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

What size AC a 2,000 sq ft house in Cambridge 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 Patuxent River NAS (25.4 mi away). Jul is the hottest month, averaging a high of 87.3°F and a low of 70.9°F, with about 26 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 2,222 kWh a year at SEER2 14.3, or roughly $474 at Maryland's average residential rate of 21.31¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Cambridge

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 Cambridge'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 Cambridge

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 Cambridge

Jul alone uses 29% of the year's cooling energy. The unit runs the equivalent of 952 hours a year at full output.

Cambridge against nearby cities

Of 23 Maryland cities here, Cambridge has the 14th highest cooling load. The range runs from Westminster at 28,000 BTU/hr to Baltimore at 39,300.

Where Cambridge's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Patuxent River NAS, 25.4 miles from the center of Cambridge at 39 ft above sea level. That is a long way for a weather station, so treat the design day as approximate: a city on the coast side or the high side of the station can run several degrees different.

No other city in this set uses Patuxent River NAS, so this sizing is Cambridge's alone.

The sun over Cambridge in late July

At 38.55°N, the noon sun on 21 July stands 71.8° above the horizon over Cambridge and the day runs 14 h 18 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.

13,262 people live in Cambridge, the 21st largest of the 23 Maryland cities sized on this site. Coordinates used: 38.5541, -76.0771.

Cambridge AC sizing questions