| Overview

True CFM (Cubic Feet per Minute) is a unit of airflow that is been used by the ASHRAE 62.1 indoor air quality standards. It mandates that a minimum amount of airflow is maintained as long as the space is occupied. One of the primary reasons for which True CFM is adopted since legacy systems and senior facility management personnel use airflow measurements in CFM for ease of understanding. This further helps in maintaining common standards of measurements across HVAC systems. It is also incorporated in accordance to the Title 24 building codes which are building energy efficiency standards designed to ensure new and existing buildings achieve energy efficiency and preserve outdoor and indoor environmental quality.

The CFM measurement is a product of the velocity volume of air in the duct and the cross-sectional area of the duct.

To calculate Air Flow in Cubic Feet per Minute (CFM), we have to determine the Flow Velocity in feet per minute, then multiply this value by the Duct Cross cross-sectional area. i.e.,
Air Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x Duct Cross Sectional Area (A)

| True CFM Introduction

As a part of the adoption towards enabling the CFM in profiles for comfort and indoor air quality standards, a toggle button is introduced in the terminal profiles VAV in the 75F system. 

The feature is introduced in the following terminal profile.


  • VAV Reheat with No fan


  • VAV Reheat with Series Fan


  • VAV Reheat with Parallel Fan


| Configuring CFM


Enabling the CFM for the VAV terminal profile, would introduce additional configurable parameters in the configuration screen as below:


Along with the already existing parameter in the VAV terminal profile, the user intent parameters such as K Factor, max and min CFM Cooling, max and min CFM Reheating are introduced.

The additional parameters introduced are the same for VAV Reheat with no fan and VAV Reheat with parallel fan terminal profiles.

| CFM Measurement Setup

As a part of the adoption of measuring and communicating the Airflow in CFM. At the ductwork, end of the system the following setup from the 75F system hardware is used.


75F has developed a new differential pressure sensor that connects to a pitot tube and directly measures the airflow pressure difference between velocity pressure and static pressure. This allows a translation into velocity of the airstream. Coupled with knowing the cross-sectional area of the duct, the actual cubic feet per minute flow rate can be inferred. 

| Operation

With the Introduction of CFM, the following are the changes that are made to the Algorithm and operation.

CFM for VAV damper control

  • When the system is in cooling mode the CFM parameters used are  

maxCFMCooling and  minCFMCooling (when the terminal needs cooling) 

maxCFMReHeating and minCFMReHeating (when the terminal needs reheating) 

  • When the system is in heating mode the CFM parameters used are :

maxDamperPosHeating and minCFMReHeating

Note: In case the system is in heating, the min CFM is maintained, and the damper position is used as per standard DAB operation using a normalization strategy. 

Once the current CFM per damper that is controlled by the SmartNode is calculated, the system shall adhere to the min and max values. If the CFM is below the minimum threshold, then the damper opens, and if it is above the maximum CFM the damper closes.

Note: The system does not maintain the Minimum CFM levels during auto-away and forced occupied modes.

| CFM Related Widgets & Visualization

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.

Predefined Widgets:

  • VAV

image (11).png



Portal User Intent

  • VAV



CCU Zone User Intent

  • VAV


| CFM Influence on Damper Position Calculation in VAV Terminal Profiles

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

When the System is cooling:

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.


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.


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% of 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.

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 & 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.

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 an app 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.




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