Sequence of Operation
90
RT-SVX34F-EN
pressure. If the space pressure transducer fails, the
unit will revert back to fresh air tracking control.
Economizer Operation Based on Reference
Enthalpy
Reference enthalpy is accomplished by using an Outdoor
Humidity Sensor (OHS). The reference enthalpy is field
selectable to one of five standard enthalpies. Refer to
Table 25, p. 49
for the proper potentiometer setting for
each enthalpy selection.
If the outdoor air enthalpy is greater than the selected
reference enthalpy, the economizer will not operate and
the damper will not open past the minimum position
setting.
If the outdoor air enthalpy is less than the reference
enthalpy, the dampers will modulate to maintain a 45° to
55° F minimum supply air temperature (constant volume
or variable air volume applications). The ECA modulates
the economizer dampers from minimum position to fully
open based on a 1.5° F control point below either the space
temperature setpoint for constant volume applications or
1.5° F below the discharge air temperature setpoint for
variable air volume applications. With reference enthalpy
control, reference enthalpy is not allowed if the outdoor
temperature is below 32° F. Below 32° F, dry bulb
economizer control is enabled.
If communications between the Outdoor Humidity Sensor
(OHS) and the Economizer Actuator Control (ECA) were to
fail, the economizer will operate using the dry bulb
parameters.
Economizer Operation Based on
Comparative Enthalpy
Comparative enthalpy is accomplished by using an
outdoor humidity sensor (OHS), return humidity sensor
(RHS), and the return air sensor (RAS).
If the outdoor air enthalpy is greater than the return air
enthalpy, the economizer will not operate and the damper
will not open past the minimum position setting. The
economizer will not operate at outdoor air temperatures
above 75°F.
If the outdoor air enthalpy is less than the return air
enthalpy, the dampers will modulate to maintain a 45° to
55°F supply air temperature (constant volume or variable
air volume applications). The ECA modulates the
economizer dampers from minimum position to fully
open based on a 1.5°F control point below either the space
temperature setpoint for constant volume applications or
1.5°F around the supply air temperature setpoint for
variable air volume applications. Refer to
Figure 59, p. 90
for the Humidity versus Voltage Input Values.
If either or both the return air humidity sensor (RHS) or the
return air sensor (RAS) fails, the economizer will operate
using the reference enthalpy setpoint perimeters.
Dehumidification (Modulating
Hot Gas Reheat) Sequence of
Operation
When the relative humidity in the controlled space (as
measured by the sensor assigned to space humidity
sensing) rises above the space humidity setpoint,
compressors and the supply fan will energize to reduce the
humidity in the space. All compressors on both refrigerant
circuits will be staged up during active dehumidification.
A Voyager Commercial Rooftop unit can contain one or
two refrigerant circuits. Units with dehumidification will
have one circuit with an outdoor condenser coil located in
the outdoor section for normal head pressure control and
a reheat coil located in the indoor air stream section for
supply air reheat; both coils are for the same circuit. For 40-
50 ton Voyager Commercial units the reheat circuit is
circuit# 2. For 27.5-35 ton Voyager Commercial units there
is only one circuit.
During dehumidification mode, the CLV and RHP will
modulate which will allow refrigerant to flow through both
the condenser coil and the reheat coil. The RHP will be de-
energized when in dehumidification mode.
During dehumidification mode, the Supply Air
Temperature is controlled to the Supply Air Reheat
Setpoint by controlling the reheat and cooling modulating
valve position. The range for the Supply Air Reheat
Setpoint is 65°F to 80°F and the default is 70°F. The Supply
Air Reheat Setpoint is adjusted by using a potentiometer
on the RTOM.
Figure 59.
Humidity vs. current input
