What size AC for Mountain View, CA? 2 tons for 2,000 sq ft

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

2 tons central AC or heat pump, about 21,000 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 100% 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 2 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 1,145 square feet in this climate. For a single room, size by the room: 10.5 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Moffett Fed Airfield (1.8 mi away). Sep is the hottest month, averaging a high of 77.7°F and a low of 57.3°F, with about 9 days a year at 90°F or more. That gives a design temperature of 82°F against 75°F inside.

What it costs to run

About 537 kWh a year at SEER2 14.3, or roughly $175 at California's average residential rate of 32.58¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Mountain View

Of the 21,000 BTU/hr, 5,040 comes through walls, roof and windows on a 82°F afternoon (24%), 13,000 is sun on the house (62%), 2,120 is four people and their appliances, and 800 is moisture the coil has to wring out of Mountain View'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 20,160 BTU/hr before the moisture allowance.

Other house sizes in Mountain View

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,600 BTU/hr; a 350 sq ft living room needs about 3,700 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 6,300 BTU/hr.

Month by month in Mountain View

Aug alone uses 23% of the year's cooling energy. The unit runs the equivalent of 366 hours a year at full output.

Mountain View against nearby cities

Of 376 California cities here, Mountain View has the 311th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where Mountain View's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Moffett Fed Airfield, 1.8 miles from the center of Mountain View at 39 ft above sea level. That is close enough that the station's summers are Mountain View's summers.

8 other cities use the same station and get the same climate inputs. The closest to Mountain View: Los Altos (2 mi away, 3.7 mi from the station), Sunnyvale (3 mi away, 1.8 mi from the station), Palo Alto (4 mi away, 5.3 mi from the station), East Palo Alto (6 mi away, 6.3 mi from the station), Cupertino (6 mi away, 6 mi from the station). Mountain View sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Mountain View in late July

At 37.40°N, the noon sun on 21 July stands 73.0° above the horizon over Mountain View and the day runs 14 h 12 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.

87,316 people live in Mountain View, the 93rd largest of the 376 California cities sized on this site. Coordinates used: 37.3997, -122.0793.

Mountain View AC sizing questions