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

What size AC a 2,000 sq ft house in Ottumwa 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 Ottumwa Ind Airport (5.5 mi away). Jul is the hottest month, averaging a high of 85.7°F and a low of 64.5°F, with about 19 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 1,368 kWh a year at SEER2 14.3, or roughly $199 at Iowa's average residential rate of 14.55¢ per kWh. Cooling runs 5 months of the year here.

What drives the load in Ottumwa

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 Ottumwa'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 Ottumwa

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 Ottumwa

Jul alone uses 32% of the year's cooling energy. The unit runs the equivalent of 656 hours a year at full output.

Ottumwa against nearby cities

Of 41 Iowa cities here, Ottumwa has the 10th highest cooling load. The range runs from Dubuque at 24,900 BTU/hr to Council Bluffs at 34,300.

Where Ottumwa's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Ottumwa Ind Airport, 5.5 miles from the center of Ottumwa at 834 ft above sea level. That is close enough that the station's summers are Ottumwa's summers.

1 other city uses the same station and get the same climate inputs. The closest to Ottumwa: Oskaloosa (22 mi away, 16.7 mi from the station). Ottumwa sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Ottumwa in late July

At 41.02°N, the noon sun on 21 July stands 69.4° above the horizon over Ottumwa and the day runs 14 h 31 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.

25,648 people live in Ottumwa, the 19th largest of the 41 Iowa cities sized on this site. Coordinates used: 41.0223, -92.4233.

Ottumwa AC sizing questions