What size AC for Butte-Silver Bow, MT? 2 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in Butte-Silver Bow needs

2 tons central AC or heat pump, about 24,000 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 75% 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 1,002 square feet in this climate. For a single room, size by the room: 12 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Butte Bert Mooney Airport (9 mi away). Jul is the hottest month, averaging a high of 81.3°F and a low of 45.9°F, with about 8 days a year at 90°F or more. That gives a design temperature of 86°F against 75°F inside.

What it costs to run

About 104 kWh a year at SEER2 14.3, or roughly $15 at Montana's average residential rate of 14.13¢ per kWh. Cooling runs 0 months of the year here.

What drives the load in Butte-Silver Bow

Of the 24,000 BTU/hr, 7,920 comes through walls, roof and windows on a 86°F afternoon (33%), 13,000 is sun on the house (54%), 2,120 is four people and their appliances, and 900 is moisture the coil has to wring out of Butte-Silver Bow'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 23,040 BTU/hr before the moisture allowance.

Other house sizes in Butte-Silver Bow

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 7,200 BTU/hr.

Month by month in Butte-Silver Bow

Jul alone uses 58% of the year's cooling energy. The unit runs the equivalent of 62 hours a year at full output.

Butte-Silver Bow against nearby cities

Of 8 Montana cities here, Butte-Silver Bow has the lowest cooling load. The range runs from Butte-Silver Bow at 24,000 BTU/hr to Billings at 29,500.

Where Butte-Silver Bow's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Butte Bert Mooney Airport, 9 miles from the center of Butte-Silver Bow at 5,505 ft above sea level. That is close enough that the station's summers are Butte-Silver Bow's summers.

No other city in this set uses Butte Bert Mooney Airport, so this sizing is Butte-Silver Bow's alone.

The sun over Butte-Silver Bow in late July

At 45.90°N, the noon sun on 21 July stands 64.5° above the horizon over Butte-Silver Bow and the day runs 15 h 01 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.

35,480 people live in Butte-Silver Bow, the 5th largest of the 8 Montana cities sized on this site. Coordinates used: 45.8962, -112.6601.

Butte-Silver Bow AC sizing questions