Armstrong World Industries EHU-703 - Manual (Page 38)

AIR SUPPLY HIGH LIMIT HUMIDITY SENSOR
The A 17898 (H270) includes a high limit circuit. This allows
the use of a second humidity in the supply air. Input signal
goes to terminals #8 on the humidistat. High limit set point is
preprogrammed at 85%. It can be readjusted using the
A19517 service tool. If you don’t use the high limit feature,
connect a 1 Meg
Ω Ω, ¼ watt resistor between terminals 8
and 10 (supplied with the humidistat).
Remember: this high limit function is not a safety device.
For critical situations, provide installation with normal
protections required to ensure a safe operation.
HUMIDITY SET POINT RESET BY OUTSIDE TEMPERATURE
The A17898 (H270) humidistat has an optional
outdoor temperature sensor. This sensor is used
to reset the humidity set point during the cold season
to minimize condensation on windows and building
structures.
When the outdoor temperature falls below the
selected high temperature, parameter HT (32°F in
the example Figure 41-2), the humidity set point will
start to decrease. The lowest humidity set point will
be reached at selected low temperature, parameter
LT
(-20°F).
The set point decrease from original set point down to
the lowest set point determined by the parameter rE. In the example, Figure 41-2, rE was set to
20%, therefore the humidity set point dropped from 45% to 20%.
If you don’t use the outdoor reset feature, put a jumper between terminals 6 and 9 (supplied
with the humidistat).
38
Table 4: Example showing proportional high
limit override signal in supply duct.
Control Output Le
v
e
l
Supply Humidity
00%
50%
0%
85%
Figure 38-1
LT= -20˚F
HT=32˚F
Outside Air
T
emper
ature
%RH
rE=20%
User
Setpoint
32˚F
14˚F
-4˚F
-20˚F
-40˚F
20% 25% 30% 35% 40% 45%