What size AC for Carlsbad, NM? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 41,400 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.5 tons. This is one of the climates the chart was written for.

If a contractor quotes you more than 3.5 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 580 square feet in this climate. For a single room, size by the room: 20.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Cavern City Airport (4.8 mi away). Jun is the hottest month, averaging a high of 96.2°F and a low of 65.9°F, with about 121 days a year at 90°F or more. That gives a design temperature of 104°F against 75°F inside.

What it costs to run

About 3,049 kWh a year at SEER2 14.3, or roughly $468 at New Mexico's average residential rate of 15.34¢ per kWh. Cooling runs 7 months of the year here.

What drives the load in Carlsbad

Of the 41,400 BTU/hr, 20,880 comes through walls, roof and windows on a 104°F afternoon (50%), 13,000 is sun on the house (31%), 2,120 is four people and their appliances, and 5,400 is moisture the coil has to wring out of Carlsbad's summer air (13%).

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 36,000 BTU/hr before the moisture allowance.

Other house sizes in Carlsbad

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,100 BTU/hr; a 350 sq ft living room needs about 7,200 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 12,400 BTU/hr.

Month by month in Carlsbad

Jul alone uses 24% of the year's cooling energy. The unit runs the equivalent of 1,053 hours a year at full output.

Carlsbad against nearby cities

Of 19 New Mexico cities here, Carlsbad has the 4th highest cooling load. The range runs from Las Vegas at 25,500 BTU/hr to Sunland Park at 44,800.

Where Carlsbad's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Cavern City Airport, 4.8 miles from the center of Carlsbad at 3,250 ft above sea level. That is close enough that the station's summers are Carlsbad's summers.

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

The sun over Carlsbad in late July

At 32.40°N, the noon sun on 21 July stands 78.0° above the horizon over Carlsbad and the day runs 13 h 49 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.

31,999 people live in Carlsbad, the 9th largest of the 19 New Mexico cities sized on this site. Coordinates used: 32.4008, -104.2410.

Carlsbad AC sizing questions