9
In southern climates, often the heating and air
conditioning equipment is turned off if the
residence is to be unoccupied for an extended
period of time. It is recommended that the
dehumidifier be left ON even when the air
conditioning is off. The internal fan in the
dehumidifier will move air throughout the
conditioned space to keep the humidity under
control. Since the dehumidifier adds a small
amount of heat to the space, it is recommended
that the air conditioning and the dehumidifier be
left on during unoccupied periods; however, the
set point of the air conditioning unit should be set
HIGHER than normal. The higher temperature will
lower the relative humidity. This, coupled with the
dehumidifier operation, will ensure a dry
conditioned space.
PROCESS
When the dehumidifier fan (blower) is operating,
moist air from the conditioned space is drawn into
the unit through the inlet duct. The air first passes
through the filter in the unit. Once beyond the
filter, the air passes through the cool evaporator
(cooling) coil. This coil is part of the refrigeration
process and is, therefore, cold. In all probability
the coil is colder than the dew point of the air.
Since it is below the dew point, moisture in the air
will condense on the coil fins. Once condensed
out of the circulating air, the moisture now in the
form of droplets falls to the bottom of the coil and
is collected in the drain pan. This is essentially the
same process that causes water to drip from
beneath an automobile when the air conditioning
is operating.
Once the moisture has been removed from the
air, it is somewhat cooler. However, since the air
is cooler it cannot hold as much moisture and its
relative humidity is high, even though moisture
has been removed. To solve this “problem” the air
next passes through a warm condenser (heating)
coil where it is warmed. Since the air is warmed
without adding any moisture, its relative humidity
decreases sharply. This dry air is drawn through
the fan and then expelled through the discharge
duct back to the conditioned space.
Dehumidifiers take latent heat (heat removed
from water vapor when it condenses) and convert
it to sensible heat (heat you can feel in the form of
temperature increase). This exchange causes the
discharge air temperature to be warmer than the
entering air temperature. The amount of this
temperature increase varies and is dependent
upon the load (how much dehumidification is
being done).
To ensure that the dehumidification process is
maximized, a thermostatic expansion valve is
used as part of the refrigeration system. This
device provides exacting control of the system
over a very wide range of operating temperatures.
This is superior to lower cost refrigerant control
devices such as capillary tubes which limit
efficient operation to a very narrow band of
operating temperatures.
SEQUENCE
Unit Controls
If no wall mounted dehumidistat is used, the unit
will be controlled by the integral control system.
This system operates the internal fan for 15
minutes approximately every hour to “sample” the
air in the system. The air sample passes over the
unit mounted humidity sensor. The unit mounted
control should be set at approximately mid level. If
the humidity level in the sampled air is greater
than the set point, the compressor will be turned
on, after a 3 minute delay. The fan and the
compressor will operate until the humidity level of
the air entering the dehumidifier satisfies the
humidity sensor/setpoint.
Wall Control
When this control method is selected, the unit
mounted control must be adjusted to its minimum
(lowest relative humidity) level and never
readjusted. The room dehumidistat should be set
in the comfort area at approximately 50% RH
(relative humidity). When the dehumidistat senses
that the RH in the space is above its set point, an
electrical contact within the dehumidistat closes
causing the dehumidifier to start. The unit runs to
dehumidify the air from the conditioned space
until the
dehumidistat is
satisfied, that is,
the RH has been
lowered to the set
point.
Wall Mounted
Dehumidistat
