Outside Design Temperature
In heating calculations, the outside design temperature is the cold- est outside temperature expected for a normal heating season. It is not the coldest temperature on record, but the lowest one recorded for a particular locale over a 3- to 5-year period.
Lists of outside design temperatures are published for selected localities throughout the United States (Table 4-1). If a locality is not included on the list of winter outside design temperatures, check with a local weather bureau. Using an outside design temper- ature from the nearest locality on the list can be misleading, because even closely located cities can differ widely in weather conditions as a result of different altitudes, the effects of large bodies of water, and other variables.
Caution
Make sure the HVAC contractor/installer is using the cor- rect local outside design temperature and humidity for the area. Air conditioners are often oversized by using a higher than required summer design temperature. Undersizing the air conditioner is caused by underestimating the latent load (i.e., the energy used by the unit to remove moisture from the air).