Lennox Prodigy M2 Application Manual page 94

19.1.5. Constant Air Volume (CAV) Units in Single Zone Applications with a Thermostat or Third Party
Unit Controller (2 Heat / 2 Cool)
19.1.5.1. Unit Controller Operation
When using a two−stage heat/ cool thermostat or third party unit controller with the unit controller in the thermostat mode, a
packaged rooftop unit can provide up to two stages of mechanical heating and cooling operation. To operate correctly, a
thermostat or third party unit controller must provide the following wiring connections to the unit controller:
1. Ventilation demand
2. Occupied demand
3. Heating demand one
4. Heating demand two
5. Cooling demand one
6. Cooling demand two
In this set up, either the thermostat or third party unit controller controls the rooftop unit staging and general operation. The
unit controller functions primarily to determine unit error codes, provide diagnostic information and maintain safe operation
limits.
19.1.5.2. Ventilation Demand
Upon receiving a ventilation demand from the thermostat or third party unit controller, the unit controller instructs the supply
fan to start operation. The supply fan runs at full capacity as long as a ventilation demand is present.
19.1.5.3. Cooling Demand
Upon receiving a stage one demand for cooling from the thermostat or third party unit controller, the unit controller activates
the first two compressors, providing 50% cooling capacity.
If the unit is unable to satisfy the call for cooling within a specified time period and receives a stage two cooling demand from
the thermostat or third party unit controller, the unit controller activates the third and fourth compressors, providing 100%
cooling capacity. The thermostat or third party unit controller has direct control over the rooftop unit's staging capability.
19.1.5.4. Cooling Demand With Economizer
If the unit features an economizer and outdoor air is suitable for free cooling, a call for stage one cooling will activate the
economizer. The unit will try to satisfy the cooling demand using outdoor air rather than mechanical cooling.
If the unit is unable to satisfy the call for cooling within a specified time period using the economizer and receives a stage two
call for cooling from the thermostat or third party unit controller, the unit controller activates all four compressors. This will
provide 100% cooling capacity. It is important to note that the thermostat or third party unit controller has direct control over
the rooftop unit's staging capability. While the unit controller typically has direct control over the economizer, it is possible for
a thermostat or third party unit controller to directly control this functionality.
19.1.5.5. Heating Demand (General Operation)
Upon receiving a stage one heating demand from the thermostat or third party unit controller, the unit controller activates the
unit's heating section to start operation. This activates the first two stages of mechanical heat, providing approximately 66%
heating capacity.
If the unit is unable to satisfy the call for heating within a specified time period and receives a stage two heating demand from
the thermostat or third party controller, the unit controller activates the third and fourth stages of heat, providing 100% heat-
ing capacity. It is important to note that the thermostat or third party unit controller has direct control over the rooftop unit's
staging capability.
19.1.5.6. Heating Demand (Electric)
Units feature multiple electric heat sections available in sizes from 30 kW to 180 kW (depending on unit size and voltage).
Units can provide up to two stages of mechanical heating depending on the size of the electric heater. Staged operation
occurs similar to cooling operation, with the thermostat or third party unit controller activating or deactivating sections of the
electric heater as the demand for heat increases or decreases.
19.1.5.7. Humiditrol Dehumidification Operation − Dehumidification Demand
Upon a dehumidification demand, the unit controller activates compressor number one and two. At the same time, the unit
controller uses solenoid valves to divert hot gas from compressor one and two to the first reheat coil. The cooled and dehu-
midifed air from the evaporator is then reheated as it passes through the reheat coil. The de−superheated and partially con-
densed refrigerant continues to the outdoor condenser coil where condensing is completed. The reheat coil is sized to offset
most of the first and second stages of sensible cooling effect during reheat only operation. This reduction in sensible cooling
capacity extends compressor run time to control humidity when cooling loads are light.
The unit will continue to operate in this mode until the dehumidification demand is satisfied. A heating demand will terminate
reheat operation.
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19. Sequence of Operation