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VAV Terminal Profile- Damper Position Calculation & Operation

Matt Blount
Updated

| Overview

The VAV terminal profile follows guideline 36 in all its aspects of operation. The Damper position calculation for a VAV box at the terminal side contributes to the effective operation of the system-level equipment.

The algorithm in the Airoverse VAV system ensures the same, and sequences are in place to optimally drive the damper position calculations, leading to optimal usage of system-level equipment (AHU/RTU), achieving enhanced energy savings.

For more information on the influence of Damper positions on system-level equipment refer to Damper Position & Zone Priority Driven Trim and Respond Requests

| Possible Configuration

The VAV Terminal profile comes with four possible types of configuration and controls, which are as follows.

  • Room temperature-based control
  • Airflow CFM-based control
  • Room CO2 level based control

based on the site requirement, the building configuration can be modified and the required controls can be opted for.

The Damper position calculation for different types of configuration and control is different.

| Damper Position Calculation for Room Temperature-based Control

The above configuration is a room temperature-based control. The VAV terminal sequences for the Damper position calculation for the same are like the example illustration below.

 

When the System is Cooling:

Zone/Room Demanding Cooling Zone/Room Demanding Heating

The Damper position is scaled from the Min Damper Position Cooling to Max Damper Position Cooling configured values based on the resultant Cooling Loop Output, which uses the current and Desired Cooling Temperatures.

Example: 

Let us Assume the Cooling Loop Output signal is 80%, the same is translated to Damper Positions as

  • Min Damper Position Cooling=20%
  • Max Damper Position Cooling=100%
  • Range of the Damper Position= 80%
  • 80% loop output signal scaled for Damper position range (80)= 80*80/100= 6400/100= 64%
  • Actual Damper Position= 20 (Minimum Damper Position) + 64% = 84%
  • The heating side calculates the Heating Loop Output based on the current and desired heating temperatures.
  • The Damper remains at a minimum for 0% - 50% of Heating Loop Output.
  • Also, the Reheat loop ramps up for the 0-50% of the Heating Loop Output till a maximum  Discharge Air Temperature (DAT) is reached, which is defined by the tuner parameter VAV-reheatZoneMaxDischargeTemp defaulted to 90F,
  • The Reheat Loop stops scaling once the maximum  Discharge Air Temperature (DAT) is reached
  • After 50% of the Heating Loop Output, the damper increases from the configured Min Damper Position Heating to  Max Damper Position Heating.

When the System is Heating:

Zone/Room Demanding Cooling Zone/Room Demanding Heating

The Damper position moves to the Min Damper Position Cooling configured value, not allowing further hot air into the room.

 

The Damper position is scaled from the Min Damper Position Heating to Max Damper Position Heating configured values based on the resultant Heating Loop Output, which uses the current and Desired Heating Temperatures.

Example: 

Let us Assume the Heating Loop Output signal is 60%, the same is translated to Damper Positions as

      1. Min Damper Position Heating=20%
      2. Max Damper Position Heating=100%
      3. Range of the Damper Position= 80%
      4. 60% loop output signal scaled for Damper position range (80)= 60*80/100= 4800/100= 48%
      5. Actual Damper Position= 20 (Min Damper Position Heating) + 48% = 68%
  • From 0% to 50% (which is tuner value valveActuationStartDamper PosDuringSysHeating) of the heating loop output, the damper position is proportionally scaled to let in more hot air.
  • For the remaining heating loop output which is above valveActuationStartDamper PosDuringSysHeating (50%), both the Damper position and Reheat are scaled as below.
  • Where, the remaining 50% damper position increases by 1% for every 1% heating loop increase, whereas the reheat increases by 2% for every 1% heating loop increase.

As a part of the adoption of visualizing information related the Airflow in CFM. At the portals end of the system, the following are made available:

In the VAV terminal profile, the user can track the airflow CFM setpoint in real-time against the actual CFM read from the DPS.

Also, In the VAV Terminal profile, the user can track effective DAT/DAT (Discharge Air Temperature setpoint against the CFM setpoint utilized by the airflow loop when the CFM is engaged. This is done by the logical points created in the CCU which can now be tracked on the portal using the widget.

image (11).png

| Damper Position Calculation for Room Temperature & CFM-based Control

The above configuration is a room temperature & CFM-based control. The VAV terminal sequences for the Damper position calculation for the same are like the example illustration below.

When the System is Cooling:

Zone/Room Demanding Cooling Zone/Room Demanding Heating
  • When CFM- based control is enabled, the Cooling Loop Output which uses the current and Desired Cooling Temperatures, is factored to calculate the Airflow set point between Min CFM Cooling and Max CFM Cooling configured values as below.

Example: 

Let us Assume the Cooling Loop Output signal is 80%, the same is factored to calculate the Airflow Setpoint as below:

  1. Min CFM Cooling= 50 cfm
  2. Max CFM Cooling =250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 80% Cooling Loop Output signal scaled for Airflow CFM Range (200)= 200*80/100= 16000/100= 160 cfm
  5. Actual Airflow Setpoint= 50 (Min CFM(Cooling))+ 160= 210 cfm
  • The Damper position is scaled from the Min Damper Position Cooling to the Max Damper Position Cooling configured values based on the resultant CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM,

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.
  • When CFM-based control is Enabled, the Heating Loop Output is factored to calculate the Airflow set point between Min CFM Reheating and Max CFM Reheating configured values.

Example: 

Let us Assume the Heating Loop Output signal is 20%, the same is factored to calculate the Airflow Setpoint as below:

  1. Min CFM Reheating= 50 cfm
  2. Max CFM Reheating =250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 20% Heating Loop Output signal scaled for Airflow CFM Range (200)= 200*20/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 50 (Min CFM(Cooling))+ 40= 90 cfm
  • The Damper position goes from the Min Damper Position Heating to the minimum CFM requirement based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

 

When the System is Heating:

Zone/Room Demanding Cooling Zone/Room Demanding Heating
  • When CFM-based control is Enabled, the Cooling Loop Output is factored to calculate the Airflow set point between Min CFM Cooling and Max CFM Cooling configured values.

Example: 

Let us Assume the Cooling Loop Output signal is 20%, the same is factored to calculate the Airflow Setpoint as below:

  1. Min CFM Reheating= 50 cfm
  2. Max CFM Reheating =250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 20% Cooling Loop Output signal scaled for Airflow CFM Range (200)= 200*20/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 50 (Min CFM(Cooling))+ 40= 90 cfm
  • The Damper position goes from Min Damper Position Cooling to the Minimum CFM requirement based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.
  • When CFM-based control is Enabled, the Heating Loop Output is factored to calculate the Airflow set point between Min CFM Reheating and Max CFM Reheating configured values.

Example: 

Let us Assume the Heating Loop Output signal is 80%, the same is factored to calculate the Airflow Setpoint as below:

  1. Min CFM Reheating= 50 cfm
  2. Max CFM Reheating =250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 80% Heating Loop Output signal scaled for Airflow CFM Range (200)= 200*80/100= 16000/100= 160 cfm
  5. Actual Airflow Setpoint= 50 (Min CFM(Cooling))+ 160= 210 cfm
  • The Damper position is scaled from the Min Damper Position Heating to the Max Damper Position Heating based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

 

Note: The default value of the following tuners has been changed for efficient airflow balancing to give more weightage to the Integral side of the loop and also to reduce the amount of time it takes for the integral to ramp up. The new tuner values are as follows:
  • airflowCFMproportionalKFactor = 0.2
  • airflowICFMntegralKFactor = 0.8
  • airflowCFMintegralTime = 5

Note: In the system, edits to PI Loop related tuners such as proportionalKfactor, IntegralKfactor, temperatureProportionalRange, TemperatureIntegralTime, and any other PI related tuners made through the Internal Portal are consumed on the fly without the need for a restart. This seamless process ensures that the PI Loop restarts automatically with the updated tuner values. This is also applicable for the Trim and Response loop configuration.

In the VAV terminal profile, the user can track the airflow CFM setpoint in real-time against the actual CFM read from the DPS which is visualized in CCU and Portal using the available widget.

Also, In the VAV Terminal profile, the user can track effective DAT/DAT (Discharge Air Temperature setpoint against the CFM setpoint utilized by the airflow loop when the CFM is engaged. This is done by the logical points created in the CCU which can now be tracked on the portal using the available widget.

TRUECFM.png

 

| Damper Position Calculation for Room Temperature & CO2 based control

The above configuration is a room temperature, CO2-based control. The VAV terminal sequences for the Damper position calculation for the same are like the example illustration below.

When the System is Cooling:

Zone/Room Demanding Cooling Zone/Room Demanding Heating
  • When the CO2-based control is enabled,  the Cooling Loop Output which uses the current and Desired Cooling Temperatures, is factored to calculate the new minimum damper position based on the CO2 levels breach, as below

Example: 

Let us Assume there is a CO2 breach in the system

    1. zoneCO2Threshold=800 ppm
    2. zoneCO2Target=1000 ppm
    3. Zone CO2 Breach Range  = 200 ppm
    4. Zone Current CO2=875 ppm
    5. Actual Breach= 75 ppm
    6. Breach Percentage= 75/200*100= 37.5%
  • The Damper position is scaled from the Calculated Damper Position Cooling to the Max Damper Position Cooling configured value based on the Cooling Loop Output, as below

Example: 

Let us Assume the Cooling Loop Output is 80% system

  1. Configured min damper position=20%
  2. Calculated Min Damper Position=37.5%
  3. Max Damper Position=100%
  4. Range of the Damper Position= 62.5%
  5. 80% loop output signal scaled for Damper position range (80)= 80*62.5/100= 5000/100= 50%
  6. Actual Damper Position = 37.5 (Minimum Damper Position) + 50% = 87.5%

 

  •  
  • When CO2-based control is Enabled, the Heating Loop Output is factored to calculate the new minimum damper position based on the CO2 levels breach, as below.

Example: 

Let us Assume there is a CO2 breach in the system

  1. zoneCO2Threshold=800 ppm
  2. zoneCO2Target=1000 ppm
  3. Zone CO2 Breach Range  = 200 ppm
  4. Zone Current CO2=875 ppm
  5. Actual Breach= 75 ppm
  6. Breach Percentage= 75/200*100= 37.5%
  • The Damper position goes from the Min Damper Position Heating to the New Calculated minimum damper position based on the CO2 levels breach.

 

When the System is Heating:

Zone/Room Demanding Cooling Zone/Room Demanding Heating
  • When the CO2-based control is enabled,  the Cooling Loop Output which uses the current and Desired Cooling Temperatures, is factored to calculate the new minimum damper position based on the CO2 levels breach, as below

Example: 

Let us Assume there is a CO2 breach in the system

    1. zoneCO2Threshold=800 ppm
    2. zoneCO2Target=1000 ppm
    3. Zone CO2 Breach Range  = 200 ppm
    4. Zone Current CO2=875 ppm
    5. Actual Breach= 75 ppm
    6. Breach Percentage= 75/200*100= 37.5%
  • The Damper position goes from the Minimum Damper Position Cooling to the New Calculated Damper Position based on the CO2 levels breach.

 

  •  
  • When CO2-based control is Enabled, the Heating Loop Output is factored to calculate the new minimum damper position based on the CO2 levels breach, as below.

Example: 

Let us Assume there is a CO2 breach in the system

  1. zoneCO2Threshold=800 ppm
  2. zoneCO2Target=1000 ppm
  3. Zone CO2 Breach Range  = 200 ppm
  4. Zone Current CO2=875 ppm
  5. Actual Breach= 75 ppm
  6. Breach Percentage= 75/200*100= 37.5%
  • The Damper position is scaled from the Calculated Damper Position Heating to the Max Damper Position Heating configured value based on the Cooling Loop Output, as below

Example: 

Let us Assume the Heating Loop Output is 80% system

  1. Configured min damper position=20%
  2. Calculated Min Damper Position=37.5%
  3. Max Damper Position=100%
  4. Range of the Damper Position= 62.5%
  5. 80% loop output signal scaled for Damper position range (80)= 80*62.5/100= 5000/100= 50%
  6. Actual Damper Position = 37.5 (Minimum Damper Position) + 50% = 87.5%

| Damper Position Calculation for Room Temperature, CFM, & CO2 based control

In this configuration, the Damper Position calculation would have four calculation stages.

  • The Breach percentage on the CO2 breaches, as in the above section.
  • The Breach Percentage translated to the new minimum cfm.
  • The airflow set point calculation using the new minimum and maximum airflow cfm, and the temperature-based PI loop output.
  • The CFM loop output calculation from the airflow set point and actual airflow, which is translated to the Damper position minimum and Maximum.

The considered Highest breach is used to calculate the minimum cfm.

And based on the below table the Damper positions operate.

When the System is Cooling:

Zone/Room Demanding Cooling Zone/Room Demanding Heating

The Breach Percentage translated to the new minimum cfm.

  1. Min CFM(Cooling/Heating)=50 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 75 % Breaches Transaltes to =75*200/100= 15000/100= 150 cfm
  5. The New minimum CFM= 50 (Min CFM(Cooling/Heating))+ 150= 200 cfm

The airflow set point calculation

Let us Assume the Cooling Loop output signal is 80%, the same is translated to Airflow Setpoint as

  1. New Min CFM(Cooling/Heating)=200 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 50 cfm
  4. 80% loop output signal scaled for Airflow CFM Range (50)= 50*80/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 200 (Min CFM(Cooling/Heating))+ 40= 240 cfm
  • The Damper position is scaled from the Min Damper Position Cooling to the Max Damper Position Cooling configured values based on the resultant CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

 

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

  •  

The Breach Percentage translated to the new minimum cfm.

  1. Min CFM(Cooling/Heating)=50 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 75 % Breaches Transaltes to =75*200/100= 15000/100= 150 cfm
  5. The New minimum CFM= 50 (Min CFM(Cooling/Heating))+ 150= 200 cfm

The airflow set point calculation

Let us Assume the Cooling Loop output signal is 80%, the same is translated to the Airflow Setpoint as

  1. New Min CFM(Cooling/Heating)=200 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 50 cfm
  4. 80% loop output signal scaled for Airflow CFM Range (50)= 50*80/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 200 (Min CFM(Cooling/Heating))+ 40= 240 cfm
  • The Damper position goes from the Min Damper Position Heating to the minimum CFM requirement based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

 

When the System is Heating:

Zone/Room Demanding Cooling Zone/Room Demanding Heating

 

The Breach Percentage translated to the new minimum cfm.

  1. Min CFM(Cooling/Heating)=50 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 75 % Breaches Transaltes to =75*200/100= 15000/100= 150 cfm
  5. The New minimum CFM= 50 (Min CFM(Cooling/Heating))+ 150= 200 cfm

The airflow set point calculation

Let us Assume the Cooling Loop output signal is 80%, the same is translated to Airflow Setpoint as

  1. New Min CFM(Cooling/Heating)=200 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 50 cfm
  4. 80% loop output signal scaled for Airflow CFM Range (50)= 50*80/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 200 (Min CFM(Cooling/Heating))+ 40= 240 cfm
  • The Damper position goes from Min Damper Position Cooling to the Minimum CFM requirement based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

 

The Breach Percentage translated to the new minimum cfm.

  1. Min CFM(Cooling/Heating)=50 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 200 cfm
  4. 75 % Breaches Transaltes to =75*200/100= 15000/100= 150 cfm
  5. The New minimum CFM= 50 (Min CFM(Cooling/Heating))+ 150= 200 cfm

The airflow set point calculation

Let us Assume the Cooling Loop output signal is 80%, the same is translated to the Airflow Setpoint as

  1. New Min CFM(Cooling/Heating)=200 cfm
  2. Max CFM(Cooling/Heating)=250 cfm
  3. Airflow CFM Range = 50 cfm
  4. 80% loop output signal scaled for Airflow CFM Range (50)= 50*80/100= 4000/100= 40 cfm
  5. Actual Airflow Setpoint= 200 (Min CFM(Cooling/Heating))+ 40= 240 cfm
  • The Damper position is scaled from the Min Damper Position Heating to the Max Damper Position Heating based on the CFM Loop Output, which uses the Actual Airflow Set Point and Measured Airflow CFM.

For more information on the Measured CFM calculation refer to True CFM (Cubic Feet Per Minute) VAV.

The CFM loop output calculation from the airflow CFM set point and actual airflow CFM, which is translated to the Damper position minimum and Maximum.

For effects in the system-level equipment refer to Damper Position & Zone Priority Driven Trim and Respond Requests.

 

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