What size AC for Boulder, CO? 1.5 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in Boulder 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 Boulder 14 W (15.3 mi away). Jul is the hottest month, averaging a high of 67.9°F and a low of 42.5°F, with about 0 days a year at 90°F or more. That gives a design temperature of 72°F against 75°F inside.

What it costs to run

About 0 kWh a year at SEER2 14.3, or roughly $0 at Colorado's average residential rate of 16.8¢ per kWh. Cooling runs 0 months of the year here.

What drives the load in Boulder

Of the 15,700 BTU/hr, 0 comes through walls, roof and windows on a 72°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 Boulder'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 Boulder

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 Boulder

Jan alone uses 0% of the year's cooling energy. The unit runs the equivalent of 0 hours a year at full output.

Boulder against nearby cities

Of 53 Colorado cities here, Boulder has the lowest cooling load. The range runs from Boulder at 15,700 BTU/hr to Grand Junction at 36,500.

Where Boulder's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Boulder 14 W, 15.3 miles from the center of Boulder at 9,829 ft above sea level. That is close enough that the station's summers are Boulder's summers.

No other city in this set uses Boulder 14 W, so this sizing is Boulder's alone.

The sun over Boulder in late July

At 40.02°N, the noon sun on 21 July stands 70.4° above the horizon over Boulder and the day runs 14 h 26 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.

106,803 people live in Boulder, the 12th largest of the 53 Colorado cities sized on this site. Coordinates used: 40.0244, -105.2513.

Boulder AC sizing questions