What size AC for Lewiston, ID? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 32,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 29% 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 3 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 735 square feet in this climate. For a single room, size by the room: 16.3 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Lewiston Airport (1.7 mi away). Jul is the hottest month, averaging a high of 90.8°F and a low of 60.9°F, with about 44 days a year at 90°F or more. That gives a design temperature of 96°F against 75°F inside.

What it costs to run

About 1,191 kWh a year at SEER2 14.3, or roughly $155 at Idaho's average residential rate of 13.02¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in Lewiston

Of the 32,700 BTU/hr, 15,120 comes through walls, roof and windows on a 96°F afternoon (46%), 13,000 is sun on the house (40%), 2,120 is four people and their appliances, and 2,400 is moisture the coil has to wring out of Lewiston's summer air (7%).

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 30,240 BTU/hr before the moisture allowance.

Other house sizes in Lewiston

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,400 BTU/hr; a 350 sq ft living room needs about 5,700 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 9,800 BTU/hr.

Month by month in Lewiston

Jul alone uses 37% of the year's cooling energy. The unit runs the equivalent of 522 hours a year at full output.

Lewiston against nearby cities

Of 26 Idaho cities here, Lewiston has the 11th highest cooling load. The range runs from Sandpoint at 24,000 BTU/hr to Mountain Home at 36,500.

Where Lewiston's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Lewiston Airport, 1.7 miles from the center of Lewiston at 1,430 ft above sea level. That is close enough that the station's summers are Lewiston's summers.

No other city in this set uses Lewiston Airport, so this sizing is Lewiston's alone.

The sun over Lewiston in late July

At 46.39°N, the noon sun on 21 July stands 64.0° above the horizon over Lewiston and the day runs 15 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. Long days mean the house keeps gaining heat into the evening, well after the outdoor peak.

34,693 people live in Lewiston, the 11th largest of the 26 Idaho cities sized on this site. Coordinates used: 46.3931, -116.9935.

Lewiston AC sizing questions