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22.2. CAV Units in Single-Zone Applications with a Lennox Room Sensor
22.2.1.
Unit Controller Operation (4-Heat / 4-Cool)
When using a Lennox room sensor with the M3 unit controller operating in room sensor mode, a packaged rooftop unit
can provide up to four stages each of mechanical heating and cooling operation. The room sensor provides space
temperature information to the M3 unit controller. The M3 unit controller houses all space temperature set points and
controls all rooftop unit staging and general operation functions. The M3 unit controller also determines unit error codes,
provides diagnostic information and maintains safe operation limits.
22.2.2.
Ventilation Demand
When the M3 unit controller is in room sensor control mode, the user has several different ventilation sequence of
operation scenarios to choose from. The default mode causes the M3 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 M3 unit controller
receives both a ventilation and occupied demand. This is independent of a call for either heating or cooling. When the M3
unit controller receives a ventilation demand and occupied demand is not present, the M3 unit controller will only activate
the supply fan when it receives either a heating or cooling demand.
22.2.3.
Cooling Demand
The M3 unit controller directly monitors space temperature through the room sensor. Based on this information, the M3
unit controller activates the different compressor stages to maintain the desired occupied space temperature set point.
Increasing compressor stages provides more cooling capacity while decreasing compressor stages provides less
cooling capacity. The M3 unit controller has direct control over the rooftop unit mechanical cooling staging operation. The
user has the option to configure the M3 unit controller so that if the room sensor fails, the M3 unit controller can use a
backup operation to control unit operation.
Energence units feature up to four separate compressors and refrigeration circuits that can provide up to four stages of
mechanical cooling operation.
For stage one operation, the M3 unit controller activates the first compressor (25% of total unit capacity). For stage two
operation, the M3 unit controller activates the second compressor (50% unit capacity). For stage three operation, the M3
unit controller activates the third compressor (75% total unit capacity). For stage four operation, the M3 unit controller
activates the fourth compressor (100% unit capacity). Depending on the room sensor configuration setting, occupants in
the space can change the set point. The M3 unit controller automatically recognizes this change and instructs the unit to
respond accordingly.
NOTE: Energence Standard Efficiency 3 to 6-ton units use a two-step compressor (two speeds for cooling application)
and must have two-stage cooling inputs from a thermostat, third-party DDC control or third-party DDC electronic
thermostat module control.
22.2.4.
Cooling Demand with Economizer
If the outdoor air is suitable for free cooling and the unit has an economizer, the M3 unit controller will open the economizer
and use fresh air for stage one cooling. For stage two cooling operation, the M3 unit controller activates the first
compressor. For stage three cooling operation, the M3 unit controller activates the second compressor. For stage four
cooling operation, the M3 unit controller activates the remaining compressors (number three and four). The M3 unit
controller has direct control over the rooftop unit mechanical cooling staging and economizer operation.
22.2.5.
Heating Demand (General Operation)
The M3 unit controller directly monitors space temperature through the room sensor. Based on this information, the M3
unit controller turns on or off the heating stages to maintain the desired temperature set point. Increasing heating stages
provides additional heating capacity while decreasing heating stages provides less heating capacity. The M3 unit
controller has direct control over rooftop unit mechanical heating staging operation. Some units feature four separate
heating stages that can provide up to four stages of mechanical heating operation. The specific heating capacity varies
for each stage depending on the heat source. Depending on the room sensor configuration setting, occupants in the
space can change the set point. The M3 unit controller automatically recognizes this change and instructs the unit to
respond accordingly.
22.2.6.
Heating Demand (Electric)
Energence units feature multiple electric heat sections available in sizes from 7.5 kW to 180 kW (depending on unit size
and voltage). Units can provide up to four stages of mechanical heating depending on the size of the electric heater.
Staged operation occurs similar to cooling operation, with the M3 unit controller activating or deactivating sections of the
electric heater as the demand for heat increases or decreases.
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