Outside Design Temperature
In heating calculations, the outside design temperature is the coldest 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 temperature 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.
Make sure the HVAC contractor/installer is using the correct 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).