HVAC design temperatures

What the percentages mean

• 99.6 % and 99 % heating: the outdoor temperature is at or above this value for 99.6 % (or 99 %) of the hours in a typical year — colder only about 35 (or 88) hours a year. Residential heat-loss calculations commonly use the 99 % value.
• 0.4 % and 1 % cooling: the temperature is exceeded only 0.4 % (about 35 hours) or 1 % (about 88 hours) of the year. Residential cooling loads commonly use the 1 % value. The mean coincident wet bulb is the average humidity that comes with that heat.

Where the data comes from

The values are taken from the Engineering Weather Data station sheets that the U.S. Air Force Combat Climatology Center compiled from each station’s hourly observations and NOAA’s National Climatic Data Center published (CD-ROM version 1.0, 1999). As a U.S. Government work it is in the public domain. We use only the design temperatures and the station details.

Most records end in 1996 (each station’s period is shown) and many stations are airfields, so a value can differ from today’s climate and from the conditions at your site. Many building departments specify the design temperatures to use for permits; where they do, use theirs. ASHRAE and ACCA publish their own design tables, which are copyrighted; we do not reproduce them.