What size AC for Johnston, IA? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 29,800 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 41% 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 805 square feet in this climate. For a single room, size by the room: 14.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Des Moines International Airport (11.5 mi away). Jul is the hottest month, averaging a high of 85.6°F and a low of 66.4°F, with about 22 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 1,487 kWh a year at SEER2 14.3, or roughly $216 at Iowa's average residential rate of 14.55¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Johnston

Of the 29,800 BTU/hr, 10,800 comes through walls, roof and windows on a 90°F afternoon (36%), 13,000 is sun on the house (44%), 2,120 is four people and their appliances, and 3,900 is moisture the coil has to wring out of Johnston's summer air (13%).

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 Johnston

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 Johnston

Jul alone uses 32% of the year's cooling energy. The unit runs the equivalent of 714 hours a year at full output.

Johnston against nearby cities

Of 41 Iowa cities here, Johnston has the 11th highest cooling load. The range runs from Dubuque at 24,900 BTU/hr to Council Bluffs at 34,300.

Where Johnston's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Des Moines International Airport, 11.5 miles from the center of Johnston at 939 ft above sea level. That is close enough that the station's summers are Johnston's summers.

10 other cities use the same station and get the same climate inputs. The closest to Johnston: Grimes (4 mi away, 12.6 mi from the station), Urbandale (5 mi away, 9.7 mi from the station), Clive (7 mi away, 9.3 mi from the station), Ankeny (7 mi away, 13.9 mi from the station), Des Moines (10 mi away, 3.5 mi from the station). Johnston is farther from the station than 6 of them.

The sun over Johnston in late July

At 41.69°N, the noon sun on 21 July stands 68.7° above the horizon over Johnston and the day runs 14 h 35 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.

25,022 people live in Johnston, the 20th largest of the 41 Iowa cities sized on this site. Coordinates used: 41.6919, -93.7245.

Johnston AC sizing questions