Latent Heat Load Calculator

Latent load is the cooling capacity spent condensing water vapor rather than lowering dry-bulb temperature. Because the enthalpy of vaporization of water near room conditions is about 1,060 BTU per pound, moisture removal is energetically expensive: one pint of condensate per hour represents roughly 1,050 BTU/hr of coil duty. The ventilation term, 0.68 x CFM x grain difference, is not arbitrary. It is 60 minutes per hour times standard air density of 0.075 lb per cubic foot times about 1,060 BTU per pound, divided by the 7,000 grains in a pound, which works out to 0.68 BTU/hr per CFM per grain. Occupant latent output climbs steeply with activity: roughly 200 BTU/hr per person seated at rest, 250 for office work, 350 for light walking, and 500 to 600 in kitchens or gyms. Indoor design is usually 75 degrees F and 50 percent relative humidity, about 65 grains per pound. Gulf Coast summer air can exceed 120 grains while Denver or Phoenix may sit below 40, which is why ventilation dominates the latent total in humid climates and nearly vanishes in arid ones. The limitations matter: infiltration is approximated here as a flat 10 percent of the ventilation term rather than derived from a measured envelope leakage rate, and cooking, bathing, and process moisture are excluded. Equipment must also be selected on sensible heat ratio, since an oversized unit short-cycles and leaves a space cool but clammy.

The distinction between heat that changes temperature and heat that changes phase was established by Joseph Black at Glasgow in the 1760s, who named the second kind latent heat after observing that ice absorbed large quantities of heat while staying at its melting point. The idea became engineering practice only when air conditioning was invented for humidity control rather than comfort: the apparatus Willis Carrier built in 1902 for a Brooklyn printing plant was meant to stabilize paper dimensions and ink registration, and his 1911 paper Rational Psychrometric Formulae, presented to the American Society of Mechanical Engineers, supplied the vapor-pressure relationships that put moisture removal on a numerical footing. Systematic latent load procedures followed the growth of the professional societies. The American Society of Heating and Ventilating Engineers, founded in 1894, and its successor ASHRAE, formed in 1959, published occupant heat and moisture tables together with cooling load procedures in the Handbook of Fundamentals, while the Air Conditioning Contractors of America issued Manual J for residential work, separating sensible and latent components explicitly. Attention to latent load intensified after the energy crises of the 1970s pushed designers toward tighter envelopes and, later, toward higher outdoor air rates under ASHRAE Standard 62. Buildings that were both tight and heavily ventilated developed humidity and mold complaints when equipment was sized on sensible load alone, driving adoption of energy recovery ventilators, dedicated outdoor air systems, and explicit sensible heat ratio checks.