What size AC for Mount Vernon, WA? 1.5 tons for 2,000 sq ft

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

1.5 tons central AC or heat pump, about 15,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 167% 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 1.5 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 1,526 square feet in this climate. For a single room, size by the room: 7.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Whidbey Island NAS (16.9 mi away). Aug is the hottest month, averaging a high of 69.7°F and a low of 53.9°F, with about 0 days a year at 90°F or more. That gives a design temperature of 74°F against 75°F inside.

What it costs to run

About 31 kWh a year at SEER2 14.3, or roughly $4 at Washington's average residential rate of 14.42¢ per kWh. Cooling runs 0 months of the year here.

What drives the load in Mount Vernon

Of the 15,700 BTU/hr, 0 comes through walls, roof and windows on a 74°F afternoon (0%), 13,000 is sun on the house (83%), 2,120 is four people and their appliances, and 600 is moisture the coil has to wring out of Mount Vernon'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 15,120 BTU/hr before the moisture allowance.

Other house sizes in Mount Vernon

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,200 BTU/hr; a 350 sq ft living room needs about 2,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 4,700 BTU/hr.

Month by month in Mount Vernon

Aug alone uses 45% of the year's cooling energy. The unit runs the equivalent of 28 hours a year at full output.

Mount Vernon against nearby cities

Of 91 Washington cities here, Mount Vernon has the 86th highest cooling load. The range runs from Burlington at 15,700 BTU/hr to Kennewick at 32,900.

Where Mount Vernon's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Whidbey Island NAS, 16.9 miles from the center of Mount Vernon at 47 ft above sea level. That is close enough that the station's summers are Mount Vernon's summers.

3 other cities use the same station and get the same climate inputs. The closest to Mount Vernon: Burlington (3 mi away, 17.5 mi from the station), Anacortes (15 mi away, 9.8 mi from the station), Oak Harbor (17 mi away, 4.3 mi from the station). Mount Vernon is farther from the station than 2 of them.

The sun over Mount Vernon in late July

At 48.42°N, the noon sun on 21 July stands 62.0° above the horizon over Mount Vernon and the day runs 15 h 18 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.

35,344 people live in Mount Vernon, the 35th largest of the 91 Washington cities sized on this site. Coordinates used: 48.4171, -122.3118.

Mount Vernon AC sizing questions