Lennox Prodigy M2 Application Manual page 92

19. Sequence of Operation
Control of the fans can occur based on damper position or building differential static pressure transducers located outside
the building and in the return duct. Using the differential pressure transducer allows for more precise control of building static
pressure and ultimately better performance. Control of power exhaust fans can occur through the unit controller, third party
device or separate unit controller.
19.1.3.9. Damper Position Control
Power exhaust fans (standard or high static) with damper position control use damper position to determine when to activate
fan operation. When the economizer damper is closed, the power exhaust fan will remain off. Once the economizer modu-
lates open past a pre−determined position, the power exhaust fan will turn on. This allows the unit to relieve a portion of the
incoming fresh outdoor air and help reduce building static pressure.
If using a 100% (two fans) power exhaust configuration, a second power exhaust fan will turn on once the economizer damp-
er modulates open past a second pre−determined position. Turning on the second fan will allow the unit to further reduce
building static pressure.
19.1.3.10. Differential Static Pressure Control
Power exhaust fans (standard or high static) with building differential static pressure transducer control use the actual build-
ing static pressure relative to the outdoor atmospheric pressure to activate fan operation. Based on actual building static
pressure as determined by the building differential pressure transducer, the unit controller, third party device or unit control-
ler will instruct the power exhaust fan(s) to turn on or off as needed to maintain the building static pressure setpoint.
Turning on the fans decreases building pressure, while stopping fan operation increases building pressure. Power exhaust
confirmations with two fans have two stage capability for improved building static pressure performance and enhanced con-
trol. The building pressure setpoint resides in the unit controller.
19.1.3.11. Building Pressure Control for High Static Power Exhaust Fans with Variable Frequency Drives
Energence units can control building static pressure with a high static power exhaust fan featuring a variable frequency
drive, using building differential static pressure control. This system provides precise and powerful control of building static
pressure. This system uses actual building static pressure relative to the outdoor atmospheric pressure and a variable fre-
quency drive to activate fan operation and modulate fan speed. It is important to note that the unit controller connects directly
to and controls the variable frequency drive and that the building static pressure setpoint resides in the unit controller.
Based on the actual building static pressure (as determined by the building pressure transducer) the unit controller instructs
the power exhaust fan(s) to increase or decrease speed as needed to maintain the building static pressure setpoint. Increas-
ing fan speed decreases building pressure while decreasing fan speed increases building pressure. Power exhaust configu-
rations with two fans (100% capacity) have the ability to remove more exhaust air than single fan configurations.
19.1.4. Constant Air Volume (CAV) Units in Single Zone Applications with a Lennox Zone Sensor (4
Heat / 4 Cool)
19.1.4.1. Unit Controller Operation
When using a Lennox zone sensor with the unit controller operating in zone sensor mode, a packaged rooftop unit can
provide up to four stages each of mechanical heating and cooling operation. The zone sensor provides space temperature
information to the unit controller. The unit controller houses all space temperature setpoints and controls all rooftop unit
staging and general operation functions. The unit controller also determines unit error codes, provides diagnostic informa-
tion and maintains safe operation limits.
19.1.4.2. Ventilation Demand
When the unit controller is in zone sensor control mode, the user has several different ventilation sequence of operation
scenarios to choose from. The default mode causes the unit controller to activate the supply fan when both a ventilation and
either heating or cooling demand are present. This occurs independent of receiving an occupied demand. The user can
change the default setting to allow the supply fan to run continuously when the unit controller receives both a ventilation and
occupied demand. This is independent of a call for either heating or cooling. When the unit controller receives a ventilation
demand and occupied demand is not present, the unit controller will only activate the supply fan when it receives either a
heating or cooling demand.
19.1.4.3. Cooling Demand
The unit controller directly monitors space temperature through the zone sensor. Based on this information, the unit control-
ler activates the different compressor stages to maintain the desired occupied space temperature setpoint. Increasing com-
pressor stages provides more cooling capacity while decreasing compressor stages provides less cooling capacity. The unit
controller has direct control over the rooftop unit mechanical cooling staging operation. The user has the option to configure
the unit controller so that if the zone sensor fails, the unit controller can use a backup operation to control unit operation.
Energence units feature four separate compressors and refrigeration circuits that can provide up to four stages of mechani-
cal cooling operation.
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