VAV Dual Duct Variable Flow Endpoint Calculation - 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 module generates minimum and maximum deck flows for both the cold and hot deck of a standard dual duct application. These values are based upon the input minimum and maximum flows and ventilation requirements.

Two scenarios exist that can change the Minimum and Maximum Flows for the Hot Deck and Cold Deck, based on ventilation requirements. In both cases, the controller calculates how much OA it receives from both the Hot Deck and Cold Deck, depending on the value of the CD OA Percent and HD OA Percent Network Inputs.

  1. If the Min Flow Reset Type is set to None, the OA ventilation requirements are calculated by multiplying the Occupancy Level (number of occupants) by the Ventilation Requirement (amount of OA required per occupant). The minimum values increase to ensure the OA delivered from the AHU meets the requirements of the space.

  2. If the Min Flow Reset Type is set to CO2, when the Process Variable (typically, Zone CO2) is greater than the Setpoint, the minimums increase to deliver more air to the space. The total increase in the minimum flows includes this proportional value as well as any increase to make sure that the air stream contains enough outdoor air.

The module is designed to meet the following three conditions (listed from highest priority):

  • Do not exceed the user-entered maximum flows. There is a heating maximum flow and a cooling maximum flow and under the appropriate heating/cooling conditions, these total flows are not exceeded.

  • Do not provide individual deck flows that are less than the user-entered minimum flows. This condition is obeyed unless it causes the condition above to be exceeded. In this situation, the minimum flows is adjusted down.

  • Meet the outdoor air requirements. This condition is met as long as it does not cause the above conditions to be exceeded. As part of this condition, the outdoor air requirements may be compromised when the sum of the two OA percentages is less than 10 percent. This procedure ensures that the individual deck minimums change smoothly as the outdoor air percentages change. For example, a change from 2% to 3% in outdoor air percent does not cause the minimum flow rate to jump from 500 cfm to 333 cfm.

You cannot view or modify the modules in this group’s logic.