DAB Staged RTU w/ VFD Fan - Wiring/Configuration

4 Minute Read

| Overview

The DAB staged RTU with VFD 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, and a modulating control for the Variable Frequency Drive (VFD) for the fan speed. Based on the type of RTU used and the type of conditioning and controls required, the options can be set and the used.

| How it Works

Once you select the DAB Staged RTU with VFD 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, an auxiliary relay and an Analog out is made available in the profile screen to enable and select the staged and modulating types of conditioning required based on the preferences.

The Algorithm continuously track the average current temperature of the zones mapped as terminal profiles, under the DAB Staged RTU with VFD 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 and modulating equipment.

When the Average Temperature increases above the set point or desired temperature, and the zones demand cooling the cooling stages are activated

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

| Wiring

Below is a wiring schematic for the DAB Staged RTU, that show the connection from the CM board of the CCU to the RTU.

Note: The wiring schematic would differ for different staged control options set during the configuration.

| Configuration

Below is the configuration screen for the DAB staged RTU The relays are enabled or disabled based on the preferences.

| Configuration Parameters

Once you've configured the system profile as a DAB system, the sequences follow the proprietary 75F algorithm for operation. You will need to select a few specific configuration parameters that define the physical characteristics of the RTU. 

Relays 1-6 - By default, are configured as traditional thermostat connections. However, depending on your equipment, you can reconfigure (Mapping) any of these relays to act as heating, cooling, or a 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. 

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.  

| 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 DAB Terminal Profile- Damper Position Calculation & Operation

| Sequence of Operation During Occupied

Influencers

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

• • The Operating Mode
  • The Conditioning Mode

Operating 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

Conditioning Modes

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

• • Cooling
  • Heating
  • Deadband

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 DAB Terminal Profile- Damper Position Calculation & Operation

Weighted Average Load MA (Heating /Cooling)

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

Conditioning Mode Determination 

Conditioning mode is determined using the following table:

| Sequence of Operation During Unoccupied

• • The range of heating and cooling desired temperature of the zones drifts further away due to unoccupied setbacks.
  • The device supplies 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 DAB 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 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

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.

| Control Trigger Examples

Cooling Stages

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.

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.

Fan

The fan stages are triggered as per the table below.