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Air Conditioning Alphabet – Sensible and Latent Cooling

Sensible cooling and latent cooling are, at best, confusing terms. At worst they’re useless. Air conditioners do both, so we always talk about the sensible and latent capacity of A.C. equipment compared to those loads in the building. Most of us know that a ton of cooling equals 12,000 BTUH, but it only removes about 9,000 BTUH of heat, a fact that we can “sense” with a thermometer. We can’t “sense” the other 3,000 BTUH that removes moisture, we call it “latent.” I think Willis Carrier coined the terms early in the 20th century to explain where the rest of the heat went and we have been using them ever since.

The two processes—cooling and moisture removal–are inseparable (almost) so we say “sensible and latent” in the same breath; like salt and pepper or black and white. But in good air conditioning design we treat the two separately: run a cooling load and size the air conditioner to handle it; then have a strategy for removing moisture that might involve a dehumidifier.

scale with water on one side and grains on the otherSo let’s separate the terms. Drop “sensible;” throw it away; it adds nothing. It’s just cooling, and each ton that you buy will actually do about 8,300-8,500 BTUH. Look it up in the manufacturer’s engineering performance literature.

Replacing “latent” isn’t so easy. Each ton of equipment removes about 3.0 pounds of water per hour (21,000 grains) and depresses the dew point 12 grains; but what do you call removing moisture? Being very clever, I call it “moisture removal.” Maybe we should just call it grains, not 100% scientific but by golly I’d know what you were talking about if the catalogue said “12 grains depression.”
Sensible and latent cooling got us through the first 100 years of HVAC but it’s time for them to go. File them in the same drawer as “the cat’s pajamas” and “wooden nickel.” Wouldn’t that be sensible?