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

What size AC a 2,000 sq ft house in Missoula 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 Missoula International Airport (4.5 mi away). Jul is the hottest month, averaging a high of 85.4°F and a low of 51.5°F, with about 27 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 385 kWh a year at SEER2 14.3, or roughly $54 at Montana's average residential rate of 14.13¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in Missoula

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 Missoula'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 Missoula

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 Missoula

Jul alone uses 48% of the year's cooling energy. The unit runs the equivalent of 204 hours a year at full output.

Missoula against nearby cities

Of 8 Montana cities here, Missoula has the 3rd highest cooling load. The range runs from Butte-Silver Bow at 24,000 BTU/hr to Billings at 29,500.

Where Missoula's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Missoula International Airport, 4.5 miles from the center of Missoula at 3,195 ft above sea level. That is close enough that the station's summers are Missoula's summers.

No other city in this set uses Missoula International Airport, so this sizing is Missoula's alone.

The sun over Missoula in late July

At 46.87°N, the noon sun on 21 July stands 63.5° above the horizon over Missoula and the day runs 15 h 07 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.

78,204 people live in Missoula, the 2nd largest of the 8 Montana cities sized on this site. Coordinates used: 46.8744, -114.0261.

Missoula AC sizing questions