What size AC for Chelsea, MA? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 27,300 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 54% 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 2.5 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 878 square feet in this climate. For a single room, size by the room: 13.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Boston (2.7 mi away). Jul is the hottest month, averaging a high of 82.1°F and a low of 66°F, with about 14 days a year at 90°F or more. That gives a design temperature of 87°F against 75°F inside.

What it costs to run

About 1,114 kWh a year at SEER2 14.3, or roughly $336 at Massachusetts's average residential rate of 30.19¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Chelsea

Of the 27,300 BTU/hr, 8,640 comes through walls, roof and windows on a 87°F afternoon (32%), 13,000 is sun on the house (48%), 2,120 is four people and their appliances, and 3,600 is moisture the coil has to wring out of Chelsea'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 23,760 BTU/hr before the moisture allowance.

Other house sizes in Chelsea

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,100 BTU/hr; a 350 sq ft living room needs about 4,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,200 BTU/hr.

Month by month in Chelsea

Jul alone uses 35% of the year's cooling energy. The unit runs the equivalent of 583 hours a year at full output.

Chelsea against nearby cities

Of 58 Massachusetts cities here, Chelsea has the 14th highest cooling load. The range runs from Pittsfield at 22,600 BTU/hr to Framingham at 28,000.

Where Chelsea's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Boston, 2.7 miles from the center of Chelsea at 11 ft above sea level. That is close enough that the station's summers are Chelsea's summers.

10 other cities use the same station and get the same climate inputs. The closest to Chelsea: Everett (1 mi away, 3.9 mi from the station), Malden (3 mi away, 5.4 mi from the station), Revere (3 mi away, 4.3 mi from the station), Winthrop Town (3 mi away, 2.2 mi from the station), Somerville (4 mi away, 5.1 mi from the station). Chelsea is farther from the station than 2 of them.

The sun over Chelsea in late July

At 42.40°N, the noon sun on 21 July stands 68.0° above the horizon over Chelsea and the day runs 14 h 39 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. Long days mean the house keeps gaining heat into the evening, well after the outdoor peak.

40,245 people live in Chelsea, the 37th largest of the 58 Massachusetts cities sized on this site. Coordinates used: 42.3968, -71.0313.

Chelsea AC sizing questions