VAV Staged RTU w/ VFD Fan

3 Minute Read

| How it Works

The VAV Staged RTU w/ VFD Fan 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

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.

| Wiring

| Configuration

| Configuration Parameters

Parameters	Purpose	Default Value	Values in the Drop-Down
Relay 1	To enable staged conditioning	Cooling Stage 1	Cooling Stages 1 to 5 Heating Stages 1 to 5 Fan Stages 1 to 5 Humidifier Enable Dehumidifier Enable
Relay 2	To enable staged conditioning	Cooling Stage 2
Relay 3	To enable staged conditioning	Fan Stage 1
Relay 4	To enable staged conditioning	Heating Stage 1
Relay 5	To enable staged conditioning	Heating Stage 2
Relay 6	To enable staged conditioning	Fan Stage 2
Relay 7	To enable staged conditioning	Humidifier Enable
Analog Out 2	To enable modulating Fan speed control	Fan Speed	NA

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-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.

| 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= ((zone1HeatingLoadzone1DynamicPriority + zone2HeatingLoadzone2DynamicPriority … )) /(zone1DynamicPriority + zone2DynamicPriority…)* *weightedAverageCoolingOnlyLoad= ((zone1CoolingLoadzone1DynamicPriority + zone2CoolingLoadzone2DynamicPriority … )) /(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= HeatingLoopOutputAnalogFanSpeedMultiplier.**
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: Trim & Respond Logic How are Trim & Respond Requests Generated in a VAV zone? Damper Position & Zone Priority Driven Trim and Respond Requests The calculated SAT value is mapped between the coolingSATMinimum and coolingSATMaximum tuners to generate systemCoolingLoopOutput which varies from 0 -100% to provide the cooling load requirements for the zones, with staged/modulating controls. The FanLoopOutput on the cooling side is enabled according to Guideline 36; High Performance Sequence of Operations for HVAC systems *The Guideline 36; High Performance Sequence of Operations for HVAC systems* uses a Trim and Response to calculate the System Pressure. For more information refer to the following articles. Trim & Respond Logic, How are Trim & Respond Requests Generated in a VAV zone? Damper Position & Zone Priority Driven Trim and Respond Requests
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

| 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 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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