2 Minute Read
| Overview
Here we are going to assume that the system is cooling for all examples. But if the AHU goes into heating mode, refer to DAB logic to understand how the damper positions will be set. A VAV system only goes into heating if ALL zones current temperatures are below their respective desired temperatures. Otherwise stated all zones must have a heating load before the central AHU goes into heating. For more information refer to VAV Terminal Profile Logic - Heating.
Note: Guideline 36 specifies that you set the min/max flow of your VAVs. With 75F you replace CFM with damper position as a proxy.
| Zone in Cooling
VAV damper logic has some very important distinctions over DAB damper control. Primarily you need to remember that the loop output is 0-100% as a single direct-acting PI loop. The concept of zero error at the midpoint does NOT apply here as it did with DAB.
The graphic below should be your guide to damper positions in VAV. Be aware that IAQ can be layered on top of this and can affect damper positions. For now, though, we aren't going to complicate it with that.
So let's discuss what's happening here.
Your Zone coolingLoopOutput begins when your current Temperature > coolingDesiredTemp, or as soon as you have a positive error. Once the loop begins for zone cooling mode, it is as simple as a basic PI loop to determine damper positions. So we will take an example here. Below you can see when we are at 0% coolingLoopOutput our damper position is at its cooling minimum of 20%.
As the loop increases so do our damper position. All we have done here is scale the damper position min and max to the loop Output 0-100%.
loop Output = 0% then damper Position = Min Damper Position
loop Output = 100% then damper Position = Max Damper Position
Other than this there are a set of the standard PI Loop tuners and configuration values for each of the VAV zones controlled off of the current temp-desired temp error.
| VAV Tuners
Tuners are the features of the 75F system, which are a set of predefined parameters that can be modified or changed to customize and finely tune our system's functioning to cater to specific building types and specific customer needs.
Below are tuners applicable for VAV Terminal profiles:
|
Tuner Group |
Tuner Name | Tuner Description |
Unit |
Default Value |
Min, Max, Incremental Values | Priority Array Level |
| VAV | CoolingDeadband | Range of temperatures where no cooling occurs. E.g. if SP is 70F with coolingdeadband as 2 then the range between 68 ~ 72 no cooling will occur | °F | 2 | 0, 10.0, 0.1 | Building, Module |
| VAV | CoolingDeadbandMultiplier | Tuner that manages the value of coolingdeadband. This tuner multiplies with the value of coolingdeadband and manages the value of coolingdeadband | NA | 0.5 | 0, 5.0, 0.1 | Building, Module |
| VAV | fanControlOnFixedTimeDelay | Prior to starting the fan, the damper Position is first driven fully closed to ensure that the fan is not rotating backward. Once the fan is proven on for fanControlONFixedTimeDelay, the damper override is released | min | 1 | 0, 10, 1 | Building, Module |
| VAV | HeatingDeadband | Range of temperatures where no heating occurs. E.g. if SP is 65F with heatingdeadband as 2 then the range between 63 ~ 67 no heating will occur | °F | 2 | 0, 10.0, 0.1 | Building, Module |
| VAV | HeatingDeadbandMultiplier | Tuner that manages the value of heatingdeadband. This tuner multiplies with the value of heatingdeadband and manages the value of heatingdeadband | NA | 0.5 | 0, 5.0, 0.1 | Building, Module |
| VAV | ValveActuationStart DamperPosDuringSysHeating | During system heating, this tuner determines the control signal for the valve as the damper opens from vavleActuationStartDamperPosDuringSysHeating % to maxDamperPosHeating %. The actual mapping of the valve actuator into the output signal depends on the type of actuator selected. Eg. if the max/min dampers are 100% and 40% and the calculated damper position is 80% then the v ve control signal will be 33% and for a 2-10V actuator, the output will be 4.4V | NA | 50 | 0,100, 5 | Building, Module |
| VAV | zoneCO2Target | This tuner determines target value of CO2 in a zone | ppm | 1000 | 0, 2000, 10 | Building, Module |
| VAV | zoneCO2Threshold | This tuner determines a point where system starts controlling dampers to maintain space CO2 at target value in a zone | ppm | 800 | 0, 2000, 10 | Building, Module |
| VAV | ZonePriorityMultiplier | Each zone has a priority, and this tuner manages the value of zone priority in a zone. This tuner multiplies with the value of zonepriority | NA | 1.3 | 0, 100, 1.3 | Building, Module |
| VAV | ZonePrioritySpread | This is a range of one priority where occupants in a zone that is further away from the desired are exponentially more likely to feel uncomfortable. This prevents a zone with very large temp drifts from driving system operation | NA | 2 | 0, 10, 1 | Building, Module |
| VAV | ZoneVOCTarget | This tuner determines the target value of VOC in a zone | ppb | 500 | 0, 1000, 10 | Building, Module |
| VAV | ZoneVOCThreshold | This tuner determines a point where the system starts controlling dampers to maintain space VOC at target value in a zone | ppb | 400 | 0, 1000, 10 | Building, Module |
| VAV | reheatZoneToDATMinDifferential | When the system is in cooling and a zone requires heating, instead of opening the damper when the heating loop output for a particular zone is above 50%, now the zone damper is opened only when discharge temp is more than room temp by the value of reheatZoneToDATMinDifferential tuner | °F | 9 | 0, 20, 0.5 | Building, Module |
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