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| Overview

The VVT-C staged RTU with VFD is a profile that is used to control an RTU, which meet the HVAC demands of a space. The RTU can be a cooling only/Heating Only or Both cooling and Heating, the profile has its all to control.

The profile supports up to five stages of cooling, Heating and Fan speeds and humidification, and a modulating control for the Variable Frequency Drive (VFD) for the fan speed. Based on the type of RTU used and type of conditioning and controls required the options can be set and the used.

| How it Works

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Once you select the VVT-C Staged RTU with VFD profile from the system selection, from all the relays, analog inputs and output that are available as a part of the CM board of the CCU, six output relays, an auxiliary relay and an Analog out is made available in the profile screen to enable and select the staged and modulating types of conditioning required based on the preferences.

The Algorithm continuously track the average current temperature of the zones mapped as terminal profiles, under the VVT-C Staged RTU with VFD system profile, and looks for the change in the difference between the system level Desired Temperature and Average Current Temperatures and provides the required conditioning via Staged and modulating equipment.

When the Average Temperature increases above the set point or desired temperature, and the zones demand cooling the cooling stages are activated

And when the Average Temperature decreases below the set point or desired temperature, and the zones demand heating the Heating stages are activated.

| Wiring

Below is a wiring schematic for the VVT-C Staged RTU, that show the connection from the CM board of the CCU to the RTU.

Note: The wiring schematic would differ for different staged control options set during the configuration.

 

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| Configuration

Below is the configuration screen for the VVT-C staged RTU, The relays are enabled or disabled based on the preferences.

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Relays 1-6 - By default are configured as a traditional thermostat connections. However, depending on your equipment, you can reconfigure (Mapping) any of these relays to act as heating, cooling, or fan. 

Relay 7 - Auxiliary relay for humidifier or dehumidifier contact. This is a wet contact. If your equipment needs a dry contact, please see wiring information for which jumper to pull from the CM board. 

Test Signal - At any point you can override the value the output the algorithm decided. This is used for troubleshooting and testing your equipment. 

Analog-out2 - Fan Speed. Customized using the 'Analog-out2 During STAGE' options below. 

Analog-out2 During 'STAGE' - Directly Set the VFD fan speed during a given Stage of mechanical conditioning. Whatever you set here is the speed the VFD will run when the given stage is calling. 

Below the profile configuration you can program the output voltage for the modulating VFD control during various stages of equipment operation.  

| Operations

Cooling

When the zones demand for cooling the algorithm calculates a loop output, which is based on the Proportional and Integral loop for control signals, called cooling loop output. This loop output which is in percentage is scaled to the different stages of cooling as below.

  • Cooling loop output > 0%, Cooling Stage 1   Turns On
  • Cooling loop output > 50%, Cooling Stage 2   Turns On
  • Cooling loop output <= 0%, Cooling Stage 1 Turns Off
  • Cooling loop output <= 40%, Cooling Stage 2  Turns Off

Heating

When the zones demand for heating the algorithm calculates a loop output, which is based on the Proportional and Integral loop for control signals, called Heating loop output. This loop output which is in percentage is scaled to the different stages of Heating as below.

  • Heating loop output > 0%, Heating Stage 1 Turns On
  • Heating loop output > 50%, Heating Stage 2 Turns On
  • Heating loop output <= 0%, Heating Stage 1 Turns Off
  • Heating loop output <= 40%, Heating Stage 2 Turns Off

Recirculate

When the zone demands recirculation along with cooling, the system will call for the stage-1 fan and so on for fan speed which is also based on the loop output that is calculated.

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