What size AC for Flagstaff, AZ? 2 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in Flagstaff 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 Flagstaff Airport (3.8 mi away). Jul is the hottest month, averaging a high of 82°F and a low of 51.4°F, with about 3 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 182 kWh a year at SEER2 14.3, or roughly $28 at Arizona's average residential rate of 15.36¢ per kWh. Cooling runs 1 month of the year here.

What drives the load in Flagstaff

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 Flagstaff'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 Flagstaff

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 Flagstaff

Jul alone uses 52% of the year's cooling energy. The unit runs the equivalent of 108 hours a year at full output.

Flagstaff against nearby cities

Of 44 Arizona cities here, Flagstaff has the lowest cooling load. The range runs from Flagstaff at 24,000 BTU/hr to Bullhead City at 60,800.

Where Flagstaff's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Flagstaff Airport, 3.8 miles from the center of Flagstaff at 6,999 ft above sea level. That is close enough that the station's summers are Flagstaff's summers.

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

The sun over Flagstaff in late July

At 35.19°N, the noon sun on 21 July stands 75.2° above the horizon over Flagstaff and the day runs 14 h 02 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. Shorter days give the house a longer night to shed the day's heat.

77,539 people live in Flagstaff, the 16th largest of the 44 Arizona cities sized on this site. Coordinates used: 35.1852, -111.6207.

Flagstaff AC sizing questions