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 outer fresh air that is brought into the building. Otherwise, known as the economizer, which is a feature of HVAC systems to use cold outside air to provide free cooling to the building. The OAO has 2 main functions, Demand Control Ventilation(DCV) and economizing. Optionally, it is also used for maintaining pressure inside the building.
|How It works
The OAO combines two primary aspects for Economizer control.
Demand Control Ventilation
It is a CO2 levels based economizing, which is used to control CO2 IAQ levels in the space. If CO2 is above 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 its 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 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 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 provide 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
- Analog-in1 connects to return air CO2 sensor
- Analog-in2 connects to RTU current transformer
- Th-in1 connects to Outside Air Temperature
- Th-in2 connects to Supply Air Temperature
- One Wire sensor bus connects to Duct Temperature/Humidity sensor for Mixed Air Temperature/Humidity
- Analog-out1 connects to Outside Air Damper
- Analog-out2 connects to Return Air Damper
- Relay-1 connects to exhaust fan stage 1
- Relay-2 connects to exhaust fan stage 2
|Pairing
The 75F smart Node is used along with the CCU to achieve the Outside Air Optimization control in the buildings.
Once power is supplied to the system and all Smart Nodes are displaying 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. The device is paired via Bluetooth.
- Press the right button on the Smart Node OAO until you reach advertising mode screen.
- Click system devices option
-
Select the OAO under System Devices.
- Click Pair Module to start the pairing process.
-
Follow the pairing steps on the Smart Node.
Click Pair.
The CCU detects all the devices ready for pairing at close range, confirm the Bluetooth Pairing address of the Smart Node 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 Smart Node and click proceed.
- 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
|
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(%) |
To select a percentage value for the minimum outside air damper opening position |
0 |
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 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 |
- Set the required parameters from the configuration parameters screen, using the table provided 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.
|OAO Tuners
Tuners are the features of 75F system, that are a set of predefined parameters that can be modified or changed in order to customize and finely tune our system's functioning to cater to specific building types and specific customer needs.
Below are tuners applicable for a OAO profile:
|
Tuner Group |
Tuner Name | Tuner Description |
Unit |
Default Value |
Min, Max, Incremental Values | Priority Array Level |
| OAO | Co2DamperOpeningRate | This tuner determines the rate at which outside air damper will open to manage CO2 level based on return air CO2 level | NA | 10 | 0, 200, 10 | Building, Zone |
| OAO | EconomizingMaxHumidity | Once the outside humidity value goes beyond this value then the outside air damper will be completely closed even when free cooling is available | % | 100 | 0,100,1 | Building, Zone |
| OAO | EconomizingMaxTemperature | Once the outside temperature value goes beyond this value then the outside air damper will be completely closed even when free cooling is available | °F | 70 | -50,120,1 | Building, Zone |
| OAO | EconomizingMinHumidity | Once the outside humidity value goes below this value then the outside air damper will be completely closed even when free cooling is available | % | 0 | 0,100,1 | Building, Zone |
| OAO | EconomizingMinTemperature | Once the outside temperature value goes below this value then the outside air damper will be completely closed even when free cooling is available | °F | 0 | -50,80,1 | Building, Zone |
| OAO | EconomizingToMain CoolingLoopMap | For a fully modulating RTU the economizingLoopOutput maps to the first economizingToMainCoolingLoopMap % of the cooling loop. When this is 30, 0 to 30% of systemCoolingLoopOp gets mapped to 0 to 100% of economizingLoop | °F | 30 | 0, 100, 1 | Building, Zone |
| OAO |
EnthalpyDuct CompensationOffset |
This tuner is used in OAO as an offset on outsideEnthalpy for determining whether free cooling can be used. If sum of outsideAirEnthalpy & this tuner value is less than insideAirEnthalpy then free cooling will be used | NA | 0 | 0, 10, 0.1 | Building, Zone |
| OAO | OutsideDamper MixedAirMinimum | This tuner describes the mixed air minimum temp below which the outside damper is completely closed even when free cooling is available. | °F | 44 | 30, 60, 1 | Building, Zone |
| OAO | OutsideDamperMixedAirTarget | This tuner describes the MAT target temp below which the outside damper starts closing | °F | 50 | 30, 60, 1 | Building, Zone |
| OAO | systemPostPurgeFanSpeedTuner | Tuner that determines the at what speed the fan should be run during the purge | % | 50 | 0, 100, 1 | Building, Zone |
| OAO | systemPostPurgeOccupied TimeOffsetTuner | Tuner that determines the how long after the occupancy period the post purge will kick in | min | 20 | 0, 360, 1 | Building, Zone |
| OAO | systemPostPurgeRuntimeTuner | Tuner that determines the how long the purge that occurs after the occupancy period will last | min | 120 | 0, 360, 1 | Building, Zone |
| OAO | systemPrePurgeFanSpeedTuner | Tuner that determines the at what speed the fan should be run during the purge | min | 50 | 0, 100, 1 | Building, Zone |
| OAO | systemPrePurgeOccupied TimeOffsetTuner | Tuner that determines the how long before the occupancy period the pre purge will kick in | min | 180 | 0, 360, 1 | Building, Zone |
| OAO | systemPrePurgeRuntimeTuner | Tuner that determines the how long the purge that occurs before the occupancy period will last | min | 120 | 0, 360, 1 | Building, Zone |
| OAO | systemPurgeDabDamper MinOpenMultiplier | Parameter that determines the minimum DAB damper position during the purge and enhanced ventilation mode period | % | 1.5 | 0, 10, 0.1 | Building, Zone |
| OAO | systemPurgeVavDamper MinOpenMultiplier | Parameter that determines the minimum VAV damper position during the purge and enhanced ventilation mode period | % | 1.5 | 0, 10, 0.1 | Building, Zone |
| OAO | systemPurgeDab MinFanLoopOutput | Parameter that determines the minimum DAB fan loop output during the purge period | % | 50 | 0, 100, 1 | Building, Zone |
| OAO | systemPurgeVav MinFanLoopOutput | Parameter that determines the minimum VAV fan loop output during the purge period | % | 50 | 0, 100, 1 | Building, Zone |
| OAO | economizingDryBulbThreshold | In case If inside enthalpy is lower than the outside enthalpy, free-cooling can still be done if outsideTemp is lower than economizingDryBulbThreshold tuner | °F | 55 | 0, 70, 0.5 | Building, Zone |
|Few Important Tuners and their Usage
From the above list of tuners let us look at a few important tuners and examples, of their usage in the the algorithm and the 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 operation should not happen when outside weather parameters are beyond 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 heater is on. This condensation is acidic and will corrode the heater.
- For cooling scenario with DCV or econ this 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, where 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 its highly recommended to use them with an expert supervision preferably the 75F support team.
Mechanical Lockouts for Cooling & Heating in OAO
Mechanical lockouts are features in the 75F system, that are introduced to either enable or disable the locking out or shutting down of the cooling and heating systems in the HVAC, otherwise called the mechanical heating or cooling, and use the outside air via the economizer for cooling or heating, based on the outside air temperature.
The lockout features are also provided with an option to preset a temperature value, based on which the actual lockout is actuated or not actuated.
This protects the HVAC unit under bad weather conditions and provides energy efficiency.
Note: The lockouts do not affect any zone profiles, operation of the OAO logic, and the heatingLoopOutput or the CoolingLoopOutput in any way.
Accessing Lockout
The lockout options can be accessed from the CCU.
To access the lockout options.
From the CCU zones page.
- Click the setting
icon.
The Floor Layout screen is displayed.
- Click Settings option.
The settings screen is displayed.
- Click the Installer options icon
The Installer options screen is displayed, and under the pairing start address section, the lockout options are displayed, which can be enabled or disabled.
The toggle for the Use Outside Temp Lockout Cooling is enabled to disable cooling based on the Outside Air Temperature, and The toggle for the Use Outside Temp Lockout Heating is enabled to disable Heating based on the Outside Air Temperature. BY default the Toggle button is disabled, and can be manually enabled.
When a OAO is paired as a system profile, under a CCU as below.
The Use Outside Temp Lockout Cooling is auto enabled, disabling the mechanical cooling based on the outside air temperature as below.
The enabled cooling lockout option is provided with a option to set a lockout temperature limit, and when the Outside Air Temperature is below this set temperature, there will be no mechanical cooling, by default the temperature limit value for the cooling lockout is 50 degree Fahrenheit, and the same can be modified using the dropdown values. The auto enabled cooling lockout can be disabled manually using the toggle.
Similarly the Use Outside Temp Lockout Heating can be enabled as below.
The manually enabled heating lockout option is also provided with a option to set a lockout temperature limit, and when the Outside Air Temperature is above this set temperature, there will be no mechanical heating, by default the temperature limit value for the cooling lockout is 80 degree Fahrenheit, and the same can be modified using the dropdown values. The auto enabled heating lockout can be disabled manually using the toggle.
Formula Based Scenarios
Below are a few formula based scenarios, where either the lookout happen and the mechanical cooling or heating is not available, or the lockout do not happen and the mechanical cooling or heating is available
|
For Mechanical Cooling Lockout When the set lockout temperature is 50F, where |
For Mechanical Heating Lockout, When the set lockout temperature is 80F, where |
|
When, systemCoolingLoopOutput >0% and the useOutsideTempLockoutCooling is Enabled and, If Outside Air Temperature < outsideTempCoolingLockout, Then, Mechanical Cooling is not available |
When, systemHeatingLoopOutput > 0% and the useOutsideTempLockoutHeating is Enabled and, If Outside Air Temperature > outsideTempHeatingLockout, Then, Mechanical Heating is not available |
|
When, systemCoolingLoopOutput >0% and the useOutsideTempLockoutCooling is Enabled and, If Outside Air Temperature > outsideTempCoolingLockout, Then, Mechanical Cooling is available |
When, systemHeatingLoopOutput >0% and the useOutsideTempLockoutHeating is Enabled and, If Outside Air Temperature < outsideTempHeatingLockout, Then, Mechanical Heating is available |
Mechanical Cooling & Heating Widget
The mechanical cooling and heating can be observed under the internal portal through the Lockouts widget for the internal portal
- Login to the Internal portal
- Hover on the Lockouts Widget
The mechanical cooling and mechanical heating status can be observed.
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