What size AC for Portsmouth, NH? 2 tons for 2,000 sq ft

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

2 tons central AC or heat pump, about 24,100 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 74% 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 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 996 square feet in this climate. For a single room, size by the room: 12.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Durham 2 Ssw (9.1 mi away). Jul is the hottest month, averaging a high of 81.1°F and a low of 58.3°F, with about 8 days a year at 90°F or more. That gives a design temperature of 85°F against 75°F inside.

What it costs to run

About 530 kWh a year at SEER2 14.3, or roughly $143 at New Hampshire's average residential rate of 26.91¢ per kWh. Cooling runs 3 months of the year here.

What drives the load in Portsmouth

Of the 24,100 BTU/hr, 7,200 comes through walls, roof and windows on a 85°F afternoon (30%), 13,000 is sun on the house (54%), 2,120 is four people and their appliances, and 1,800 is moisture the coil has to wring out of Portsmouth's summer air (7%).

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 22,320 BTU/hr before the moisture allowance.

Other house sizes in Portsmouth

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,800 BTU/hr; a 350 sq ft living room needs about 4,200 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 7,300 BTU/hr.

Month by month in Portsmouth

Jul alone uses 39% of the year's cooling energy. The unit runs the equivalent of 314 hours a year at full output.

Portsmouth against nearby cities

Of 11 New Hampshire cities here, Portsmouth has the 9th highest cooling load. The range runs from Claremont at 24,000 BTU/hr to Manchester at 27,200.

Where Portsmouth's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Durham 2 Ssw, 9.1 miles from the center of Portsmouth at 63 ft above sea level. That is close enough that the station's summers are Portsmouth's summers.

No other city in this set uses Durham 2 Ssw, so this sizing is Portsmouth's alone.

The sun over Portsmouth in late July

At 43.06°N, the noon sun on 21 July stands 67.3° above the horizon over Portsmouth and the day runs 14 h 43 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.

22,938 people live in Portsmouth, the 7th largest of the 11 New Hampshire cities sized on this site. Coordinates used: 43.0606, -70.7803.

Portsmouth AC sizing questions