Lennox Prodigy M2 Application Manual page 97

3. Heating demand
4. Cooling demand
In this configuration, the unit controller controls the rooftop staging and general operation. The third party unit controller only
informs the unit controller controller as to whether or not there is a specific demand. For example, if the third party unit con-
troller passes along a demand for cooling, then the unit controller increases or decreases cooling stages to maintain the
discharge supply air temperature setpoint. Along with controlling the rooftop unit, the unit controller also determines error
codes, provides diagnostic information and maintains safe operating limits.
19.1.8.2. Unit Sequence of Operation
Constant air volume units in bypass zoning applications featuring a third party unit controller with the unit controller operating
in thermostat mode, configured for discharge air temperature control have the same basic heating and cooling unit se-
quence of operations as constant air volume units in single zone applications featuring a third party unit controller, with the
unit controller operating in thermostat mode with discharge air temperature control.
For specific unit sequence of operation information refer to the Constant Air Volume Units in Single Zone Applications with a
Thermostat or Third Party Unit Controller and the Unit Operating in Discharge Air Temperature Control section.
The following sequence of operation information is specific to constant air volume units in bypass zoning applications.
19.1.8.3. Supply Duct Bypass Damper
To maintain accurate supply duct static pressure control, constant volume units in bypass zoning applications typically fea-
ture a bypass damper between the supply and return air ducts. In this scenario, the supply duct static pressure transducer
and damper connect directly to the third party unit controller. Based on actual static pressure relative to setpoint, the unit
controller either modulates open or modulates closed the bypass damper.
If the damper modulates further closed, the static pressure in the supply air duct increases. If the damper modulates further
open, the static pressure in the supply air duct decreases. The unit controller does not have any direct control over the
bypass damper in this scenario.
19.1.9. Variable Air Volume (VAV) Units in Zoning Applications with a Third Party Unit Controller and
the Unit Operating in Discharge Air Temperature Control (4 Heat / 4 Cool)
19.1.9.1. Unit Controller Operation
When using a third party unit controller with the unit controller operating in thermostat mode configured for discharge air
temperature control, a packaged rooftop unit can provide up to four stages of mechanical heating and cooling operation. To
operate correctly, a third party unit controller must provide the following wiring connections to the unit controller:
1. Ventilation demand
2. Occupied demand
3. Heating demand
4. Cooling demand
In this configuration, the unit controller controls the rooftop unit staging and general operation. The third party unit controller
only informs the unit controller as to whether or not there is a specific demand. For example, if the unit controller passes
along a demand for cooling, then the unit controller increases or decreases cooling stages to maintain the discharge supply
air temperature setpoint. In this mode, the unit controller also maintains the supply duct static pressure by directly controlling
the supply fan variable frequency drive. Along with providing control of the rooftop unit, the unit controller determines error
codes, supplies diagnostic information and maintains safe operating limits.
19.1.9.2. Ventilation Demand
Upon receiving a ventilation demand from the unit controller, the unit controller instructs the supply fan variable frequency
drive to start the supply fan and maintain a constant supply duct static pressure setpoint. The unit controller has direct con-
trol over the variable frequency drive and supply fan.
The unit controller receives supply duct static pressure information directly from a supply duct static pressure sensor located
approximately three quarters down the length of the longest main supply duct. Based on information from the supply duct
static pressure transducer relative to setpoint, the unit controller instructs the variable frequency drive to either increase or
decrease the speed of the supply fan. Speeding up the supply fan will increase the supply duct static pressure while slowing
down the supply fan speed will decrease the supply duct static pressure. For increased flexibility, the unit controller has
separate, adjustable static pressure setpoints for ventilation, cooling, heating and smoke alarms. These set points reside in
the unit controller memory, have factory default settings and may be adjusted in the field prior to start−up.
19.1.9.3. Cooling Demand
Upon receiving a cooling demand from the unit controller, the unit controller instructs the unit to maintain a cooling discharge
air temperature setpoint. The unit controller has direct control over the rooftop unit staging. The unit controller receives
discharge supply air temperature information directly from the supply duct temperature sensor located in the supply duct
system. Based on this information, the unit controller turns on or off the different compressor stages to maintain the dis-
charge supply air temperature setpoint (55°F default). Increasing compressor stages provides more cooling capacity while
decreasing compressor stages provides less cooling capacity.
Page 97
19. Sequence of Operation
PRODIGYT M2 UNIT CONTROLLER