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| 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 75F 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
  • Room VOC 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 Minimum to the Maximum position ( Configured parameters), based on the resultant PI loop (Cooling), that uses the current and desired temperatures (Cooling).

Example: 

Let us Assume the PI loop output signal is 80%, the same is translated to Damper Positions as

  • Min Damper Position=20%
  • Max Damper Position=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%

On the heating side the PI loop (Heating Loop Output), which uses the current and desired temperatures (heating) is used to determine the DAT setpoint.

The Loop output signal is scaled for the SAT setpoint (which is equal to DAT and considered as the heating minimum endpoint, before the reheat loop triggers)  and reheatZoneMaxDischargeTemp ( heating maximum endpoint) to determine the DAT setpoint.

Example: 

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

  • SAT SP=70 F
  • reheatZoneMaxDischargeTemp=90F
  • DAT Range = 20F
  • 50% heating loop output will be yielding max DAT SP of 90F     

The Damper remains at a minimum from 0% - 50% of the Heating Loop Output, till a maximum Discharge Air Temperature (DAT) is reached. This is defined by the tuner parameter VAV-reheatZoneMaxDischargeTemp, which is by default 90F.

The Reheat loop output triggers, that use the DAT set point and actual DAT for the loop output calculation, as shown above.

For 50% and above of the Heating Loop Output (when the DAT is more than the zone temperature+ reheatZoneToDATMinDifferential), the damper position is proportionally scaled for the rest of the Loop output if the required conditioning is not achieved.

The damper increases from 20%(the configured), or calculated min damper position whichever is higher, to 100% when the heating loop output is 100%

 

| 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.

Zone/Room Demanding Cooling Zone/Room Demanding Heating

On the cooling side the PI loop (Cooling Loop Output), which uses the current and desired temperatures (Cooling) is used to determine the Airflow setpoint.

The Loop output signal is scaled for configured Minimum and Maximum CFM to determine the Airflow setpoint.

Example: 

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

  • Min CFM(Cooling)=50 cfm
  • Max CFM(Cooling)=250 cfm
  • Airflow CFM Range = 200 cfm
  • 80% loop output signal scaled for Airflow CFM Range (200)= 200*80/100= 16000/100= 160 cfm
  • Actual Airflow Setpoint= 50 (Min CFM(Cooling))+ 160= 210 cfm

On the heating side the PI loop (Heating Loop Output), which uses the current and desired temperatures (heating) is used to determine the DAT setpoint.

The Loop output signal is scaled for the SAT setpoint (which is equal to DAT and considered as the heating minimum endpoint, before the reheat loop triggers)  and reheatZoneMaxDischargeTemp ( heating maximum endpoint) to determine the DAT setpoint.

Example: 

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

  • SAT SP=70 F
  • reheatZoneMaxDischargeTemp=90F
  • DAT Range = 20F
  • 50% heating loop output will be yielding max DAT SP of 90F  

 

Zone/Room Demanding Cooling Zone/Room Demanding Heating

The Damper position is scaled from the Minimum to the Maximum position ( Configured parameters), based on the resultant CFM Loop Output Signal, which uses the Airflow Setpoint and calculated actual CFM, as shown above.

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

The Damper remains at a minimum from 0% - 50% of the Heating Loop Output, till a maximum Discharge Air Temperature (DAT) is reached. This is defined by the tuner parameter VAV-reheatZoneMaxDischargeTemp, which is by default 90F.

The Reheat loop output triggers, that use the DAT set point and actual DAT for the loop output calculation, as shown above.

For 50% and above of the Heating Loop Output (when the DAT is more than the zone temperature+ reheatZoneToDATMinDifferential), the damper position is proportionally scaled for the rest of the Loop output if the required conditioning is not achieved.

The damper increases from 20%(the configured), or calculated min damper position whichever is higher, to 100% when the heating loop output is 100%

 

| Damper Position Calculation for Room Temperature & CO2/VOC based control

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

The Damper position is scaled from the Minimum to the Maximum position ( Configured parameters), based on the resultant PI loop (Cooling), that uses the current and desired temperatures (Cooling/Heating).

Where the minimum position is altered based on the breach in CO2 or VOC levels. below is an example of the same

Example: 

Let us Assume there is a CO2 breach in the system

  • zoneCO2Threshold=800 ppm
  • zoneCO2Target=1000 ppm
  • Zone CO2 Breach Range  = 200 ppm
  • Zone Current CO2=875 ppm
  • Actual Breach= 75 ppm
  • Breach Percentage= 75/200*100= 37.5%

The new minimum damper position is 37.5%, using the same the actual damper position is calculated as below.

Let us Assume the temperature-based PI loop output signal is 80%, the same is translated to Damper Positions as follows

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

The same applies to VOC breaches as well:

Example: 

Let us Assume there is a VOC breach in the system

  • zone VOC Threshold=400 ppb
  • zone VOC Target=500ppb
  • Zone VOC Breach Range  = 100 ppb
  • Zone Current VOC =475 ppb
  • Actual Breach= 75 ppb
  • Breach Percentage= 75/100*100= 75%

The new minimum damper position is 75%, using the same the actual damper position is calculated as below.

Let us Assume the temperature-based PI loop output signal is 80%, the same is translated to Damper Positions as follows

  • Configured min damper position=20%
  • Calculated Min Damper Position=75%
  • Max Damper Position=100%
  • Range of the Damper Position= 25%
  • 80% loop output signal scaled for Damper position range (80)= 80*25/100= 2000/100= 20%
  • Actual Damper Position = 20 (Minimum Damper Position) + 75% = 95%

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

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

  • The Breach percentage on the CO2& VOC 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.

Below is an Example calculation for the same

Example:

The Breach percentage on the CO2 breach

Let us Assume there is a CO2 in the system

  • zoneCO2Threshold=800 ppm
  • zoneCO2Target=1000 ppm
  • Zone CO2 Breach  = 200 ppm
  • Zone Current CO2=825 ppm
  • Actual Breach= 25 ppm
  • Breach Percentage= 25/200*100= 12.5%

The Breach percentage on the VOC breach

Let us Assume there is a VOC breach in the system

  • zone VOC Threshold=400 ppb
  • zone VOC Target=500ppb
  • Zone VOC Breach Range  = 100 ppb
  • Zone Current VOC =475 ppb
  • Actual Breach= 75 ppb
  • Breach Percentage= 75/100*100= 75%

Note: When both CO2 and VOC are enabled for control, the highest breach percentage among both is considered for the minimum damper position, for the actual damper position calculation.

As per the above, let us consider the Breach Percentage from VOC = 75% for the new minimum cfm calculation

The Breach Percentage translated to the new minimum cfm.

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

The airflow set point calculation

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

  • New Min CFM(Cooling/Heating)=200 cfm
  • Max CFM(Cooling/Heating)=250 cfm
  • Airflow CFM Range = 50 cfm
  • 80% loop output signal scaled for Airflow CFM Range (50)= 50*80/100= 4000/100= 40 cfm
  • Actual Airflow Setpoint= 200 (Min CFM(Cooling/Heating))+ 40= 240 cfm

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

| Damper Position Calculation & System Operation When System is Heating

When the system is heating:

Zone/Room Demanding Cooling  Zone/Room Demanding Heating

The Damper position moves to the minimum position, not allowing further hot air into the room, and the Active airflow setpoint will be no higher than the minimum endpoint (minimum Cfm)

When the profile is configured for only Temperature and Cfm-based control or

When the CO2 & VOC are configured, they do not influence the control (No breaches).

 

When the CO2 &  VOC breach influences the control of the operation  the step for the System in cooling mode is followed where the breach contributes to the minimum Cfm

From 0% to 50% (which is tuner value valveActuationStartDamperPosDuringSysHeating) 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 valveActuationStartDamperPosDuringSysHeating (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.

 

 

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