, the noon sun on 21 July stands 85.7° above the horizon over Marathon and the day runs 13 h 19 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.

10,039 people live in Marathon, the 172nd largest of the 173 Florida cities sized on this site. Coordinates used: 24.7323, -81.0251.

Marathon AC sizing questions