Outside Air Optimizer (OAO) - Economizer Control

10 Minute Read

| Overview

The Outside Air Optimization is a software profile supported by the 75F system. It controls the outside air damper to regulate the amount of fresh air that is brought into the building. Otherwise, known as the economizer, which is a feature of HVAC systems that uses cold outside air to provide free cooling to the building. The OAO has 2 main functions: 

• Economizing, based on Demand Control Ventilation(DCV) 

• Economizing, based on Comparative or Dry Bulb Enthalpy

| How It Works

The OAO combines two primary aspects for Economizer control.

Demand Control Ventilation

It is an occupancy & CO2 level-based economizing, which is used to control CO2 levels in space. If CO2 is above the threshold that is set, then the OAO control opens the outside air damper proportionately. DCV essentially sets a minimum damper position for the outside air, and it's applicable only during cooling.

Comparative Enthalpy/ Dry Bulb free cooling control

It is a temperature and humidity-based economizing condition where the enthalpy of the outdoor air and the indoor air is derived to make the decision on whether to bring in outside air for free cooling, where Enthalpy determines the total amount of energy in the air based on humidity and temperature.

Enthalpy determines the total amount of energy in the air and is based on both humidity and temperature. While there is no simple math to determine enthalpy, the psychometric chart shown below is used to discover how much energy the air has.

The 75F Outside Air Optimization (OAO) kit includes a 75F economizer controller, CO2 sensor, Mixed Air Temperature (MAT) sensor, two 10K thermistors for SAT and OAT, and cloud computing software with predictive analytics that provides optimum positioning of the outside air damper on package HVAC units throughout the year using demand-controlled ventilation (DCV) and differential enthalpy. Economizer logic is enhanced by predictive analytics to yield increased energy savings and comfort.

| Wiring

Below are the connection details for the wiring schematic shown above.

• Universal In 1 connects to RTU current transformer 

• Universal In 2 connects to the return air CO2 sensor   

• Universal In 3 connects to Supply Air Temperature   

• Universal In 4 connects to Outside Air Temperature 

• The two-wire sensor bus connects to the Duct Temperature/Humidity sensor for Mixed Air Temperature/Humidity.

• Analogue-out1 connects to the Outside Air Damper 

| Pairing

The 75F SmartNode is used along with the CCU to achieve the Outside Air Optimization control in the buildings.

Once power is supplied to the system, all SmartNodes display the default room.

From the zones page of the CCU:

• Click the settings   icon.

The floor layout screen is displayed.

• Disconnect the tablet from the CM and carry it with you. The device is paired via Bluetooth.

• Press the right button on the SmartNode OAO until you reach the advertising mode screen.
                                                  

• Click the System Config option

• Select the OAO under System Devices..

• Click Pair OAO.

• Click Pair to start the pairing process. 

• Follow the pairing steps on the SmartNode.

The CCU detects all the devices ready for pairing at close range, confirms the Bluetooth Pairing address of the SmartNode that needs to be paired, and select the device

The pairing pin entry screen is displayed.

• Enter the pin displayed on the advertising screen in the SmartNode and click proceed.

• The pairing process will take a few more seconds and then complete, displaying “Success” on the screen. 

| Configuration

The OAO system profile configuration screen is displayed.

| Configuration Parameters

• Set the required parameters from the configuration parameters screen, using the table provided as a reference.

• Click Set to confirm the configuration changes made.

The OAO profile is configured and the same can be visualized in the CCU and portals, for more information on the operation of the OAO refer to CCU Outside Air Optimizer (OAO) System Profile - Sequence of Operation.

| Sequence of Operation

The sequence of operation is completely driven based on Occupancy modes, CO2 levels, and the Outside and Inside Enthalpies (product of Temperature & Humidity).

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 Preconditioning

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.

On the cooling side when the zones demand cooling and the conditions for free cooling are favorable as per the | Criteria Favouring OAO Economization.

• The Economization is enabled with the  economizingLoopOutput = coolingLoopOutput * 100/
  EconomizingToMainCoolingLoopMap, where the preconditioning-based desired temperatures will drive the Heating/Cooling valve & Fan operation.

• The DCV-based economization is disabled.

| 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.

If the zones demand cooling and if the criteria for the free cooling are favorable as per the | Criteria Favouring OAO Economization.

• The Economization is enabled with the  economizingLoopOutput = coolingLoopOutput * 100/
  EconomizingToMainCoolingLoopMap, where the Occupancy-based desired temperatures will drive the Heating/Cooling valve & Fan operation.
• The DCV-based economization is enabled with the dcvLoopOutput = (
  Average zone Sensor CO2 - Average zone CO2 Threshold) / CO2DamperOpeningRate)

When zones demand heating Economization is disabled.

| 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 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 auto away setback temperature range.

If the zones demand cooling and if the criteria for the free cooling are favorable as per the | Criteria Favouring OAO Economization.

• The Economization is enabled with the  economizingLoopOutput = coolingLoopOutput * 100/
  EconomizingToMainCoolingLoopMap, where the Occupancy-based desired temperatures will drive the Heating/Cooling valve & Fan operation.

• The DCV-based economization is disabled.

When zones demand heating Economization is disabled.

| 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, if the zone demands cooling the Economization is enabled as in the occupied mode.

| Sequence of Operation During 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
  • DCVLoopOutput
  • EconomizingLoopOutput

| 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.

| Exhaust Fan Control

Exhaust stages trigger based on the economizing loop output. Stage 1 triggers when the economizing loop output is at 50% (Default). Stage 2 triggers when the economizing loop is at 90% (Default).

The exhaust threshold percentage for the economizing loop can be defined during the configuration using the Exhaust Fan Stage 1(Threshold) and Exhaust Fan Stage 2 (Threshold).

There is also a hysteresis associated with it to prevent short cycling of the equipment, which can be defined using the parameter Exhaust Fan Hysteresis (%) during configuration.

Example:

For default hysteresis of 5% the Exhaust Fan stage 2 deactivates at 90-5 = 85% of the Economizing loop output. And Exhaust Fan stage 1 deactivates at 50-5 = 45% of the Economizing loop output.

| Operation Mode Summary

| Economizer Operation

Outside Air Optimization (OAO), as we know, is bringing in the outside air into the room space when conditions are favorable for free cooling.

The OAO control trigger with the CPU & Economizer is driven by the outsideAirFinalLoopOutput

The factors contributing to the outsideAirFinalLoopOutput include Outside Air Temperature & Humidity, CO2 Threshold, outsideAirMinOpen, and Outside and Mixed Air Temperature (MAT) parameters.

The table below talks about the possible scenarios when the OAO is enabled, the corresponding contributors, and the resultant loop outputs:

Loop Outputs	Parameters Involved	Conditions	Loop Output Caculation	Outside Air Final Output
Outide Air Loop Output (Dry Bulb Economizing)	Outside Air Temperature economizingDryBulbThreshold economizingMinTemperature economizingMaxTemperature	outsideAirTemperature > economizingMinTemperature(0) , and outsideAirTemperature < economizingDryBulbThreshold	Economizing LoopOutput= Loop calulation which uses the outside air temperature sensed and 55°F with economizingDryBulbThreshold for the  error calculation	If coolingLoopOutput > 0 and economizing is enabled:  outsideAirLoopOutput = economizingLoopOutput, dcvLoopOutput or outsideMinDamperOpen (whichever is greatest)  Else  outsideAirLoopOutput = dcvLoopOutput or outsideMinDamperOpen tuner (whichever is greatest)
Outide Air Loop Output (Enthalpy Economizing)	Outside Air Temperature Outside Air Humidity economizingMinHumidity  economizingMaxHumidity enthalpyDuctCompensationOffset Calculated Outside Enthalpy Calculated Inside Enthalpy	calculated outside enthalpy (60) < calculated outside enthalpy (55) + enthalpyDuctCompensationOffset	Economizing Loop Output = Loop calculation, which uses the outside Enthalpy and Inside Enthalpy for the error
DCV Loop Output	zoneCO2 zoneCO2Threshold) co2DamperOpeningRate OAO Damper Min Open	zoneCO2 > zoneCO2Threshold	dcvLoopOutput = (zoneCO2 – zoneCO2Threshold) / co2DamperOpeningRate
MAT Safety Checks	oaoDamperMatTarget MAT Min MAT Max	MAT between 50 and 70 – equal to loop o/p	LoopOutput = outsideAirMinOpen
		MAT Between 44 and 50 – formula

The table below talks about the possible scenarios when the OAO is enabled, the corresponding contributors, and the resultant loop outputs:

| OAO Controls and Operation Illustration

| Calculating Economizer Load

We already learned to calculate the coolingLoopOutput for DAB and VAV. Once you know that, calculating the economizerLoopOutput is pretty simple. When an economizer is available, it acts as the 1st stage of cooling and increases the total # of stages available by 1. This changes the calculation for staging on compressors. 

coolingLoopOutput = 0 Then economizerLoopOutput = 0
coolingLoopOutput = economizingToMainLoopMap (30) Then economizerLoopOutput = 100
If we expand an example of a two-stage mechanical cooling system.
coolingLoopOutput > 33 Then coolingStage1 = ON
coolingLoopOutput > 66 Then coolingStage2 = ON

If the economizer becomes unavailable, the cooling stages would stage on as a 2-stage system again at 0% and 50%. 

The Same applies to OAO combined with the Analog cooling in the profiles with modulating control.

When only Economizing is active, and cooling loop output < 30%  CCU will not operate the cooling loop; the command to operate the Outside Air damper based on the Economizer loop is enabled.