What size AC for Nampa, ID? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 35,000 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 20% 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 686 square feet in this climate. For a single room, size by the room: 17.5 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Boise Air Terminal (16.2 mi away). Jul is the hottest month, averaging a high of 92.7°F and a low of 61.9°F, with about 55 days a year at 90°F or more. That gives a design temperature of 99°F against 75°F inside.

What it costs to run

About 1,364 kWh a year at SEER2 14.3, or roughly $178 at Idaho's average residential rate of 13.02¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Nampa

Of the 35,000 BTU/hr, 17,280 comes through walls, roof and windows on a 99°F afternoon (49%), 13,000 is sun on the house (37%), 2,120 is four people and their appliances, and 2,600 is moisture the coil has to wring out of Nampa's summer air (7%).

Heat through the shell is the biggest share here, so insulation and air sealing cut the size you need more than shading would. Sensible heat totals 32,400 BTU/hr before the moisture allowance.

Other house sizes in Nampa

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,600 BTU/hr; a 350 sq ft living room needs about 6,100 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 10,500 BTU/hr.

Month by month in Nampa

Jul alone uses 37% of the year's cooling energy. The unit runs the equivalent of 557 hours a year at full output.

Nampa against nearby cities

Of 26 Idaho cities here, Nampa has the 4th highest cooling load. The range runs from Sandpoint at 24,000 BTU/hr to Mountain Home at 36,500.

Where Nampa's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Boise Air Terminal, 16.2 miles from the center of Nampa at 2,823 ft above sea level. That is close enough that the station's summers are Nampa's summers.

7 other cities use the same station and get the same climate inputs. The closest to Nampa: Meridian (8 mi away, 8.6 mi from the station), Star (9 mi away, 15.7 mi from the station), Middleton (9 mi away, 21.2 mi from the station), Kuna (9 mi away, 8.7 mi from the station), Eagle (11 mi away, 12.4 mi from the station). Nampa is farther from the station than 6 of them.

The sun over Nampa in late July

At 43.59°N, the noon sun on 21 July stands 66.8° above the horizon over Nampa and the day runs 14 h 46 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.

117,350 people live in Nampa, the 3rd largest of the 26 Idaho cities sized on this site. Coordinates used: 43.5864, -116.5639.

Nampa AC sizing questions