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CCU VAV System Profile - Sequence of Operation

Matt Blount
Updated

| Description of Equipment

The CCU VAV System profile is a system-level equipment (AHU/ RTU) controller that provides heating and cooling equipment controls for conditioning.

Following are the system-level profiles that are available under the VAV

  • VAV Staged RTU
  • VAV Fully Modulating AHU
  • VAV Staged RTU with VFD Fan
  • VAV Advanced Hybrid AHU

The profile can be selected and used based on the type of equipment controlled.

| Schematics

VAV Staged RTU

 

VAV Fully Modulating AHU

 

VAV Staged RTU with VFD Fan

 

VAV Advanced Hybrid AHU

 

| Control System Object List

 Object Name Type Input/Output Type/ Default Value
Current Temp Input (Terminal Side) Average Current Temperature of Zones
Humidity Input (Terminal Side) Average Humidity of Zones
Zone Base Priority User Intent (Terminal Side) NA
coolingSATMinimum (SATspMin) Tuner 55 F
coolingSATMaximum (SATspMin) Tuner 65 F
coolingPreconditioningRate Tuner 15 mins
heatingPreconditioningRate Tuner 15 mins
zonePriorityMultiplier Tuner 1.30
zonePrioritySpread Tuner 2
Cooling Desired Temperature User Intent (Terminal side) NA
Heating Desired Temperature User Intent (Terminal side) NA
Conditioning Mode User Intent NA
Operation Mode Calculated Input NA
Fan Mode User Intent NA
Occupancy Mode

Input

 Schedule / Terminal OWI Sensors
Cooling Stage 1 Output Control Mote Relay
Cooling Stage 2 Output Control Mote Relay
Cooling Stage 3 Output Control Mote Relay
Cooling Stage 4 Output Control Mote Relay
Cooling Stage 5 Output Control Mote Relay
Heating Stage 1 Output Control Mote Relay
Heating Stage 2 Output Control Mote Relay
Heating Stage 3 Output Control Mote Relay
Heating Stage 4 Output Control Mote Relay
Heating Stage 5 Output Control Mote Relay
Fan Speed 1 Output Control Mote Relay
Fan Speed 2 Output Control Mote Relay
Fan Speed 3 Output Control Mote Relay
Fan Speed 4 Output Control Mote Relay
Fan Speed 5 Output Control Mote Relay
Humidifier Output Control Mote Relay
Dehumidifier Output Control Mote Relay
Modulating Cooling Output Control Mote AnalogOut
Modulating Heating Output Control Mote AnalogOut
Modulating Fan Speed Output Control Mote AnalogOut
coolingPreconditioningRate Tuner 15 mins
heatingPreconditioningRate Tuner 15 mins
zonePriorityMultiplier Tuner 1.30
zonePrioritySpread Tuner 2

 

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.

Step Formula/Calculation
Heating Load/Cooling Load

When the zone's current temperatures fall below the HeatingDesiredTemperatures they contribute to the zone heating load

  • zoneHeatingLoad= zoneTargetTemperature- currentTemperature
  • zoneCoolingLoad= currentTemperature- zoneTargetTemperature 
Dynamic Priority (Heating/Cooling)
  • zoneDynamicPriority = zoneBasePriority*((zonePriorityMultiplier )^(zoneCoolingLoad/zonePrioritySpread)), 
  • zoneDynamicPriority = zoneBasePriority*((zonePriorityMultiplier )^(zoneHeatingLoad/zonePrioritySpread)), 

Note: The exponent (zoneHeatingLoad/zonePrioritySpread) or (zoneCoolingLoad/zonePrioritySpread) will be limited to the max value of 10. This will prevent a zone with large temperature drift (probably because of faulty sensors) from driving system operation.
Weighted Average (Heating/Cooling)
  • weightedAverageHeatingOnlyLoad= ((zone1HeatingLoad*zone1DynamicPriority + zone2HeatingLoad*zone2DynamicPriority … )) /(zone1DynamicPriority + zone2DynamicPriority…)
  • weightedAverageCoolingOnlyLoad= ((zone1CoolingLoad*zone1DynamicPriority + zone2CoolingLoad*zone2DynamicPriority … )) /(zone1DynamicPriority + zone2DynamicPriority…)
Weighted Average Post ML (Machine Learning) (Heating/Cooling
  • weightedAverageHeatingOnlyLoadPostML = weightedAverageHeatingOnlyLoad + buildingLoadOffsetML 
  • weightedAverageCoolingOnlyLoadPostML = weightedAverageCoolingOnlyLoad + buildingLoadOffsetML 

Weighted Average MA (Moving Average)

(Heating/Cooling)
  • weightedAverageHeatingOnlyLoadMA = weightedAverageHeatingOnlyLoadPostML
  • weightedAverageCoolingOnlyLoadMA = weightedAverageCoolingOnlyLoadPostML
is then put through a 15-minute moving average filter (which takes the last 15, minute readings and then averages them) to reduce sudden swings to get the weightedAverageLoadMA)

 

Operating Mode Determination 

Conditioning mode is determined using the following table:

If  Then
weightedAverageCoolingOnlyLoadMA is zero and weightedAverageHeatingOnlyLoadMA is positive.
  • Heating conditioning mode is enabled via HeatingLoopOutput which uses the weightedAverageHeatingOnlyLoadMA as the process variable and Zero as the control variable, to give an output that ranges from 0-100%, to provide the heating load requirements for the zones, with staged/modulating controls.
  • The FanLoopOutput is enabled based on the HeatingLoopOutput using the formula FanLoopOutput= HeatingLoopOutput*AnalogFanSpeedMultiplier.

weightedAverageHeatingOnlyLoadMA is zero.

weightedAverageCoolingOnlyLoadMA is positive.
  • Cooling Conditioning mode is enabled
  • The systems operate as per Guideline 36: High-Performance Sequence of Operations for HVAC systems.
  • The Guideline 36 uses a Trim and Response to calculate the Supply Air Temperature (SAT). For more information refer to the following articles:

weightedAverageHeatingOnlyLoadMA is zero.

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 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

Modes & 

Operation

 Pre-Conditioning Occupied Unoccupied
Auto

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput

 

 

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput
Heat Only

HeatingLoopOutput FanLoopOutput

HeatingLoopOutput FanLoopOutput

 

HeatingLoopOutput FanLoopOutput
Cool Only

CoolingLoopOutput

FanLoopOutput

CoolingLoopOutput

FanloopOutput

 

CoolingLoopOutput

FanLoopOutput
Off

All Loops Deactivated

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