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22.3.4.
Bypass Zoning Applications (4-Heat / 4-Cool)
22.3.4.1.
Unit Controller Operation
When using a third-party controller and the M3 unit controller is operating in thermostat mode and configured for
discharge air temperature control, a packaged rooftop unit can provide up to four stages of mechanical heating and
cooling operation. To operate correctly, a third-party controller must provide the following wiring connections to the M3
unit controller:
1. Ventilation demand
2. Occupied demand
3. Heating demand
4. Cooling demand
In this configuration, the M3 unit controller controls the rooftop staging and general operation. The third-party controller
only informs the M3 unit controller as to whether or not there is a specific demand. For example, if the third-party controller
passes along a demand for cooling, then the M3 unit controller increases or decreases cooling stages to maintain the
discharge supply air temperature set point. Along with controlling the rooftop unit, the M3 unit controller also determines
error codes, provides diagnostic information and maintains safe operating limits.
22.3.4.2.
Unit Sequence of Operation
Constant air volume units in bypass zoning applications featuring a third-party controller with the M3 unit controller
operating in thermostat mode, configured for discharge air temperature control 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 with discharge air temperature control.
For specific unit sequence of operation information refer to the constant air volume units in single-zone applications with a
wired thermostat or third-party controller and the unit operating in discharge air temperature control section.
The following sequence of operation information is specific to constant air volume units in bypass zoning applications.
22.3.4.3.
Supply Duct Bypass Damper
To maintain accurate supply duct static pressure control, constant volume units in bypass zoning applications typically
feature 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 modulates closed the bypass damper.
If the damper modulates further 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.
