valent VX Series, Installation, Operation And Maintenance Manual

Page: 18 / 72

Dedicated Outdoor Air Unit 18
Frost Control
Extremely cold outdoor air temperatures can cause
moisture condensation and frosting on the energy
recovery wheel. Frost control is an optional feature that
will prevent/control wheel frosting. Three options are
available:
1. Timed exhaust frost control
2. Electric preheat frost control
3. Modulating wheel frost control
All of these options are provided with a thermodisc
mounted in the outdoor air intake compartment and a
pressure sensor to monitor pressure drop across the
energy wheel.
An outdoor air temperature of below 5
° F and an
increase in pressure drop would indicate that frost is
occurring. Both the pressure sensor and the outdoor air
thermodisc must trigger in order to initiate frost control.
The two sensors together ensure that frost control is
only initiated during a real frost condition.
Timed exhaust frost control
includes a timer in
addition to the thermodisc and wheel pressure sensor.
When timed exhaust frost control is initiated, the timer
will turn the supply blower off. Time exhaust using
default timer setting will shut down the supply fan for
5 minutes every 30 minutes to allow exhaust to defrost
energy wheel. Use the test procedure in the Optional
Start-Up Accessories section for troubleshooting.
Electric preheat frost control includes an electric
heater (at outdoor air intake) in addition to the
thermodisc and pressure sensor on wheel. When
electric preheat frost control is initiated, the electric
preheater will turn on and warm the air entering the
energy wheel to avoid frosting. Use the test procedure
in the Optional Start-Up Accessories section for
troubleshooting.
Modulating wheel frost control includes a variable
frequency drive (VFD) in addition to the thermodisc and
pressure sensor. When modulating wheel frost control
is initiated, the VFD will reduce the speed of the wheel.
Reducing the speed of the energy wheel reduces its
effectiveness, which keeps the exhaust air condition
from reaching saturation, thus, eliminating condensation
and frosting. If the outdoor air temperature is greater
than the frost threshold temperature OR the pressure
differential is less than the set point, the wheel will run
at full speed. If the outdoor air temperature is less than
5
° F AND the pressure differential is greater than the
set point, the wheel will run at reduced speed until the
pressure differential falls below the set point. The VFD
will be fully programmed at the factory.
Optional Component Operation
Rotation Sensor
The rotation sensor monitors energy wheel rotation. If
the wheel should stop rotating, the sensor will close a
set of contacts in the unit control center. Field-wiring of
a light (or other alarm) between terminals R and 12 in
the unit control center will notify maintenance personnel
when a failure has occurred.
Dirty Filter Sensor
Dirty filter sensors monitor pressure drop across the
outdoor air filters, exhaust air filters, or both. If the
pressure drop across the filters exceeds the set point,
the sensor will close a set of contacts in the unit control
center. Field-wiring of a light (or other alarm) to these
contacts will notify maintenance personnel when
filters need to be replaced. The switch has not been
set at the factory due to external system losses that
will affect the switch. This switch will need minor field
adjustments after the unit has been installed with all
ductwork complete. The dirty filter switch is mounted in
the exhaust inlet compartment next to the unit control
center or in unit control center.
Microprocessor Control
The microprocessor controller is specifically designed
and programmed to optimize the performance of
the unit with supplemental heating and cooling. This
option ensures that the outdoor air is conditioned to
the desired discharge conditions. The controller and
accompanying sensors are factory-mounted, wired and
programmed. Default settings are pre-programmed, but
are easily field-adjustable.
The microprocessor controller can be interfaced with
a Building Management System through LonWorks®,
BACnet®, or ModBus.
Please refer to the Installation, Operation and
Maintenance manual for detailed information.
Unoccupied Recirculation Damper
The unoccupied recirculation option provides a
recirculation damper from the return air intake to the
supply airstream. If equipped with a room temperature
and/or relative humidity sensor, the unit can cycle on to
maintain unoccupied room set points if there is a call
for unoccupied heating, cooling, or dehumidification.
During the unoccupied mode, the supply and exhaust
(if equipped) fans will be off unless there is a call for
cooling, heating, or dehumidification in the space.
Vapor Tight Lights
Vapor tight lights provide light to each of the
compartments in the energy recovery unit. The lights
are wired to a junction box mounted on the outside of
the unit. The switch to turn the lights on is located in
the unit control center. The switch requires a separate
power source to allow for power to the lights when the
unit main disconnect is off for servicing.