Effects Of Humidity; Design Procedures; General - Honeywell AUTOMATIC CONTROL Engineering Manual

INDOOR AIR QUALITY FUNDAMENTALS
Failing to maintain the temperature will affect the relative
humidity which can have an adverse effect on the growth of
viruses, mold, mildew, and bacteria. A temperature increase of
5 to 7 degrees can double the levels of formaldehyde.

EFFECTS OF HUMIDITY

Controlling relative humidity between 40 and 60 percent
has a beneficial impact on health and therefore, productivity.
Figure 2 shows the effect of relative humidity on bacteria,
viruses, fungi, mites, respiratory infections, allergic rhinitis,
asthma, chemical reactions, and ozone production. The choice
DECREASE IN BAR WIDTH
INDICATES DECREASE
IN EFFECT
BACTERIA
VIRUSES
FUNGI
MITES
RESPIRATORY
INFECTIONS #
ALLERGIC RHINITIS
AND ASTHMA
CHEMICAL
REACTIONS
OZONE
PRODUCTION
# INSUFFICIENT DATA
ABOVE 50% RH
The relative humidity in the supply air duct after the
humidifier will approach 100 percent and condense unless
controlled. Condensation in the duct promotes the growth of
mold and fungi and has been the source of air quality problems
in many buildings. Proper maintenance is essential for the
control of contaminants and bioaerosols. Assuming that return
air relative humidity is the same as space relative humidity is a
common mistake. They are the same only if the two
temperatures are the same. Maintaining the relative humidity
below 60 percent frequently requires dehumidification by the
cooling coil resulting in condensation of moisture on the coils.
If this moisture is not removed through proper drainage it
promotes the growth of mold and fungi and has been the source
of air quality problems in many buildings.
ENGINEERING MANUAL OF AUTOMATION CONTROL
OPTIMUM RELATIVE HUMIDITY RANGES FOR HEALTH
0 10 20
Fig. 2. Effect Of Changes in Relative Humidity on Various Items.
of the optimum zone is a compromise. It is informative to note
the overlap of ozone production and chemical reactions, since
ozone is such an active oxidizer. Ozone may not be generated
at high humidity, but if present it will certainly react. Also
note that bacteria, viruses, fungi mites, and chemical reactions
all thrive in the high moisture content found in the drain pans,
humidifier discharges, and on the cooling coils if they are
dehumidifying. Some state building codes prohibit
humidification above 30 percent space levels in the winter
except for special occupancy uses.
Refer to ASHRAE Standard 55-1992 Thermal Environmental
Conditions for Human Occupancy for more information.
OPTIMUM ZONE
30 40 50 60 70
RELATIVE HUMIDITY, PERCENT

DESIGN PROCEDURES

GENERAL

ASHRAE Standard 62-1989 sets the IAQ standard for design
professionals by specifying the minimum ventilation rates and
IAQ that are acceptable to human occupants and is intended to
minimize the potential for adverse health effects. This standard
defines two methods that can be used to achieve these goals.
— Ventilation Rate Procedure: Provides ventilation air of
the specified quality and quantity to the space.
— Indoor Air Quality Procedure: Controls known and
specifiable contaminants within the space.
160
80 90 100
M10481