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

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

2.5 tons central AC or heat pump, about 26,200 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 60% 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 916 square feet in this climate. For a single room, size by the room: 13.1 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Great Falls Airport (4.4 mi away). Jul is the hottest month, averaging a high of 84.3°F and a low of 51.4°F, with about 22 days a year at 90°F or more. That gives a design temperature of 89°F against 75°F inside.

What it costs to run

About 390 kWh a year at SEER2 14.3, or roughly $55 at Montana's average residential rate of 14.13¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in Great Falls

Of the 26,200 BTU/hr, 10,080 comes through walls, roof and windows on a 89°F afternoon (38%), 13,000 is sun on the house (50%), 2,120 is four people and their appliances, and 1,000 is moisture the coil has to wring out of Great Falls'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 25,200 BTU/hr before the moisture allowance.

Other house sizes in Great Falls

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 4,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 7,900 BTU/hr.

Month by month in Great Falls

Jul alone uses 44% of the year's cooling energy. The unit runs the equivalent of 213 hours a year at full output.

Great Falls against nearby cities

Of 8 Montana cities here, Great Falls has the 6th highest cooling load. The range runs from Butte-Silver Bow at 24,000 BTU/hr to Billings at 29,500.

Where Great Falls's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Great Falls Airport, 4.4 miles from the center of Great Falls at 3,667 ft above sea level. That is close enough that the station's summers are Great Falls's summers.

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

The sun over Great Falls in late July

At 47.50°N, the noon sun on 21 July stands 62.9° above the horizon over Great Falls and the day runs 15 h 12 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.

60,013 people live in Great Falls, the 3rd largest of the 8 Montana cities sized on this site. Coordinates used: 47.5025, -111.3000.

Great Falls AC sizing questions