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 are designed to ensure new and existing buildings achieve energy efficiency and preserve outdoor and indoor environmental quality.
The CFM measurement is a product of velocity volume of air in the duct and the cross-sectional area of the duct.
| 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 of DAB in the 75F system.
The feature is introduced in the following terminal profile.
| Configuring CFM
Enabling the CFM for the DAB system profile supplied, the terminal profile, would introduce additional configurable parameters in the configuration as below:
Along with the already existing parameter in the DAB terminal profile, the user intent parameters such as K Factor and min CFM for IAQ is introduced.
| CFM Measurement Setup
As a part of the adoption towards measuring and communicating the Airflow in CFM. At the Duct work, 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.
With the Introduction of CFM, following are the changes that are brought into the Algorithm and operation of the terminal profiles for VAN and DAB.
CFM for DAB damper control
When the system is operating in DAB mode,
The Algorithm tries to maintain the min CFM (minCFMIAQ) value.
- If the CFM is below the minimum threshold, then the damper opens up to maintain the min CFM (minCFMIAQ) value
- If the CFM is above the target, then the damper closes to maintain the min CFM (minCFMIAQ) value
Note: The system does not maintain the Minimum IAQ levels while in auto away or forced occupied mode.
| CFM Related Widgets & Visualization
As a part of the adoption towards visualizing information related the Airflow in CFM. At the portals and the CCU end of the system, the following are made available.
Portal User Intent
CCU Zone User Intent