Specifications

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Humidistat and transmitter locations are critical
Humidistat or transmitter location has a significant impact on
humidifier performance. In most cases, it is recommended that you
do not interchange duct and room humidity devices. Room humidity
devices are calibrated with zero or little airflow; whereas duct humidity
devices require air passing across them. Recommended humidistat and
transmitter locations (see Figure 13-1):
A This is the ideal sensing location because this placement ensures the
best uniform mix of dry and moist air with stable temperature control.
B This location is acceptable, but the room environment can affect
controllability such as when the humidistat or transmitter is too close
to air grilles, registers, or heat radiation from room lighting.
C This location is acceptable because it provides a good uniform mixture
of dry and moist air, but if an extended time lag exists between
moisture generation and sensing, make sure the control contractor
extends the sampling time.
D This location behind a wall or partition is acceptable for sampling
the entire room if the sensor is near an air exhaust return outlet. This
location is also typical of humidistat or transmitter placement for
sampling a critical area.
E These locations are not acceptable because they may not represent
actual overall conditions in the space.
F These locations are not acceptable. Do not place humidistats or
transmitters near windows, door passageways, or areas of stagnant
airflow.
G This is the best location for a duct high limit humidistat or humidity
transmitter.
Figure 13-1:
Recommended humidistat and transmitter locations
Outside air
Relief air
Return air
Air handling unit
8' to 12'
(2.4 m to 3.7 m)
minimum
Humidifier dispersion
assembly
Turning vanes
Window
Doorway
Window
Point of vapor absorption
Vapor absorption has taken
place
Airflow switch or
differential pressure switch
(sail type recommended
for VAV applications)
High limit humidistat or
high limit transmitter (set at
90% RH maximum) for VAV
applications
Damper control
C
A
E
F
D
B
EF
G
F
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Wall or
partition
DC-1084M
VM-IOM-1108.pdf 15 4/21/2010 1:36:21 PM