Economizer Application/Operation - Greenheck HRE-20 Installation, operationand Maintenance Manual

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Economizer Application/Operation

The energy recovery wheel operation can be altered to take advantage of economizer operation (free cooling) .
Two modes are available: 1) De-energizing the wheel or 2) Modulating the wheel. A field supplied call for cool
(Y1) is required .
De-energizing the wheel is accomplished with a signal from a Temperature or Enthalpy sensor mounted in
the supply air inlet compartment . This Primary sensor will de-energize the energy wheel when the outdoor air
temperature (factory default is 65ºF) or enthalpy (factory default is the "D" setting) is below the field adjustable
set point . An Override temperature sensor is also furnished in the supply air inlet compartment to deactivate
economizer mode . The Override (with field adjustable set point) is set at some temperature lower than the
Primary sensor (factory default is 50ºF) . Effectively, the two sensors create a deadband where the energy
recovery wheel will not operate and free cooling from outside can be brought into the building unconditioned .
Testing
Temperature Sensor with Override
• Turn both Temperature and Override thermostats down as low as they go. The
wheel should be rotating .
• Turn the Temperature sensor up as high as it goes, and keep the Override sensor
as low as it will go . The wheel should stop rotating .
• Turn both sensors as high as they will go. The wheel should start rotating.
• Set the Temperature sensor at desired point for economizer operation to begin.
Set the Override sensor at desired point for economizer operation to end (factory
default is 65ºF and 50ºF, respectively) .
Enthalpy Sensor with Override
• Turn unit power off. Disconnect C7400 solid state enthalpy sensor from terminal
So on the enthalpy controller . Also, disconnect the 620 ohm resistor from
terminal Sr on the enthalpy controller . Turn unit power on . The LED on the
enthalpy controller should light and the energy recovery wheel should not rotate .
• Turn unit power off. Reconnect 620 ohm resistor to terminal Sr on the enthalpy
controller . Turn unit power on . The LED on the enthalpy controller should not
light and the energy recovery wheel should energize and rotate .
If the steps above provide the results described, the enthalpy
economizer is working properly .
• Turn unit power off. Reconnect C7400 solid state enthalpy sensor
to terminal So .
Modulating the Wheel
In applications in which an internal heat gain is present in the space,
the rotational speed of the energy wheel may be modulated (via variable
frequency drive) to avoid overheating the space during the winter . The speed of the energy wheel will be
controlled in response to the discharge temperature setpoint .
Sequence of Operation: The variable frequency drive is fully programmed at the factory (refer to VFD section
for more information) . A "call for cool" must be field wired to the unit (terminals provided in unit - refer to wiring
diagram in unit control center) to allow for initiation of economizer mode . When the space calls for cooling,
factory supplied controls will drive the following wheel operations:
T > T
OA RA
T < T and T
OA RA
T < T and T
OA RA
where (T ) is the outdoor air temperature set point, (T
OA
the supply air discharge thermostat setpoint (nominal 60¡F Ð not adjustable) .
: Wheel runs at full speed (maximum energy recovery)
> T : Wheel is stopped (no energy recovery)
OA SA
< T : Wheel will modulate to maintain discharge temperature
OA SA
) is the return air temperature set point, and (T
RA
Temperature Sensor
with Override
Enthalpy Sensor
with Override
Enthalpy
Controller
) is
SA
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