What size AC for SeaTac, WA? 2 tons for 2,000 sq ft

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

2 tons central AC or heat pump, about 21,000 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 100% 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 1,145 square feet in this climate. For a single room, size by the room: 10.5 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Seattle Tacoma Airport (0.8 mi away). Aug is the hottest month, averaging a high of 77.6°F and a low of 57.2°F, with about 4 days a year at 90°F or more. That gives a design temperature of 82°F against 75°F inside.

What it costs to run

About 333 kWh a year at SEER2 14.3, or roughly $48 at Washington's average residential rate of 14.42¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in SeaTac

Of the 21,000 BTU/hr, 5,040 comes through walls, roof and windows on a 82°F afternoon (24%), 13,000 is sun on the house (62%), 2,120 is four people and their appliances, and 800 is moisture the coil has to wring out of SeaTac'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 20,160 BTU/hr before the moisture allowance.

Other house sizes in SeaTac

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,600 BTU/hr; a 350 sq ft living room needs about 3,700 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 6,300 BTU/hr.

Month by month in SeaTac

Aug alone uses 37% of the year's cooling energy. The unit runs the equivalent of 227 hours a year at full output.

SeaTac against nearby cities

Of 91 Washington cities here, SeaTac has the 41st highest cooling load. The range runs from Burlington at 15,700 BTU/hr to Kennewick at 32,900.

Where SeaTac's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Seattle Tacoma Airport, 0.8 miles from the center of SeaTac at 369 ft above sea level. That is close enough that the station's summers are SeaTac's summers.

8 other cities use the same station and get the same climate inputs. The closest to SeaTac: Tukwila (3 mi away, 2.8 mi from the station), Burien (3 mi away, 2.6 mi from the station), Des Moines (4 mi away, 3.9 mi from the station), Kent (5 mi away, 6 mi from the station), Federal Way (9 mi away, 9.3 mi from the station). SeaTac sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over SeaTac in late July

At 47.44°N, the noon sun on 21 July stands 63.0° above the horizon over SeaTac and the day runs 15 h 11 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.

34,024 people live in SeaTac, the 37th largest of the 91 Washington cities sized on this site. Coordinates used: 47.4434, -122.2983.

SeaTac AC sizing questions