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22.3.2.5.
Heating Demand (General Operation)
Upon receiving a heating demand from a thermostat or a third-party controller, the M3 unit controller instructs the unit to
maintain a heating discharge air temperature set point. The M3 unit controller has direct control over the rooftop unit
heating staging operation. The M3 unit controller receives discharge supply air temperature information directly from the
temperature sensor located in the supply duct. Based on this information, the M3 unit controller activates the different
heating stages to maintain the discharge supply air temperature set point (110°F default). Turning on additional heating
stages increases the heating capacity, while turning off heating stages decreases the heating capacity. The heating
discharge air temperature set point resides in the M3 unit controller, has a factory default setting, and may be adjusted at
start up. The user can adjust the set point either locally or remotely with Lennox L Connection Network unit controller
software or at the M3 unit controller. The user can not adjust the set point through a third-party control device or software
program.
22.3.2.6.
Heating Operation (Electric)
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 to
maintain the discharge air temperature set point.
22.3.3.
Bypass Zoning Applications (2-Heat / 2-Cool)
22.3.3.1.
Unit Controller Operation
When using a third-party M3 unit controller and the M3 unit controller is operating in the thermostat mode, a packaged
rooftop unit can provide up to two stages of mechanical heating and cooling operation. To operate correctly, a M3 unit
controller must provide the following wiring connections to the M3 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 configuration the third-party controller will control the rooftop unit staging and general operation. The M3 unit
controller functions primarily to determine unit error codes, provide diagnostic information and maintain safe operating
limits.
22.3.3.2.
Unit Sequence Of Operation
Constant air volume units in bypass zoning applications featuring a Lennox or third-party controller with the M3 unit
controller operating in thermostat mode, have the same basic heating and cooling unit sequence of operations as
constant air volume units in single-zone applications featuring a third-party controller, with the M3 unit controller
operating in thermostat mode. For specific information, refer to the constant air volume in single-zone applications with a
wired thermostat or third-party controller section.
The following sequence of operation information is specific to constant air volume units in bypass zoning applications.
22.3.3.3.
Supply Duct Bypass Damper
To maintain accurate supply duct static pressure control, constant volume units in bypass zoning applications use 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 M3 unit controller. Based on actual static pressure relative to set point, the M3 unit
controller either modulates open or closes the damper. If the damper 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.
