Chapter 26 of the 1997 ASHRAE Handbook—Fundamentals is a popular source for tabular, climatic data that represents the outdoor design conditions for many locations. Sensible design conditions for cooling systems appear under the heading “Cooling DB/MWB” (cooling dry bulb/mean coincident wet bulb). The tables also indicate the frequency of occurrence for each condition. For example, weather conditions exceed the values listed in the “0.4%” column for just 35 hours in an average year.

Latent design conditions, labeled “DP/ HR and MDB” (dew point/mean coincident dry bulb and humidity ratio), were added to the 1997 edition of the handbook to aid the design of dehumidification systems.

Because the peak sensible load rarely occurs at the same time as the peak latent load, cooling equipment that is selected and controlled to deliver full capacity at sensible design is likely to deliver less-than-required capacity at latent design. Therefore, system performance must be analyzed at sensible and latent design conditions, based on the moisture control needs of the application.

As the table below illustrates, latent design conditions can be similar in many locations. Ignoring system operation at latent design can lead to poor dehumidification in buildings across the country, not just in the South and Southeast.

U.S. Cooling and Dehumidification Design Conditions, 0.4% Frequency of Occurrencea
	Cooling DB/MWB		DP/HR and MDB
Station	DB	MWB	DP	HR	MDB
Chicago, Illinois	91°F	74°F	74°F	130	84°F
Dayton, Ohio	90°F	74°F	73°F	129	82°F
Harrisburg, Pennsylvania	92°F	74°F	74°F	130	82°F
Jacksonville, Florida	96°F	76°F	76°F	138	84°F
aExcerpt from Table 1B in Chapter 26 of the 1997 ASHRAE Handbook—Fundamentals.

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