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| Description of the Equipment

The CCU Outside Air Optimizer (OAO) is a system-level equipment controller, that can be combined alongside any other system profile (VAV/DAB). This combination can be an additional energy-saving measure, where this profile helps the system utilize the outside air for conditioning spaces when conditions are favorable.

The OAO not only utilizes available free cooling but also helps the system (AHU/RTU) relax controls during the free cooling utilization, thus leading to more than considerable energy savings for a building.

The OAO can be combined with all the system profiles under the VAV and DAB.

| How it Works

The OAO combines two primary aspects for Economizer control.

Demand Control Ventilation

It is a CO2 level-based economizing used to control CO2 IAQ levels in space. If the CO2 level is above the threshold set, the OAO control opens the outside air damper proportionally.

DCV sets a minimum damper position for the outside air, and it's applicable for both cooling and heating.

Comparative Enthalpy/ Dry Bulb free cooling control

It is a temperature and humidity-based economizing condition where the enthalpy of the outdoor air and indoor air is derived to decide whether to use outside air for free cooling.

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

Let’s take an example of a building that has an inside temperature of 72°F with 40% humidity, while the air outside is cooler at 60°F with 65% humidity. In the chart below we plot the enthalpy as downward sloping diagonal lines from left to right. We can see that 60°F with 65% humidity has less enthalpy than 72°F air with 40% humidity. So according to the graph, the outdoor enthalpy is less than the indoor enthalpy, so bringing in fresh air to cool the building (instead of mechanical cooling) makes sense.

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For more information refer to the following articles.

| Schematic

CCU OAO

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| Control System Object List

 Object Name Type Input/Output Type/Default Value
Inside Current Air Temperature Sensor Input  Average of zones Current Temperatures
Inside Current Air Humidity Sensor Input Average of zones Humidity 
Outside Current Air Temperature Sensor Input  Current Measured Outside Air Temperature
Outside Current Air Humidity Sensor Input Current Measured Outside Air Humidity
CO2 Sensor Input Average CO2 of Zones
CO2 Threshold User Intent NA
Co2DamperOpeningRate Tuner 10% opening /100ppm
EconomizingMaxHumidity Tuner 100 % RH
EconomizingMaxTemperature Tuner 70 F
EconomizingMinHumidity Tuner 0 % RH
EconomizingMinTemperature Tuner 0 F
EconomizingToMain CoolingLoopMap Tuner 30 F

EnthalpyDuct

CompensationOffset

Tuner 0
OutsideDamper MixedAirMinimum Tuner 44 F
OutsideDamperMixedAirTarget Tuner 50 F
systemPostPurgeFanSpeedTuner Tuner 50 %
systemPostPurgeOccupied TimeOffsetTuner Tuner 180 Mins
systemPostPurgeRuntimeTuner Tuner 120 Mins
systemPrePurgeFanSpeedTuner Tuner 50 %
systemPrePurgeOccupied TimeOffsetTuner Tuner 180 Mins
systemPrePurgeRuntimeTuner Tuner 120 Mins
systemPurgeDABDamper MinOpenMultiplier Tuner 1.50
systemPurgeVAVDamper MinOpenMultiplier Tuner 1.50
systemPurgeDab MinFanLoopOutput Tuner 50 %
systemPurgeVav MinFanLoopOutput Tuner 50 %
economizingDryBulbThreshold Tuner 55 F
Outside Damper Control  Output Smartnode Analog Out
Exhaust Fan Stages Output Smartnode Relay

 

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

 

| Configuration Parameters

Parameter

Purpose

Default Value

Values in the drop-down list

Outside Damper at Min Drive (V)

To select a voltage value for the min outside air damper opening position, at minimum economizing requirement.

 

2V

Range from 0V to 10V in 1V increments 

The outside damper output signal will modulate between outsideDamperAtMinDrive and outsideDamperAtMaxDrive as the oaoFinalLoopOutput changes from 0% to 100% depending on the conditions.   

Outside Damper at Max Drive (V)

To select a voltage value for the max outside air damper opening position, at maximum economizing requirement.

10V

 

Range from 0V to 10V in 1V increments  

Return Damper at Min Drive (V)

To select a voltage value for the min return air damper opening position, at minimum economizing requirement.

2V

 

Range from 0V to 10V in 1V increments  

Return Damper at Max Drive (V)

To select a voltage value for the max return air damper opening position, at maximum economizing requirement.

2V

Range from 0V to 10V in 1V increments

The return air damper works opposite to the outside air damper and is calculated as follows  

returnAirFinalOutput = 100 - outsideAirFinalLoopOutput   

Outside Damper Min Open During Recirculate (%) To set an outside air minimum damper position during the recirculate 25% Range from 0% to 100% in 1% increments 
Outside Damper Min Open During Conditioning (%) To set an outside air minimum damper position during conditioning 5% Range from 0% to 100% in 1% increments 
Outside Damper Min Open During Fan Low (%) To set an outside air minimum damper position during low fan speed 20% Range from 0% to 100% in 1% increments 
Outside Damper Min Open During Fan Medium (%) To set an outside air minimum damper position during medium fan speed 15% Range from 0% to 100% in 1% increments 
Outside Damper Min Open During Fan High (%) To set an outside air minimum damper position during high fan speed 10% Range from 0% to 100% in 1% increments 

Return Damper at Min Open(%) 

To select a percentage value for the minimum return air damper opening position

0

Range from 0 to 100 in 1% increments 

Exhaust Fan Stage 1 Threshold (%)

To select a percentage value for the exhaust fan stage 1 threshold

50

Range from 0 to 100 in 1% increments 

Exhaust Fan Stage 2 Threshold (%)

To select a percentage value for the exhaust fan stage 2 threshold

90

Range from 0 to 100 in 1% increments 

Current Transformer Type

To select the type of current transformer used in the setup

0-20 (A)

0-10 (A)

0-20 (A)

0-50 (A)

CO2 Threshold  (ppm)

To select a CO2 threshold value 

1000

Range from 0 to 2000 in 10 ppm increments 

Exhaust Fan Hysteresis (%)

To select a hysteresis percentage value for the Exhaust Fan

5

Range from 0 to 100 in 1% increments

Use Per Room CO2 Sensing

To enable or disable the use of per-room CO2 sensing values to determine the loop output

Disabled

NA

Smart Purge Outside Damper Min open

To select a value for the minimum outside air damper opening position during the smart purge operation in the profile.

100

Range from 0 to 100 in increments of 1

Enhanced Ventilation Outside Damper Min Open 

To select a value for the minimum outside air damper opening position during the enhanced ventilation operation in the profile.

50

Range from 0 to 100 in increments of 1

 

| Few Important Tuners and Usage

From the above list of tuners let us look at a few important tuners and examples, of their usage in the algorithm and how they affect the functioning of the profile.

Economizing Operation Limit Tuners

  • economizingMaxHumidity  (default 100) 
  • economizingMaxTemperature (default 70) 
  • economizingMinHumidity (default 0) 
  • economizingMinTemperature (default 0) 

Economizing operations should not happen when outside weather parameters are beyond the limits specified above. 

These tuners are used to determine if outside weather is suitable for economizing.

Mixed Air Temperature (MAT) safety regulation Tuner 

  • outsideDamperMixedAirTarget 

The above-mentioned tuner plays an important role in the operation of the OAO profile. If the mixed air temperature which is a sensed value, is below the value assigned to the outsideDamperMixedAirTarget, then the outside damper is position brought to minimum. This holds applicable for both DCV and Economizing operation scenarios

  • For DCV driven scenario in heating mode this prevents condensation of flue gases when the heater is on. This condensation is acidic and will corrode the heater.
  • the For the scenariosscenario with DCV or econ will cause super cold supply air to come in and that is very uncomfortable. 

Dry Bulb economizing

  • economizingDryBulbThreshold (default value of 55F)  

 This tuner is used to favor the Dry bulb economizing of the OAO profile, were at outside air temperatures below this threshold, the system will start economizing irrespective of the comparative enthalpy of the outside and inside air.  

As mentioned, above every tuner has its unique role to play in the operation of the profile, but it's highly recommended to use them with expert supervision preferably the 75F support team.

| Criteria Favouring OAO Economization

The following are the criteria for Economizer control to enable in a profile.

Enthalpy- Based Criteria

  • If the outsideAirEnthalpy + EnthalpyDuctCompensationOffset is less than insideAirEnthalpy.
  • If the outside air temperatures are below the economizingDryBulbThreshold (default 55F) tuner value, the system will start economizing irrespective of the comparative enthalpy of the outside and inside air.
  • Only if the Temperature and Humidity are within the following range.
    • Tuners, economizingMinHumidity (default 0) to economizingMaxHumidity  (default 100)
    • Tuners, economizingMinTemperature (default 0) to economizingMaxTemperature (default 70).

Mixed Air Temperature (MAT) Based

  • If the Mixed Air temperature (MAT) is greater than OutsideDamperMixedAirTarget tuner value.

Note: If the MAT < outsideDamperMixedAirTarget, then the damper will begin to close to its minimum regardless of its calculated position.

Demand Control Ventilation (DCV) Based

Now that you understand when Free cooling is available or not, the final addition is how the OA damper position is affected by IAQ, also referred to as Demand Control Ventilation (DCV).
Simply put, DCV operation changes the minimum OA damper position. 

If zoneSensorCO2 (Average of zones CO2) > zoneCO2Threshold

  • dcvCalculatedMinDamper = (returnCO2 or Zone CO2 - co2Threshold)/co2DamperOpeningRate

    Example:
    Zone / Return CO2 = 800ppm
    co2Threshold = 500ppm
    co2DamperOpeningRate = 10%/100ppm

    dcvCalculatedMinDamper = (800-500)/10 = 30%

Note:

All the below-mentioned criteria need to be true for an economizer to enable

  • If insideAirEnthalpy < outsideAirEnthalpy then economizingAvailable = True
  • If MAT > ousideDamperMixedAirMinimum then economizingAvailable = True
  • If economizingMinTemperature < OAT < economizingMaxTemperature Then economizingAvailable = True
  • If economizingMinHumidity <outsideHumidity < economizingMaxHumidity Then economizingAvailable= True

If any of the 4 statements above are False, then economizing will not be available.

| 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.

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 autoaway 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

Modes & 

Operation

Pre-Conditioning Occupied Unoccupied
Auto

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput

EconomizingLoopOutput

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput

EconomizingLoopOutput

DCVLoopOutput

 

HeatingLoopOutput FanLoopOutput

CoolingLoopOutput

FanloopOutput

EconomizingLoopOutput

Heat Only

HeatingLoopOutput FanLoopOutput

HeatingLoopOutput FanLoopOutput

 

HeatingLoopOutput FanLoopOutput

Cool Only

CoolingLoopOutput

FanLoopOutput

EconomizingLoopOutput

CoolingLoopOutput

FanloopOutput

EconomizingLoopOutput

DCVLoopOutput

 

CoolingLoopOutput

FanLoopOutput

EconomizingLoopOutput

Off

All Loops Deactivated

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