CCU Outside Air Optimizer (OAO) System Profile - Sequence of Operation – Airoverse

| 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.

For more information refer to the following articles.

| Schematic

CCU OAO

| 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 Airoverse 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 the 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( Weighted Average CO2 for zones/ Return Air CO2) > zoneCO2Threshold

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

Note:

Based on whether the usePerRoomCO2Sensing configuration parameter is enabled or disabled, the CO2 used for the above formula varies as below:

If the usePerRoomCO2Sensing is enabled	Then the weighted average CO2 for the zones is considered against the zoneCO2Threshold for the Economization action.
If the usePerRoomCO2Sensing is disabled	Then the CO2 from the Return Air CO2 sensor is considered against the zoneCO2Threshold for the Economization action.

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.

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 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

| 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.