Reznor FX05 - Dehumidification (Reheat); Control Adjustment Guidelines for a Reheat System

Form CP-MAPS-D12 with FX05, Page 12
Control Adjustment
Guidelines for a
Reheat System
Dehumidication
(Reheat)
Reheat control is enabled based on the outdoor dewpoint temperature setpoint
(dP1). If the outside air dewpoint is above the setpoint (dP1 = 60ºF), BO-4 will
be ON activating the reheat system. The reheat circuit adds 7 to 17°F to the
discharge air temperature after it leaves the main cooling section of the unit.
The reheat control output will be OFF based upon any of the following condi-
tions:
The space temperature sensor calls for the unit to provide space cooling.
In this mode the reheat is OFF. Once space cooling (SSP) is satised,
reheat can restart.
2. The unit is equipped with a space humidistat with setpoint control. If the
humidistat is not calling for dehumidication, thus the switch is open to BI-
2, the reheat circuit is OFF.
3. If the outdoor air temperature is below the low ambient lockout (roL), the
reheat circuit will be OFF. The rol default value is 65ºF.
4. If the outdoor air temperature is above the high ambient lockout (roH), the
reheat circuit will be OFF. The roH default value is 120ºF.
5. If the unit is in unoccupied mode and the (rHE) reheat value is NO, reheat
is not allowed in the unoccupied mode. If the rHE default value is YES,
reheat is allowed in the unoccupied mode based upon space humidistat. If
a space humidistat is not installed, the contact is wired closed thus allow-
ing reheat in the unoccupied mode.
6. If the outdoor air temperature sensor fails, cooling, reheat, and heating
equipment are not allowed to operate.
The discharge air temperature can be adjusted when the reheat point is ON.
When reheat is ON, the discharge air temperature can be reset as follows.
dCH = dCH + rSP
NOTE: If outdoor reset is used, dCL must replace dCH)
NOTE: It is important that rSP be used when the dcH value is lower than
65°F. Because the reheat system adds 7 to 17°F to reheat, a dcH set-
point of 55°F must discharge 40°F from the main coil to maintain a 55°F
setpoint value. This low discharge temperature may damage the unit.
There are many ways to combine settings based on each application. The type
of reheat application will determine the setting for dCH and rSP. The typical
application requires rSP to be 0°F when dCH is 70°F. The unit will discharge a
dewpoint of 52 to 55°F at 70°F discharge air temperature.
1.
EXAMPLE 1) Neutral Air Application (corridor or kitchen makeup air)
Since the unit reheat override is based primarily on space temperature, the reheat override parameter roH
will be adjusted outside of operating range. Below are the proposed settings:
dP1 = 58; reheat enabled above 58°F dewpoint
roL = 58; reheat compressor locked out below 58°F dry bulb
oC1 = 65; cooling compressors locked out below 65°F (Assumes reheat pump alone is adequate.)
En1 = 29; 1st stage cooling compressor enabled above 29 btu/lb
oH1 = 70; Heating locked out above 70°F ambient
dCH = 70; cooling duct discharge air setpoint with rSP = 0 (use dCL with outdoor reset)
rSP = 0; Regardless of ambient dewpoint, the duct discharge air temperature setting is 70°F.
roH = 115; High ambient reheat override is set where reheat is not disabled.
rCP = 55; if the space temperature exceeds the cooling setpoint, reheat is disabled and discharge air is
supplied at 55°F.
NOTE: Does not apply to
Option AU25 reheat.