VAV Advanced Hybrid AHU

4 Minute Read

| How it Works

The VAV Advanced Hybrid AHU 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 

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 and also determines the # of requests the zone sends when it has demand.  As the load in the spaces shifts, the system will determine if an additional cooling, heating or fan speed is needed. 

Cooling - When modulating cooling, as your cooling loop output (0-100%) rises above 0%, the 0-10v analog signal will modulate with it.  When staging cooling, the algorithm will divide your stages evenly among the cooling loop output scale.  

5-stages of cooling:

• Cooling loop output > 0%, Cooling Stage 1   Turns On
• Cooling loop output > 20%, Cooling Stage 2   Turns On
• Cooling loop output > 40%, Cooling Stage 3   Turns On
• Cooling loop output > 60%, Cooling Stage 4   Turns On
• Cooling loop output > 80%, Cooling Stage 5   Turns On

• Cooling loop output <= 70%, Cooling Stage 5  Turns Off
• Cooling loop output <= 50%, Cooling Stage 4  Turns Off
• Cooling loop output <= 30%, Cooling Stage 3  Turns Off
• Cooling loop output <= 10%, Cooling Stage 2  Turns Off
• Cooling loop output <= 0%, Cooling Stage 1  Turns Off

Heating - When modulating heating, as your heating loop output (0-100%) rises above 0%, the 0-10v analog signal will modulate with it.  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 > 20%, Heating Stage 2   Turns On
• Heating loop output > 40%, Heating Stage 3   Turns On
• Heating loop output > 60%, Heating Stage 4   Turns On
• Heating loop output > 80%, Heating Stage 5   Turns On

• Heating loop output <= 70%, Heating Stage 5  Turns Off
• Heating loop output <= 50%, Heating Stage 4  Turns Off
• Heating loop output <= 30%, Heating Stage 3  Turns Off
• Heating loop output <= 10%, Heating Stage 2  Turns Off
• Heating loop output <= 0%, Heating Stage 1  Turns Off

Recirculate -  When recirculating with a VFD the drive will modulate to maintain the Static Duct pressure setpoint based on the # of requests from the zones as outlined in GPC 36. 

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

Analog-out1 -Cooling output scaled to the cooling load. 0Vdc = No cooling demand. 10Vdc = Max cooling demand. 

Analog-out2 -Fan Speed. Scaled based on the demand for pressure. NOTE: The output is meant to be used as a Duct Static Pressure Setpoint, and tied to a VFD or Unit Controller (in tandem with a duct static pressure sensor) to modulate the actual and correct VFD speed (%).

Analog-out3 -Heating. Output scaled to the heating load. 0Vdc = No heating demand. 10Vdc = Max heating demand. 

Analog-out4 -Composite. This assumes no demand at the mid point. Very configurable. (E.g. 5Vdc = No demand. 0Vdc = Max Cooling demand. 10Vdc = Max Heating Demand) 

Analog-out1-3 at Min and Max -Set the minimum output at minimum load or maximum load. Typically synced with your equipment. 

Analog-out4 at Min and Max -Here you need to be careful because it is so customizable. Your Min can be more than the max values depending on unit configuration. What you need to know is that however you set this up, must be matched up with your unit. (A common example would be an RTU where you are setting up a DAT reset. AO4 @ Min Cooling = 5, AO4 @ Min Heating = 5, AO4 @ Max Cooling = 0, and AO4 @ Max Heating = 10V. Then you'd need to sync your DAT setpoints at the RTU to match. 0Vdc = 55F and 10Vdc = 100F).

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.