| Overview

In many VVT applications, we have found the dynamic loads of the spaces to exceed that of native VVT's ability to redirect the airflow. This means there are zones with competing loads for cooling and heating and the targetCumulativeDamperPos = 70% keeps the zone dampers open too far to satisfy the comfort.

We have already begun deploying the Smart Node PI loop controller as a standalone bypass damper, ensuring the unit's back pressure never reaches dangerous levels when we set the targetCumulativeDamperPos to a much lower value. For more information refer to Bypass Damper Using the Generic PI Loop.
The PI loop profile has been very beneficial in this way but has limits and does not allow the system to operate as a whole to take the most advantage of having a bypass damper installed.

| Wiring Schematic


  • Wire the Analog out and the 24-volt power supply from the SmartNode to the bypass damper actuator.
  • Wire the input from the Differential Pressure Sensor to the Analog in of the SmartNode, and the common 24-volt power supply from the SmartNode to power the Differential Pressure Sensor.

| Bypass Damper System Device

A bypass damper is added as a System Device just like the OAO, similar to how OAO operates in conjunction with the system, as below.

Bypass Damper.PNG

| Configuration Parameter

Parameter Description
Pressure Sensor Type To select a pressure sensor type
Damper Type To define the damper type in terms of voltage
Damper Min Position (%) To define a minimum damper position for the device
Damper Max Position (%) To define a maximum damper position for the device
SAT Min Threshold (°F) To define a supply air temperature minimum threshold
SAT Max Threshold (°F) To define a supply air temperature maximum threshold
Expected Pressure Error (InH2O) To define a pressure error in InH2O to be used by the bypass loop output
Expected Pressure Error (InH2O) To define a pressure setpoint in InH2O to be used by the bypass loop output

| Operations

Bypass Loop Output

When system FanLoopOutput = 0 then damperPosition = minDamperPosition


When fanLoopOutput > 0 the bypassLoopOuput is enabled which uses staticDuctPressureSensor as the control variable and staticDuctPressureSp as the setpoint.


damperPosition = bypassLoopOutput(%)*100 respecting damperMinPosition and damperMaxPositions.


Supply Air Temp Equipment Lockouts

Adding bypass dampers introduces risk to customers' HVAC equipment by assuming pressure will always be under control.

But in extreme cases or when a bypass damper actuator fails or the damper gets stuck open, that could cause severe harm to the unit.

So, the following lockouts are in place:

  • If bypassSupplyAirTemp > satMaxThreshold for more than satLimitBreachTime then bypassHeatingLockout is enabled.
  • If bypassSupplyAirTemp < satMinThreshold for more than satLimitBreachTime then bypassCoolingLockout is enabled.

When either of the above conditions clear conditioning resumes.

| Failsafe Mode

When the SmartNode loses the connection with the CCU, all the PI loop controls and SAT safety checks, are handled by the SmartNode itself.




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