What size AC for Modesto, CA? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 37,300 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 643 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 Modesto Airport (2.8 mi away). Jul is the hottest month, averaging a high of 95°F and a low of 62.7°F, with about 90 days a year at 90°F or more. That gives a design temperature of 102°F against 75°F inside.

What it costs to run

About 2,119 kWh a year at SEER2 14.3, or roughly $690 at California's average residential rate of 32.58¢ per kWh. Cooling runs 6 months of the year here.

What drives the load in Modesto

Of the 37,300 BTU/hr, 19,440 comes through walls, roof and windows on a 102°F afternoon (52%), 13,000 is sun on the house (35%), 2,120 is four people and their appliances, and 2,800 is moisture the coil has to wring out of Modesto's summer air (8%).

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 34,560 BTU/hr before the moisture allowance.

Other house sizes in Modesto

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 Modesto

Jul alone uses 26% of the year's cooling energy. The unit runs the equivalent of 812 hours a year at full output.

Modesto against nearby cities

Of 376 California cities here, Modesto has the 95th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where Modesto's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Modesto Airport, 2.8 miles from the center of Modesto at 87 ft above sea level. That is close enough that the station's summers are Modesto's summers.

5 other cities use the same station and get the same climate inputs. The closest to Modesto: Ceres (3 mi away, 2 mi from the station), Riverbank (7 mi away, 7 mi from the station), Turlock (12 mi away, 9.8 mi from the station), Oakdale (12 mi away, 11.1 mi from the station), Patterson (14 mi away, 15 mi from the station). Modesto is farther from the station than 1 of them.

The sun over Modesto in late July

At 37.64°N, the noon sun on 21 July stands 72.8° above the horizon over Modesto 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.

220,592 people live in Modesto, the 19th largest of the 376 California cities sized on this site. Coordinates used: 37.6375, -121.0030.

Modesto AC sizing questions