Cooling Load Calculator

A cooling load is the rate at which heat must be extracted from a space to hold a setpoint, and it is assembled by adding independent gain paths rather than applying one equation. Conduction through opaque walls and roof follows Q = U x A x Delta-T, where U is the assembly heat transfer coefficient in BTU per hour per square foot per degree F; an insulated wood-frame wall lands near 0.08 and an insulated attic ceiling near 0.05, while a clear double-pane window is closer to 0.50. Solar gain is handled separately as the solar heat gain coefficient times glass area times incident intensity, typically 150 to 250 BTU per hour per square foot on an unshaded sunlit facade, and it usually dwarfs the window's conduction term. Internal gains are added from occupancy, lighting power density, and plug loads; ASHRAE assigns a seated adult roughly 245 BTU per hour sensible plus 155 latent, and offices commonly run 1 to 1.5 watts per square foot for each of lighting and equipment. Infiltration uses the sensible heat factor 1.08 applied to leakage airflow. Capacity is expressed in tons, where one ton equals 12,000 BTU per hour, a figure inherited from the heat needed to melt a ton of ice in 24 hours. Well-built homes commonly land between 400 and 600 square feet per ton. The main caveat is thermal lag: heavy masonry absorbs solar energy and releases it hours later, so instantaneous heat gain and cooling load are not the same quantity, and simplified summations like this one typically fall within 15 to 25 percent of a full ACCA Manual J result.

Mechanical cooling of buildings began with a humidity problem rather than a comfort one. In 1902 Willis Carrier designed an air treatment apparatus for the Sackett-Wilhelms lithography plant in Brooklyn to stabilize paper dimensions, and in 1911 he presented Rational Psychrometric Formulae to the American Society of Mechanical Engineers, giving engineers the mathematical basis for relating dry-bulb temperature, moisture content, and enthalpy. The ton of refrigeration entered the vocabulary from the nineteenth-century natural ice trade, and it survived as the sizing unit long after ice deliveries ended. Methods for predicting the load evolved through successive ASHRAE handbooks. Early practice used steady-state conduction plus crude solar allowances. In the late 1960s Gene Mitalas and Douglas Stephenson at the National Research Council of Canada developed the transfer function approach, which represented a wall as a series of weighting factors capturing its thermal storage. The 1977 Fundamentals volume popularized the simplified cooling load temperature difference and cooling load factor tables derived from it, which dominated hand calculation for two decades. Work presented in the late 1990s by Jeffrey Spitler and colleagues produced the radiant time series method, and the 2001 Handbook of Fundamentals adopted the heat balance method as the reference procedure with radiant time series as its simplified companion. On the residential side, the Air Conditioning Contractors of America codified room-by-room procedure in Manual J, which remains the basis for code-accepted equipment sizing in most US jurisdictions.