Heat Load Calculations of Tables Used in Air Conditioning

Outdoor Environment

• One of the most important tables that HVAC installers use to calculate the size of an air conditioner is based on the external environment of the location where the air conditioner will be installed. Locations that have long, hot summers require a greater air conditioning capacity because the units have to run longer to cool the air. The manual recommends keeping homes at temperatures between 75 and 78 degrees and provides tables that give typical temperatures for towns and cities, recommending that users subtract 75 degrees from their normal summer temperatures to establish a base.

Windows and Doors

• Next to the outdoor temperatures of the location, the number of windows in a home and their size and quality are the next most important calculation because the heat transfer through windows can be significant. The manual takes into account the directions the windows face and how much direct sunshine goes through them. The authors of Manual J call this the heat transfer multiplier. This multiplier also factors in the square footage of the exterior doors, as well as the door types. For example, a solid wood door resists heat better than a metal door.

Walls, Roof and Floors

• The manual also takes into account the resistance to heat from the walls, windows and floors. Insulation levels vary and the technical term for the degree of insulation that keeps heat out is called the R-value. Higher R-values help the building resist temperature gains due to hot air penetrating through the exterior surfaces. Adding more insulation, especially to the roof, keep homes cooler, especially in sunny places. The kind of flooring in the home also makes a difference. For example, carpeting helps keep the heat from the foundation from seeping into the home. Manual J also includes air infiltration from doors, windows and electrical outlets (more often an issue in older, poorly constructed homes).

Latent Heat

• The latent heat calculation is a combination of lifestyle habits and the number of occupants inside the home. Appliances such as refrigerators, stoves and ovens create heat, as do light bulbs, computers and televisions. In addition, people continually emit body heat. Therefore, a home with four occupants needs more cooling power than a home with just one person.