Rev 2.0
63-7048 1998 Honeywell Inc.
Page 17 of 22
2. Humidity Control Located Too Close to Bypass Duct
A second problem that can occur in many installations is a result of the humidity
control being located too close to the bypass duct. We always recommend
mounting the humidity control as far upstream from the bypass duct as possible
(> 24” is desired). However, in many installations the desired separation is not
possible, and compromises have to be made. The result is that, in many cases,
the hot air coming from the bypass duct heats up the return air duct and spills
upstream toward the humidistat. This affects the relative humidity sensed by the
control in a manner similar to the first problem discussed above. In fact, this
problem can be even more severe. The effect of this heating of the humidistat is
to lower the sensed relative humidity.
-20
-10
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Window Temp.
Outdoor Temp.
Dewpoint
Furnace Cycles (W)
Humidifier Cycles
Fan Cycles (G)
Duct RH
Room Temp.
Duct Temp.
Time
Temperature (oF) / Relative Hum idity (%)
Digitial Output
Figure 12 – Variation in Duct Temperature and Relative Humidity
(Bypass Air Affecting Sensor)
March 11, 1998 – Minneapolis, Minnesota
Figure 12 shows this in practice. This data was also taken from one of our field
test sites, during a long recovery period from night setback. Again, note the
large variations in temperature and relative humidity in the duct, while the