Lennox Prodigy M2 Application Manual page 91

19. Sequence of Operation
The unit controller controls modulation by adjusting either one or both of the gas burner sections. Upon receiving a heating
demand, the unit controller will bring on both gas burner sections at 100%. When the discharge air temperature reaches the
setpoint (default 110°F), the unit controller will modulate both gas burner sections by the same amount between 100% and
50% to maintain the setpoint. If less heat is required to maintain the setpoint, the unit controller will turn off the second gas
burner section and modulate the first gas burner section between 100% and 50% (50% to 25% of total unit capacity).
The basic operation of modulating gas remains the same regardless of unit type or unit controller mode. Gas heat modula-
tion requires the necessary mechanical components, a discharge air temperature sensor located in the supply duct and a
single heating demand to the unit controller.
19.1.3.2. Occupied Demand
Upon receiving occupied and ventilation demands from the Lennox or third party unit controller, the unit controller adjusts the
fresh air damper to either a fixed minimum position or allows it to modulate based on a CO2 sensor (demand control ventila-
tion). The CO2 sensor can be wired directly to the unit controller, to another controller that can monitor the sensor and pass a
signal to the unit controller for damper control, or to both the unit controller and another device for monitoring through the
desired man−machine interface while the unit controller maintains damper control.
During morning warm−up the unit controller keeps the fresh air damper closed based on unit controller configuration set-
tings. Setpoints for minimum and maximum damper position and CO2 control reside in the unit controller memory, have
factory default settings, and may be adjusted at start up. The user can change these settings either locally or remotely
through Lennox' L Connection Network® Unit Controller Software. The user will not have the ability to adjust the settings
through third party software or control devices.
19.1.3.3. Demand Control Ventilation
Demand control ventilation is used in applications where the demand for fresh outdoor air fluctuates during the occupied
time period. Using a CO2 sensor connected directly to the unit controller, the unit can intelligently increase or decrease the
amount of fresh outdoor air by changing the outdoor air damper position. The unit controller has two operation modes avail-
able, setpoint or proportional, to control the outdoor air damper position.
19.1.3.4. Fresh Air Tempering (FAT)
In applications with large outdoor air requirements, Fresh Air Tempering is used to minimize temperature fluctuations in the
conditioned space. The unit controller controls discharge air temperature by energizing heating or cooling in response to the
discharge supply air duct temperature. Fresh air tempering only occurs when there is no heating or cooling demand from the
occupied space. The user must configure the unit controller to turn on the fresh air tempering option.
Heating is energized when discharge air temperature falls below fresh air heating setpoint (60°F default) and terminates
when the return air temperature is less than the setpoint. Cooling is energized when discharge air temperature rises above
fresh air cooling setpoint (80°F default) and terminates when the return air temperature is greater than the setpoint. FAT will
operate up to four stages of heating and cooling to maintain discharge air temperature. Standard heating and cooling de-
mands will override FAT heating and cooling demands.
19.1.3.5. Hot Gas Bypass
By selecting the hot gas bypass option, the unit can operate in low airflow applications down to 12.5% of nominal capacity.
As the suction line pressure decreases and the potential for coil frosting increases, the mechanical system bypasses hot
refrigerant gas from the first stage compressor discharge line back to the suction line. The hot gas increases the pressure of
the suction line and reduces the compressor capacity. A de−superheater valve bypasses refrigerant from the liquid line and
mixes it with the hot gas before entering the suction line to maintain the setpoint suction gas superheat entering the com-
pressor.
19.1.3.6. Discharge Air Cooling Reset Operation
Discharge air cooling reset operation saves energy by gradually increasing the discharge air setpoint as outside air temper-
ature decreases. This operation also reduces the potential for overcooling if the zoning system is misapplied, has an abnor-
mal condition, or has a dominant zone. The unit controller has various advanced discharge air cooling reset options which
can be selected at start up and are based on either return air temperature, outside air temperature, or both return and out-
door air temperature.
19.1.3.7. Discharge Air Heating Reset Operation
Discharge air heating reset operation saves energy by gradually decreasing the discharge air setpoint as outside air temper-
ature increases. This operation reduces the potential for overheating if the zoning system is misapplied, has an abnormal
condition, or has a dominant zone. The unit controller has various advanced discharge air heating reset options which can
be selected at start up and are based on either return air temperature, outside air temperature or both return and outdoor air
temperature.
19.1.3.8. Building Pressure Control for Standard or High Static Power Exhaust Fans
Energence units can control building static pressure with either a standard or high static power exhaust fan. Each fan type is
available in either a 50% (one fan) or 100% (two fans) configuration. Standard static power exhaust fans use a propeller
while high static power exhaust fans use a centrifugal blower. All units featuring power exhaust fans must also have an
economizer for proper operation.
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PRODIGYT M2 UNIT CONTROLLER