What size AC for Blackfoot, ID? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 28,500 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 48% 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 843 square feet in this climate. For a single room, size by the room: 14.2 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Blackfoot Fire Dept (0.2 mi away). Jul is the hottest month, averaging a high of 86.9°F and a low of 56°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 652 kWh a year at SEER2 14.3, or roughly $85 at Idaho's average residential rate of 13.02¢ per kWh. Cooling runs 3 months of the year here.

What drives the load in Blackfoot

Of the 28,500 BTU/hr, 12,240 comes through walls, roof and windows on a 92°F afternoon (43%), 13,000 is sun on the house (46%), 2,120 is four people and their appliances, and 1,100 is moisture the coil has to wring out of Blackfoot's summer air (4%).

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 Blackfoot

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 5,000 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,500 BTU/hr.

Month by month in Blackfoot

Jul alone uses 42% of the year's cooling energy. The unit runs the equivalent of 327 hours a year at full output.

Blackfoot against nearby cities

Of 26 Idaho cities here, Blackfoot has the 18th highest cooling load. The range runs from Sandpoint at 24,000 BTU/hr to Mountain Home at 36,500.

Where Blackfoot's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Blackfoot Fire Dept, 0.2 miles from the center of Blackfoot at 4,499 ft above sea level. That is close enough that the station's summers are Blackfoot's summers.

No other city in this set uses Blackfoot Fire Dept, so this sizing is Blackfoot's alone.

The sun over Blackfoot in late July

At 43.19°N, the noon sun on 21 July stands 67.2° above the horizon over Blackfoot 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.

13,224 people live in Blackfoot, the 21st largest of the 26 Idaho cities sized on this site. Coordinates used: 43.1946, -112.3468.

Blackfoot AC sizing questions