Actonnage Calculator

A ton of refrigeration is a unit inherited from the ice trade. Melting one short ton of ice, 2,000 pounds at a latent heat of fusion near 144 BTU per pound, absorbs 288,000 BTU, and spreading that over 24 hours gives 12,000 BTU per hour. A 3-ton system therefore delivers 36,000 BTU per hour of nominal capacity, and residential split systems are sold in half-ton steps from 1.5 to 5 tons. The square-footage method used here starts from roughly 20 BTU per square foot and adjusts for climate, envelope quality, and ceiling height, since cooling acts on a volume of air rather than a floor plane; about 12.5 percent is added per foot of ceiling above 8 feet. The rule of thumb usually lands between 400 and 600 square feet per ton, but it is an estimate rather than a design. It ignores window area and orientation, solar gain, infiltration rate, duct losses in unconditioned space, occupancy, and internal gains from appliances, all of which a room-by-room load calculation treats explicitly. Oversizing is the more common error and the more damaging one. An oversized unit satisfies the thermostat quickly, short cycles, and never runs long enough to remove latent heat, leaving a cold but clammy house along with extra compressor wear. Undersizing shows up differently, as continuous operation that never reaches setpoint on the hottest design days.

Mechanical cooling of buildings begins with Willis Carrier, who in 1902 designed an air treatment system for the Sackett and Wilhelms lithography plant in Brooklyn to stabilize the humidity that was distorting color printing, and who patented an Apparatus for Treating Air in 1906. His 1911 paper Rational Psychrometric Formulae gave engineers the mathematics of moist air and made cooling loads calculable rather than empirical. Capacity was expressed in tons because buyers were already accustomed to purchasing natural harvested ice by the ton, so machine output was quoted as the equivalent tonnage of ice melted per day. Standardized residential load calculation came much later. The Air Conditioning Contractors of America developed Manual J as the industry procedure for residential heating and cooling loads, now in its eighth edition, computing gains room by room from construction assemblies, glazing area and orientation, and infiltration rather than from floor area alone. Efficiency regulation followed the National Appliance Energy Conservation Act of 1987, which introduced the seasonal energy efficiency ratio, with federal minimums rising from 10 SEER in 1992 to 13 SEER in 2006 and 14 SEER in 2015 for much of the country. In 2023 the Department of Energy shifted to the SEER2 metric, based on a test procedure using higher external static pressure that better reflects real installed duct conditions.