Operations Common To All Rooftop Units; Occupied Demand; Demand Control Ventilation (Dcv); Fresh Air Tempering (Fat) - Lennox PRODIGY2.0 Application Manual

AFTERMARKET
RTU
CONTROLLER OR
THERMOSTAT
22.1.3.

Operations Common to All Rooftop Units

The following sequence of operation information applies to all rooftop units regardless of M3 unit controller control mode,
unit type or zoning application.
22.1.3.1.

Occupied Demand

Upon receiving occupied and ventilation demands from the a zoning control the M3 unit controller adjusts the fresh air
damper to either a fixed minimum position or allows it to modulate based on a carbon dioxide sensor (demand control
ventilation). The carbon dioxide sensor can be wired directly to the M3 unit controller, to another controller that can
monitor the sensor and pass a signal to the M3 unit controller for damper control, or to both the M3 unit controller and
another device for monitoring through the desired man-machine interface while the M3 unit controller maintains damper
control.
During morning warm-up the M3 unit controller keeps the fresh air damper closed based on M3 unit controller
configuration settings. Set points for minimum and maximum damper position and carbon dioxide control reside in the M3
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
ability to adjust the settings through third-party software or control devices.
22.1.3.2.

Demand Control Ventilation (DCV)

Demand control ventilation is used in applications where the demand for fresh outdoor air fluctuates during the occupied
time period. Using a carbon dioxide sensor connected directly to the M3 unit controller, the unit can intelligently increase
or decrease the amount of fresh outdoor air by changing the outdoor air damper position. The M3 unit controller has two
operation modes available, set point or proportional, to control the outdoor air damper position.
22.1.3.3.

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 M3 unit controller controls discharge air temperature by energizing heating or cooling in response
to the discharge air temperature. Fresh air tempering only occurs during occupied periods when the blower is running and
when there is no heating or cooling demand from the space. The user must configure the M3 unit controller to turn on the
fresh air tempering options:
To enable FAH, go to SETUP > TEST & BALANCE > DAMPER > (NOTE: There are several settings to select in this path
before reaching the desired setting ) FRESH AIR HEATING ENABLE FAH = YES or NO. After enabling FAH, the next
screen will allow the FAH set point to be configured. FAH set point range is between 40.0°F to 62.0°F.
To enable FAC, go to SETUP > TEST & BALANCE > DAMPER > (NOTE: There are several settings to select in this path
before reaching the desired setting ) FRESH AIR COOLING ENABLE FAC = YES or NO. After enabling FAC, the next
screen will allow the FAC set point to be configured. FAH set point range is between 60.0°F to 90.0°F.
Heating is energized when discharge air temperature falls below FAH set point and terminates when the discharge air
temperature rises above the set point plus the heating dead-band (FAH default). Cooling is energized when discharge air
temperature rises above FAC set point and terminates when the discharge air temperature falls below the set point minus
the cooling dead-band (parameter 171, 10°F default). Fresh air tempering will operate up to four stages of heating and
cooling to maintain discharge air temperature. Standard heating and cooling demands will override FAT heating and
cooling demands. Also, when parameter 113 (EN RET AIR TMP LMT) is set to 1 (ENABLE), the return air temperature
limits set in parameter 114 (COOL RET AIR LIMIT) and 115 (HEAT RET AIR LIMIT) are enforced on FAT operation as
well.
NOTE: On Energence Ultra-High Efficiency 3 to 6-ton units, the unit controller modulates the variable speed compressor
to maintain the desired discharge air temperature.
24VAC
R C G
24VAC
COMMON
VENTILATION DEMAND
HEATING DEMAND
COOLING DEMAND
OCCUPIED DEMAND
Figure 30. Variable Air Volume Unit in Zoning Application
THERMOSTAT G 24VAC SENSOR
SENSOR
IAQ
L
W1 W2 Y1 Y2 OCP O R C
AI1 HUM
TMP
®
unit controller Software. The user will not have the
85
OUTPUTS SMOKE INPUTS HUMIDISTAT
24VAC
DI1
D01 D02 DI4 C
R C DI2 R DI3 R