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

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

2.5 tons central AC or heat pump, about 29,500 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 42% 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 812 square feet in this climate. For a single room, size by the room: 14.8 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Billings International Airport (1.3 mi away). Jul is the hottest month, averaging a high of 87.3°F and a low of 59.3°F, with about 32 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 865 kWh a year at SEER2 14.3, or roughly $122 at Montana's average residential rate of 14.13¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Billings

Of the 29,500 BTU/hr, 12,240 comes through walls, roof and windows on a 92°F afternoon (41%), 13,000 is sun on the house (44%), 2,120 is four people and their appliances, and 2,200 is moisture the coil has to wring out of Billings's summer air (7%).

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 Billings

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,200 BTU/hr; a 350 sq ft living room needs about 5,200 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 8,900 BTU/hr.

Month by month in Billings

Jul alone uses 41% of the year's cooling energy. The unit runs the equivalent of 418 hours a year at full output.

Billings against nearby cities

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

Where Billings's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Billings International Airport, 1.3 miles from the center of Billings at 3,590 ft above sea level. That is close enough that the station's summers are Billings's summers.

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

The sun over Billings in late July

At 45.79°N, the noon sun on 21 July stands 64.6° above the horizon over Billings and the day runs 14 h 60 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.

121,483 people live in Billings, the largest of the 8 Montana cities sized on this site. Coordinates used: 45.7885, -108.5525.

Billings AC sizing questions