Viconics VWZS Application Guide - Manual (Page 21)

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3) RTU Controllers VZ7656X1000W Operation
The following information needs to be carefully read and properly understood if proper system
commissioning is to be achieved.
Unlike low end commercial or residential zoning controllers which typically only use two position demand or
non- demand logic to initialize heating and cooling functions, Viconics VZ7656X1000X uses local PI zone
demand(s) to operate heating and cooling stages. Accurate temperature control in the zones is achieved by
the time proportional control algorithm. This enables performances and control sequences much closer to
what is normally found in DDC application specific control devices.
The operation of the RTU controller is linked with the operation of the attached zone controllers. Although
the controller it will operate in a stand-alone mode if the communication network is down, normal operation
of the system as a whole requires that communication with the attached zone controllers is functional.
3A) Operation Data Exchanged
Independently of the network layer being BACnet MS-TP or Wireless, the flow of data exchanged between
the zones and the RTU controller can be summarized as follow:
Heating and cooling demand data is first exchanged from the ZN controllers to the RTU controller:
• Current PI heating demand ( output value is based on PI heating weight configuration )
• Current PI cooling demand ( output value is based on PI cooling weight configuration )
Each voting controller will also calculate is demand values based on their current occupancy mode and
setpoints currently in use: Unoccupied, Stand-By or Occupied.
Based on the control type configuration (CntrlTyp), the RTU controller will calculate the resulting heating
and cooling zone demands. (See section 2A) Demand Based Heating and Cooling System.
Proper action to the heating or cooling stages using the time proportional control algorithm is accomplished
based on heating or cooling values.
- If resulting calculated PI heating demand > resulting calculated PI cooling demand, then ZN
sequence is heating
- If resulting calculated PI cooling demand > resulting calculated PI heating demand, then zone
sequence is cooling
- If resulting calculated PI cooling demand = resulting calculated PI heating demand, then zone
sequence stays in last current mode