Humid or Dry: How Summer Moisture Changes the AC Size You Need

Two kinds of heat

An air conditioner removes two things: heat you can feel (sensible) and water vapour (latent). The thermostat only reads the first. In a humid summer a real share of the coil's work goes into turning vapour back into water that drains away.

Reading humidity from the normals

NOAA normals do not include dew point at every station, but overnight lows do the job: humid air holds heat after dark, dry air drops it. ACSizeCheck adds a moisture allowance on top of sensible load by the hottest month's average low:

Same heat, different size

Cities with the same design-day temperature can still need different units. New York, NY and Eugene, OR both design for 89°F, but New York's 70.1°F nights put its load at 30,700 BTU/hr against 26,200 in Eugene, where nights fall to 51.5°F. Houston, TX and El Paso de Robles (Paso Robles), CA both design for 101°F, but Houston's 76.7°F nights put its load at 44,000 BTU/hr against 35,200 in El Paso de Robles (Paso Robles), where nights fall to 53.2°F. Philadelphia, PA and Bend, OR both design for 93°F, but Philadelphia's 69.4°F nights put its load at 34,300 BTU/hr against 29,200 in Bend, where nights fall to 48.5°F.

What to do about it

In humid climates, oversizing does the most damage, because a short-cycling unit removes even less water. A variable-speed unit that runs long at low output dehumidifies best. In dry climates the opposite is true: almost all the load is sensible, and an evaporative cooler can be an option where nights fall below about 60°F.

Where to go next

See how far apart the answers run: Bullhead City, AZ, Houston, TX, San Francisco, CA.

Is My Contractor's AC Tonnage Right? How to Check a Quote · Window AC and Mini-Split BTU by Room, Adjusted for Climate