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

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

2 tons central AC or heat pump, about 21,800 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 93% 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 1,102 square feet in this climate. For a single room, size by the room: 10.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Seattle Boeing Field (4.1 mi away). Aug is the hottest month, averaging a high of 77.4°F and a low of 58.1°F, with about 3 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 368 kWh a year at SEER2 14.3, or roughly $53 at Washington's average residential rate of 14.42¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in Mercer Island

Of the 21,800 BTU/hr, 5,040 comes through walls, roof and windows on a 82°F afternoon (23%), 13,000 is sun on the house (60%), 2,120 is four people and their appliances, and 1,600 is moisture the coil has to wring out of Mercer Island's summer air (7%).

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 Mercer Island

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 3,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 6,500 BTU/hr.

Month by month in Mercer Island

Aug alone uses 37% of the year's cooling energy. The unit runs the equivalent of 242 hours a year at full output.

Mercer Island against nearby cities

Of 91 Washington cities here, Mercer Island has the 27th highest cooling load. The range runs from Burlington at 15,700 BTU/hr to Kennewick at 32,900.

Where Mercer Island's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Seattle Boeing Field, 4.1 miles from the center of Mercer Island at 25 ft above sea level. That is close enough that the station's summers are Mercer Island's summers.

3 other cities use the same station and get the same climate inputs. The closest to Mercer Island: Seattle (7 mi away, 5.4 mi from the station), Bainbridge Island (14 mi away, 11.2 mi from the station), Port Orchard (19 mi away, 14.8 mi from the station). Mercer Island sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Mercer Island in late July

At 47.56°N, the noon sun on 21 July stands 62.8° above the horizon over Mercer Island and the day runs 15 h 12 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.

25,302 people live in Mercer Island, the 45th largest of the 91 Washington cities sized on this site. Coordinates used: 47.5640, -122.2312.

Mercer Island AC sizing questions