What size AC for South San Francisco, CA? 1.5 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in South San Francisco needs

1.5 tons central AC or heat pump, about 18,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 124% more than this climate needs. An oversized unit cools the air fast, shuts off before it pulls moisture out, and leaves the house cold and clammy.

If a contractor quotes you more than 1.5 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 1,282 square feet in this climate. For a single room, size by the room: 9.4 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at San Francisco International Airport (2.5 mi away). Sep is the hottest month, averaging a high of 74.8°F and a low of 55.9°F, with about 4 days a year at 90°F or more. That gives a design temperature of 79°F against 75°F inside.

What it costs to run

About 252 kWh a year at SEER2 14.3, or roughly $82 at California's average residential rate of 32.58¢ per kWh. Cooling runs 1 month of the year here.

What drives the load in South San Francisco

Of the 18,700 BTU/hr, 2,880 comes through walls, roof and windows on a 79°F afternoon (15%), 13,000 is sun on the house (70%), 2,120 is four people and their appliances, and 700 is moisture the coil has to wring out of South San Francisco's summer air (4%).

Sun is the biggest share here, more than heat through the walls, so shading west windows and a light roof cut the size you need more than extra insulation would. Sensible heat totals 18,000 BTU/hr before the moisture allowance.

Other house sizes in South San Francisco

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,400 BTU/hr; a 350 sq ft living room needs about 3,300 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 5,600 BTU/hr.

Month by month in South San Francisco

Sep alone uses 28% of the year's cooling energy. The unit runs the equivalent of 192 hours a year at full output.

South San Francisco against nearby cities

Of 376 California cities here, South San Francisco has the 340th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where South San Francisco's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is San Francisco International Airport, 2.5 miles from the center of South San Francisco at 10 ft above sea level. That is close enough that the station's summers are South San Francisco's summers.

11 other cities use the same station and get the same climate inputs. The closest to South San Francisco: San Bruno (4 mi away, 3.6 mi from the station), Millbrae (4 mi away, 2.5 mi from the station), Burlingame (5 mi away, 2.1 mi from the station), Pacifica (6 mi away, 6.2 mi from the station), Hillsborough (7 mi away, 4.3 mi from the station). South San Francisco is farther from the station than 1 of them.

The sun over South San Francisco in late July

At 37.66°N, the noon sun on 21 July stands 72.7° above the horizon over South San Francisco and the day runs 14 h 13 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.

64,660 people live in South San Francisco, the 138th largest of the 376 California cities sized on this site. Coordinates used: 37.6551, -122.3762.

South San Francisco AC sizing questions