Variable Speed Return Fan (MASD) - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Controller Tool Help
Product
Controls > Control Tools > Controller Configuration Tool
Document type
User Guide
Document number
LIT-12011147
Version
13.1
Revision date
2019-12-04

This option is only available if the Supply Fan is configured as Variable Speed. Variable speed return fans provide three options for control strategy.

Volume Matching: This option controls the speed of the variable volume return fan to match the speed of the supply fan by measuring the supply fan airflow and the return fan airflow. The return fan airflow setpoint is the supply airflow minus a constant. When the system is unoccupied, the constant is changed to zero since no fresh air is used.

There are two options for measuring the airflow, Velocity Pressure (Pitot Tube) or Flow Measurement (Scaled). Velocity Pressure adds an input for the velocity pressure and performs the calculation, provided you enter the K factor and duct area for your system. Flow Measurement supports direct flow reading flow measurement devices. Either input must be scaled to match your sensing device. If you select Separate Occupied and Unoccupied Flow Setpoints, this option allows you to use a different differential setpoint when unoccupied.
Note: When you select a Volume Matching return fan and select either Separate Proportional Outputs (3 AOs) or EAD & RAD Proportional, the outdoor air volume is controlled by the differential flow between the Supply and Return Fans, and the supply air temperature is controlled by modulating the return and exhaust air dampers.

Building Static Pressure: This option adds an input for building static pressure. The return fan is controlled to maintain the building static pressure setpoint (network input BLDGP-SP). This scheme maintains slight positive pressure in the building compared to the exterior static pressure.

Track Supply Fan: This option simply echoes the supply fan command to the return fan. This requires that the return fan be sized and balanced to provide less flow than the supply fan.