VAV Staged RTU

4 Minute Read

| Overview

The VAV staged RTU 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. Based on the type of RTU used and type of conditioning and controls required, the options can be set and used.

| How it Works

Once you select the VAV Staged RTU 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 and an auxiliary relay is made available in the profile screen to enable and select the stages of conditioning required, based on the preferences.

The VAV Staged RTU System profile uses advanced sequences that are fully outlined in ASHRAE GPC 36. For a comprehensive understanding of how GPC 36 works, please read the document in full. Here we will discuss some basic field level understanding for a quick and dirty understanding of what should be expected.

For the basics on Trim and Respond logic, please reference either GPC36 or our outline here: Trim and Respond Overview

The Algorithm continuously track the average current temperature of the zones mapped as terminal profiles, under the VAV Staged RTU 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 equipment.

When the Average Temperature increases above the set point or desired temperature and the zones demand cooling the cooling stages are activated, and the stages of cooling are scaled for a zone loop output which is a PI loop output.

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

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

Each zone is configured for parameters such as size of damper and min/max damper positions as input for tracking in the algorithms.  A zone is assigned different priority levels based on individual needs - low, medium, high, or no priority; this lets the user designate which zones are more important than others for the operation of the AHU.  With this in mind, the system will actively monitor the average zone temperature while weighing individual priority levels.  As the load in the spaces shifts, the system will determine if an additional stage of cooling, heating or fan speed is needed.  

Cooling - When staging cooling, the algorithm will divide your stages evenly among the cooling loop output scale.  

• 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 staging heating, the algorithm will divide your stages evenly among the heating loop output scale.  

• 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 recirculating, the system will call for the stage-1 fan.

| Wiring

| Configuration

Once you've configured the system profile as a VAV system, the sequences for VAV applications in GPC36 will be followed. What you will need to do is select a few specific configuration parameters that define the physical characteristics AHU. 

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 the wiring information for which jumper to pull from the CM board. 

Test Signal - This is used for troubleshooting and testing the equipment if the configuration and the field setup are in sync, and CCU is communicating properly with the controlled equipment. Also at any point, you can override the value of the output the algorithm decides. 

We can use the test signal for relays to turn them on or for analog out to ramp up and see if the equipment works according to commands from CCU.

Test Signal Time Out - once enabled it will be active for an hour if the screen is not changed, after an hour of no interaction on the screen.  zone screen displays and the test signal will be OFF.

If the screen changes from the test signal configuration screen it will be timed out in one minute (time for the algorithm to run next).

Note: The test signal for modulating output is in deci volts ranging from 0.0dV to 100.0dV.