VAV Terminal Profile Logic & Tuners - Heating

3 Minute Read

| Zone in Heating Mode, System Cooling

Alright, Cooling is the easy part. Now let's dissect how a zone operates when the zone requires heating, but the AHU is providing cooling. This is what I consider to be the most difficult section of the course. When a zone requires heating there will be 2 PI loops running using the same set of core PI loop tuners. For more information refer to VAV Terminal Profile Logic -Cooling.

Loop 1: The Zone Loop

Loop 2: The Discharge Air (DAT) Loop

| Zone Heating Loop

The zone loop is calculated with a standard 75F PI loop. Control Variable = Desired Temp, Process Variable = Current zone temp. The Loop output is exactly as it would be with our typical PI loop you learned about earlier.  However, the damper position is scaled a bit differently. 

Damper Position - 
The damper position is scaled proportionally to your the top half of your loop output. Meaning: 

IF DAT > current temp + reheatZoneToDATMinDifferential
0-50%loop Output = Minimum Damper Pos
51-100%loopOutput = scaled damper position
100% Loop Output = Maximum Damper Pos 

IF DAT < currentTemp + reheatZoneToDATMinDifferential
0-100% Loop Output = Minimum Damper Position

| Zone Heating DAT Loop

The DAT loop for VAV adds a new concept to the equation that we are calling Dynamic Setpoint. In all examples so far the Setpoint is either fixed or only changes at very infrequent intervals based on schedule or user intent. The DAT loop on the other hand uses the Zone Loop output to calculate the Heating Discharge Air Temp Set Point (DAT SP) which is the Setpoint for the second loop, the DAT Loop. See in the example that the DAT SP can change every minute potentially, especially during a heating event.  

Even though the DAT Loop uses the same logic as the Zone Loop and other Reverse acting PI loops in our system, it is the most complicated to internalize since the Setpoint can be constantly changing. 

The DAT SP is scaled from AHU SAT SP to snHeatingAirflowTemp
Zone Loop Output = 0 = DATSP = AHU SAT SP
Zone Loop Output = 100 = DATSP = snHeatingAirflowTemp

DATSP = Setpoint for DAT Loop

From here you can use the standard PI Loop Calculations to determine what your Reheat Output will be over time. Just plugging in DAT as our current temp and DAT SP as your desired temperature. 

| Zone Heating Complete

When you put it all together it looks like the below over some time. Take some time to internalize what is happening. We will get on to how to use the tuners for this in a little while. 

| 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: