What size AC for Knightdale, NC? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 37,800 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.1 tons. This is one of the climates the chart was written for.

If a contractor quotes you more than 3.5 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 635 square feet in this climate. For a single room, size by the room: 18.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Raleigh Airport (17.4 mi away). Jul is the hottest month, averaging a high of 90.8°F and a low of 70.2°F, with about 52 days a year at 90°F or more. That gives a design temperature of 97°F against 75°F inside.

What it costs to run

About 2,660 kWh a year at SEER2 14.3, or roughly $397 at North Carolina's average residential rate of 14.94¢ per kWh. Cooling runs 7 months of the year here.

What drives the load in Knightdale

Of the 37,800 BTU/hr, 15,840 comes through walls, roof and windows on a 97°F afternoon (42%), 13,000 is sun on the house (34%), 2,120 is four people and their appliances, and 6,800 is moisture the coil has to wring out of Knightdale's summer air (18%).

Heat through the shell is the biggest share here, so insulation and air sealing cut the size you need more than shading would. Sensible heat totals 30,960 BTU/hr before the moisture allowance.

Other house sizes in Knightdale

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,800 BTU/hr; a 350 sq ft living room needs about 6,600 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 11,300 BTU/hr.

Month by month in Knightdale

Jul alone uses 27% of the year's cooling energy. The unit runs the equivalent of 1,007 hours a year at full output.

Knightdale against nearby cities

Of 91 North Carolina cities here, Knightdale has the 22nd highest cooling load. The range runs from Boone at 21,700 BTU/hr to Lumberton at 40,200.

Where Knightdale's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Raleigh Airport, 17.4 miles from the center of Knightdale at 395 ft above sea level. That is close enough that the station's summers are Knightdale's summers.

11 other cities use the same station and get the same climate inputs. The closest to Knightdale: Wendell (8 mi away, 25.1 mi from the station), Raleigh (9 mi away, 8.9 mi from the station), Rolesville (9 mi away, 17.9 mi from the station), Garner (10 mi away, 16.9 mi from the station), Wake Forest (12 mi away, 15.9 mi from the station). Knightdale is farther from the station than 8 of them.

The sun over Knightdale in late July

At 35.79°N, the noon sun on 21 July stands 74.6° above the horizon over Knightdale and the day runs 14 h 04 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.

21,829 people live in Knightdale, the 47th largest of the 91 North Carolina cities sized on this site. Coordinates used: 35.7917, -78.4965.

Knightdale AC sizing questions