Tons of Refrigeration from BTU/hr

The ton of refrigeration is a fossil from the ice trade. Before mechanical cooling, a building's capacity was quoted in tons of ice delivered per day, so when compressors arrived the industry defined one ton as the rate of cooling that melts one short ton (2,000 lb) of ice in 24 hours: 2,000 lb × 144 BTU/lb ÷ 24 hr = 12,000 BTU/hr. In SI that is 3,516.85 W, and a "5-ton" residential condenser is a 17.6 kW machine.

Rules of thumb built on it are everywhere — 400 ft² per ton for an average house, 3 gpm of condenser water per ton, 12,000 BTU/hr per ton of nominal capacity — but note that nominal tonnage is a model-number convention, not a measurement. A "3-ton" unit at AHRI conditions might make 34,600 BTU/hr, and at 105 °F outdoors considerably less. The trap that trips apprentices is confusing the ton of refrigeration with the ton of weight or with a "ton" of heating: an oil-fired boiler is never rated in tons. Quick conversion: 60,000 BTU/hr ÷ 12,000 = 5 tons = 17.58 kW.