What size AC for Bozeman, MT? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 27,000 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 56% 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 890 square feet in this climate. For a single room, size by the room: 13.5 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Bozeman Gallatin Field Airport (8.9 mi away). Jul is the hottest month, averaging a high of 85.4°F and a low of 49.4°F, with about 23 days a year at 90°F or more. That gives a design temperature of 90°F against 75°F inside.

What it costs to run

About 292 kWh a year at SEER2 14.3, or roughly $41 at Montana's average residential rate of 14.13¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in Bozeman

Of the 27,000 BTU/hr, 10,800 comes through walls, roof and windows on a 90°F afternoon (40%), 13,000 is sun on the house (48%), 2,120 is four people and their appliances, and 1,000 is moisture the coil has to wring out of Bozeman'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 25,920 BTU/hr before the moisture allowance.

Other house sizes in Bozeman

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,000 BTU/hr; a 350 sq ft living room needs about 4,700 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,100 BTU/hr.

Month by month in Bozeman

Jul alone uses 50% of the year's cooling energy. The unit runs the equivalent of 155 hours a year at full output.

Bozeman against nearby cities

Of 8 Montana cities here, Bozeman has the 4th highest cooling load. The range runs from Butte-Silver Bow at 24,000 BTU/hr to Billings at 29,500.

Where Bozeman's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Bozeman Gallatin Field Airport, 8.9 miles from the center of Bozeman at 4,431 ft above sea level. That is close enough that the station's summers are Bozeman's summers.

1 other city uses the same station and get the same climate inputs. The closest to Bozeman: Belgrade (9 mi away, 0.9 mi from the station). Bozeman is farther from the station than 1 of them.

The sun over Bozeman in late July

At 45.68°N, the noon sun on 21 July stands 64.7° above the horizon over Bozeman and the day runs 14 h 59 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.

57,894 people live in Bozeman, the 4th largest of the 8 Montana cities sized on this site. Coordinates used: 45.6832, -111.0538.

Bozeman AC sizing questions