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Rev 2.0
63-7048 1998 Honeywell Inc.
Page 15 of 22
Making the Best of a Duct Mounted Humidity Control
One of the things we learned early on in this development was that a return air duct is not
the best place to mount a humidistat that controls only by relative humidity. There are
two major factors that can dramatically affect the accuracy of standard humidistats:
1.
Ambient Temperature of the Equipment Room
If the temperature in the area where the HVAC equipment is installed differs
much from the living space temperature, errors in sensed RH can develop.
Remember earlier in this paper, in Figure 1, we talked about how relative
humidity is very dependent on ambient temperature. We have seen installations
where the ambient temperature at the humidity control is very different from the
living space.
One example of this is when the equipment is installed in a relatively confined
furnace room. During periods when the furnace is running a lot, the temperature
in that room can exceed 80
F, when the temperature in the living space is only
72
F. This 8
F difference in temperature causes the relative humidity sensed
by the humidistat to be 11 % lower (35% in the duct vs. 46% in the living space,
for a dewpoint of 50
F) than the relative humidity in the living space.
Another example is when the equipment is installed in an unfinished area of the
house. In this case the temperature in that location can be much cooler than the
temperature in the living space. If the temperature at the humidity control was
64
F, the relative humidity sensed by the humidistat would be 14 % higher
(60% in the duct vs. 46% in the living space, for a dewpoint of 50
F) than the
relative humidity in the living space.
Once the system fan starts, the temperature in the duct begins to move toward
the living space temperature, but it takes several minutes before the error is
eliminated. In these cases the errors in sensed humidity will cause the
humidifier operation to be affected, either supplying too much or not enough
humidity.
Figure 11 shows the first example in practice. This data was taken from one of
our field test sites, during a night setback period with no calls for heat, while
running in the CIRC mode, controlled by a Perfect Climate Comfort Center
(10 minutes of system fan running each half-hour).