VAV Fully Modulating AHU

3 Minute Read

| How it Works

The VAV Fully Modulating 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.

Cooling Mode - The system will call for cooling when there are enough requests for it. The AHU will maintain a SAT which is reset based on OAT. So in the fully modulating profile the AO1 will modulate up and down to maintain the cooling SAT that is set by the # of requests. 

Heating Mode - The system will only go into heating mode if all zones are requiring heating. This will typically be during the morning warm up or after any major setpoint changes. When the unit needs heating AO3 will scale up due to the load and follow the heating sequences as if it were a Fully Modulating DAB unit. 

Recirculation Mode -AO2 will modulate up and down to maintain the static duct pressure setpoint based on the number of requests received from the zones. 

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

| Wiring

| Configuration

| Configuration Parameters

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

| Control System Object List

Note: For a comprehensive list of tuners and their details refer to Tuners Complete List

| Sequence of Operation

The sequence of operation is completely driven based on Occupancy mode, and Zone Loads (Cooling/Heating). 

Let us understand the Sequence of operations based on the occupancy modes, and how the other factors contribute to the sequence of operations during different types of occupancy modes.

| Occupancy Modes

The occupancy mode (Occupied or Unoccupied) shall be determined through a user-adjustable, graphical, seven-day schedule with a holiday schedule.

Based on the above aspects factoring into the occupancy the following can be the possible applicable occupancy modes:

• Pre- Conditioning
• Occupied
• Unoccupied

| Sequence of Operation During Pre-Conditioning

Pre-conditioning is a state just before the building enters the scheduled occupancy. It starts at more like the unoccupied state, the desired temperatures drift to unoccupied setbacks.

The preconditioning uses an algorithm to start the conditioning before the building is scheduled for occupancy, from the heating side or cooling side, to bring the building to an optimum level ( within the occupied heating & cooling desired temperatures) exactly at the time of occupancy start.

A coolingPreconditioningRate and heatingPreconditiningRate tuners defaulted to 15 minutes, are used alongside the occupied heating/cooling desired temperature breach to determine when the pre-conditioning should start.

Example Calculation:

When,

• • The heating & cooling desired temperatures (Average of zones needing conditioning) are 70F and 74F for the building.
  • The operating mode is determined based on the weighted average calculation as shown in section | Sequence of Operation During Occupied

Let us assume:

• • • The operating mode determined is cooling.
    • The current temperature (Average of zones needing cooling conditioning) is 76F
    • 8:00 AM is the time the building is scheduled to be occupied.

Then the preconditioning start time is calculated as follows:

Cooling Desired Temperature Breach= 76-74=2F

Preconditioning duration= coolingPreconditioningRate (mins) * Cooling Desired Temperature Breach

                                        = 15*2

                                        = 30 mins

The preconditioning starts 30 minutes before the building is scheduled to be occupied, which is = 7:30 AM.

The zones operate the damper positions optimally to receive the required conditioning to satisfy their load requirements to maintain a temperature within the deadband.

For more information on the damper operations, refer to VAV Terminal Profile- Damper Position Calculation & Operation.

| Sequence of Operation During Occupied

During the scheduled Occupied times, the AHU/RTU is influenced by two aspects of the system.

• The Operating Mode
• The Conditioning Mode

Conditioning Modes

The Operating mode is a user intent parameter that the user can specify. There are four types of operating modes.

• Off
• Auto
• Cooling
• Heating

Operating Modes

The Conditioning mode is a derived parameter. There are three types of conditioning modes.

• Cooling
• Heating
• Off

Based on the influence of the aspects mentioned above, the device supplies the cooling/ heating load requirements, and the zones operate the damper positions optimally to receive the required conditioning to satisfy their load requirements and maintain a temperature within the deadband.

For more information on the damper operations, refer to VAV Terminal Profile- Damper Position Calculation & Operation.

Weighted Average Load MA (Heating /Cooling)

weightedAverageHeatingOnlyLoadMA/ weightedAverageCoolingOnlyLoadMA,  is a result of the following steps.

Operating Mode Determination 

Conditioning mode is determined using the following table:

| Sequence of Operation During UnOccupied

• The range of Heating Desired Temperature and Cooling Desired Temperature of the zones drifts further away to unoccupied setback.
• The device supplies for the cooling/Heating load requirements of the zones, and the zones operate the damper positions optimally to receive the required conditioning to satisfy their load requirements to maintain the room temperature within the newly set autoaway setback temperature range, for more information on the damper operations refer to VAV Terminal Profile- Damper Position Calculation & Operation.
• If weightedAverageHeatingOnlyLoadMA/ weightedAverageCoolingOnlyLoadMA, is a positive value. The system operates in the determined conditioning mode to supply the zone cooling/heating load requirements, as shown in the | Sequence of Operation During Occupied
• If weightedAverageHeatingOnlyLoadMA & weightedAverageCoolingOnlyLoadMA, is zero. The system is within the deadband, there is neither a cooling load nor a heating load, hence there is no conditioning that occurs at the AHU end, except for a minimum mechanical or free ventilation.
•  

| Sequence of Operation During Emergency Conditioning

When zone limits are violated, and the recorded temperature is within the zone limit plus leeway the conditioning will happen in the direction of zone load, as shown in the cooling conditioning, or Heating conditioning mode.

| Sequence of Operation During Zone Temp Dead

And, When the zone temperature breaches the zone limits, beyond the leeway limits all the following loops are disabled:

• • CoolingLoopOutput
  • HeatingLoopOutput
  • FanLoopOutput

| Humidifier Control

The humidifier is turned ON whenever the humidity level for the system drops below the targetMinInsideHumidty set. The humidifier will be turned OFF after being turned on when the humidity levels go humidityHysteresis above the targetMinInsideHumidty. Humidity control will not be maintained during UNOCCUPIED or VACATION modes.

| DeHumidifier Control

If the dehumidifier is selected, it turns ON whenever the humidity level for the system goes above the targetMaxInsideHumidty set. The dehumidifier will be turned OFF after being turned on when the humidity drops humidityHysteresis below the targetMaxInsideHumidty. Dehumidifier control will not be maintained during UNOCCUPIED or VACATION modes.

Note: The humidity level for the system is either the average level reported by all the zones serviced by the RTU/AHU.

| Operation Mode Summary

| Control Trigger Examples

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 modulating cooling

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 modulating cooling.

Fan

The modulating fan control is triggered as per cooling or heating loop output..

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.