Relative Humidity is “Relative”

Psychrometrics can be a complicated field of study. However, a psychrometric chart can be a simple, invaluable tool to help HVAC engineers and owners understand the relationship between air temperature and humidity.  Relative humidity (RH) is a term that most people are familiar with, but its relationship with dry bulb (DB) temperature is typically under-appreciated.  RH is the ratio of actual water in a given air sample compared to the water in a saturated sample at the same temperature and pressure. In simpler terms, the % RH gives a measure of how close an air sample (which could be your space) is to being completely saturated with water. Warmer air (higher DB) can hold more water than cooler air (lower DB). Therefore, air temperature directly affects RH.  Below are two instances where understanding the DB temperature and RH relationship is critical:

  • Unoccupied Summer Conditions:  School classrooms are an example of spaces that have low internal heat gains during hot and humid outdoor conditions over summer break. These spaces can experience high RH conditions for a number of reasons. Sometimes the occupant/building operator’s reaction is to lower the temperature setpoint of the room to “wring out” more moisture. However, this practice usually has the opposite effect. If a space is at 75 deg F, 60% RH and the room setpoint is changed to 70 deg F, the space relative humidity can raise to 70% RH unless actions are taken to remove the moisture in the air.
  • Limits of Cooling Medium:  Typical direct expansion and chilled water HVAC systems can generate cooling supply air that ranges in temperature from 52 to 58 deg F DB.  During maximum cooling conditions, the supply air leaves the cooling coil at near saturated conditions, or 100% RH.  The lowest possible RH in the room served by this supply air is directly related to the temperature of the space (assuming no additional moisture loss in the space itself).

Room Temperature                      Lowest Possible RH*

75 deg F                                               46.8%

72 deg F                                               51.8%

68 deg F                                               59.3%

65 deg F                                               65.8%

*based on a 55 deg F DB, 54 deg F WB supply air condition

Space temperature is a critical parameter to consider when you are trying to reduce or maintain a low relative humidity in a space. Any solution to correct space humidity problems should address removing the water vapor from the air, not just changing the space temperature. If you need assistance in evaluating humidity concerns in your facility, please do not hesitate to contact Douglas C. Barnhart, PE at 717-845-7654 or dcb@ba-inc.com.

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