What size AC for Rockville, MD? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 33,400 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 26% 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 719 square feet in this climate. For a single room, size by the room: 16.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Rockville 1 Ne (1.2 mi away). Jul is the hottest month, averaging a high of 87.1°F and a low of 68.8°F, with about 23 days a year at 90°F or more. That gives a design temperature of 92°F against 75°F inside.

What it costs to run

About 1,885 kWh a year at SEER2 14.3, or roughly $402 at Maryland's average residential rate of 21.31¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Rockville

Of the 33,400 BTU/hr, 12,240 comes through walls, roof and windows on a 92°F afternoon (37%), 13,000 is sun on the house (39%), 2,120 is four people and their appliances, and 6,000 is moisture the coil has to wring out of Rockville's summer air (18%).

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 27,360 BTU/hr before the moisture allowance.

Other house sizes in Rockville

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,500 BTU/hr; a 350 sq ft living room needs about 5,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 10,000 BTU/hr.

Month by month in Rockville

Jul alone uses 31% of the year's cooling energy. The unit runs the equivalent of 808 hours a year at full output.

Rockville against nearby cities

Of 23 Maryland cities here, Rockville has the 12th highest cooling load. The range runs from Westminster at 28,000 BTU/hr to Baltimore at 39,300.

Where Rockville's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Rockville 1 Ne, 1.2 miles from the center of Rockville at 440 ft above sea level. That is close enough that the station's summers are Rockville's summers.

1 other city uses the same station and get the same climate inputs. The closest to Rockville: Gaithersburg (5 mi away, 4.1 mi from the station). Rockville sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Rockville in late July

At 39.08°N, the noon sun on 21 July stands 71.3° above the horizon over Rockville and the day runs 14 h 21 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.

68,417 people live in Rockville, the 4th largest of the 23 Maryland cities sized on this site. Coordinates used: 39.0837, -77.1556.

Rockville AC sizing questions