What size AC for Herndon, VA? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 32,300 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 30% 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 3 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 743 square feet in this climate. For a single room, size by the room: 16.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Washington Dulles International Airport (4.1 mi away). Jul is the hottest month, averaging a high of 87.6°F and a low of 66.8°F, with about 33 days a year at 90°F or more. That gives a design temperature of 93°F against 75°F inside.

What it costs to run

About 1,713 kWh a year at SEER2 14.3, or roughly $290 at Virginia's average residential rate of 16.95¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Herndon

Of the 32,300 BTU/hr, 12,960 comes through walls, roof and windows on a 93°F afternoon (40%), 13,000 is sun on the house (40%), 2,120 is four people and their appliances, and 4,200 is moisture the coil has to wring out of Herndon's summer air (13%).

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 28,080 BTU/hr before the moisture allowance.

Other house sizes in Herndon

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,400 BTU/hr; a 350 sq ft living room needs about 5,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 9,700 BTU/hr.

Month by month in Herndon

Jul alone uses 31% of the year's cooling energy. The unit runs the equivalent of 759 hours a year at full output.

Herndon against nearby cities

Of 39 Virginia cities here, Herndon has the 23rd highest cooling load. The range runs from Christiansburg at 24,900 BTU/hr to Chesapeake at 42,100.

Where Herndon's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Washington Dulles International Airport, 4.1 miles from the center of Herndon at 294 ft above sea level. That is close enough that the station's summers are Herndon's summers.

5 other cities use the same station and get the same climate inputs. The closest to Herndon: Vienna (9 mi away, 10.5 mi from the station), Fairfax (9 mi away, 9.8 mi from the station), Leesburg (13 mi away, 13.3 mi from the station), Manassas Park (14 mi away, 11.4 mi from the station), Manassas (16 mi away, 13.1 mi from the station). Herndon sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Herndon in late July

At 38.97°N, the noon sun on 21 July stands 71.4° above the horizon over Herndon and the day runs 14 h 20 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.

25,180 people live in Herndon, the 24th largest of the 39 Virginia cities sized on this site. Coordinates used: 38.9704, -77.3871.

Herndon AC sizing questions