Airflow CFM & Reheat Loops on SmartNode

| Overview

TrueCFM ensures the required airflow to the zones under the influence of temperature loads and CO2 respectively as prescribed by ASHRAE Guidelines 36. The control sequence is as prescribed for a VAV zone with reheat for a system with Cool Only AHU.

With the Airflow CFM loop on the Edge (SmarNode) device, the reaction time for the resultant loop for the rapid airflow changes, contributing to the damper opening, is greatly reduced from what was existing.

Running CFM on the edge is expected to help mitigate short cycling issues, as the control loops operate every 10 seconds. However, the effectiveness will depend on the field team’s ability to fine-tune the system.

To complement the CFM loops on SmartNode, the Reheat loop for the VAV terminal is available on the SmartNode.

The Airflow CFM & Rehat loops run once in 10 seconds, delivering optimal damper positions & heating for the required rapid changes in the airflow CFM & Room Temperatures.

| Applicability

The CFM & Reheat loop on the Edge is only applicable for the VAV-based terminal profiles that use SmartNode as follows.

• SmartNodeVAV No Fan Terminal Profile
• SmartNodeVAV Parallel Fan Terminal Profile
• SmartNodeVAV Series Fan Terminal Profile
• SmartNodeActive Chilled Beam (ACB) Terminal Profile

The CFM loops & Reheat loops on SmartNode are enabled, only when the system operates in the Cooling Mode.

Note: The CFM loop on the Edge does not apply to the VVT-C profile as the VVT-C terminal logic influences the system algorithm which considers the damper positions of all other zone VAVs fed by the system.

The VAV terminal profile that uses VVT-C terminal logic when the system is heating and the zone the trueCFM loop would continue running on CCU instead of the edge.

| CFM & Reheat Loops on SmartNode

The CFM & Reheat Loops on SmartNode & HelioNode are supported by a few process changes:

• The runPILoopOnNode field is introduced with the settings 2 messages, as below, that is enabled or disabled based on whether the CFM-based control is enabled for the profile with the system operating mode based in cooling.

• The Control message from CCU will communicate the CFM & DAT setpoints to the SmartNode, along with the CCU calculated damper position as below.

• Based on whether the runPILoopOnNode field in the settings 2 messages is enabled or not, the SmartNode run the PI loops to determine the calculated damper positions as shown below, or simply use the CCU calculated damper position to set the damper & Reheat positions.

• The calculated damper position on the edge controlling the damper opening is communicated back to the CCU via regular updates and displayed across CCU, portals, and apps UI and historized across widgets

Note: The system operating mode is not communicated as an extra field in the settings message but is instead conveyed through the “Run PI on Edge” field. This means that the system operating mode is determined by the value of the PI loop on edge bit: if the value is set to 1, the system is in cooling; otherwise, it is in heating.

Hence an extra field in the settings message communicates the system state.

| Damper Position Demarcation

Two new points are introduced to accommodate the calculated damper position in the system to visualize in portals and widgets, as below:

Calculated Damper Position

Calculated Reheat Position

 .

SN/HN calculated the Damper position in Widgets

| Fail Safe Behavior

When the system is heating, the VAV terminal runs the VVT-C logic, at this time when the SN enters Fail Safe mode in a connected to CCU environment, the DAB logic is run on the edge meeting the minimum CFM requirement of the space using the following.

When the device enters fail-safe mode and the system state is in cooling, only the CFM loop will run on the edge along with the temperature loop. All loop parameters will follow the settings specified in CCU settings 1 and 2. Additionally, configurations such as minimum and maximum CFM will be shared with the device through these settings.

| Settings Message Changes

Run PI loop on Edge is a field in the setting message that plays a vital role in the CFM loop on edge, this is leveraged for use, to decide whether to run the CFM loop on the edge device or use the controls from the loops run on CCU. 

The DAT Setpoint and CFM setpoint will be a part of the controls message as the controls message changes and regular update will contain calculated damper and reheat positions.

The Tuner propagation & consumption on the firmware is on the fly without any delay, for the CFM loop on edge, as shown below in the settings 2 message

| Assumptions & Nuances

• There can be scenarios where the Supply air temperature sensor is not connected to the thermistor 2, in this case, there is no way with which the device be it SN to determine whether the system state is heating or cooling it is assumed that the system is always cooling when no sensor is connected to Th2.
• Currently, irrespective of the system state the firmware algo in Fail Safe runs the GPC 36 loops always where the cooling and reheat loop will be driven as such as in a cooling state of the system. But in the event of the CFM on edge based on the system state the algo handling on the failsafe will be switching between GPC 36 and Non-GPC 36 typical temperature PI of DAB for heating and cooling and CFM loop running respecting the minCFM heating or cooling as per zone state.