How do I tune the CFM loop for a trueCFM VAV?

Tuning the PI loop for an independent pressure controlled (true CFM) VAV can be a little daunting. However it is absolutely critical to getting your system to behave correctly. What you will typically see out of the box is something like this. The damper slamming back and forth trying to find balance in the PI loop and your CFM wildly fluctuating due to the rapid changes in damper position. Since the 75F loops run once a minute, we need to slow down our response, and give most of the weight to the integral side of the PI loop. 

| Step 1 - Change PI Tuners at the Building Level

Give more weight to the Integral side of the loop and also reduce the amount of time it takes for the integral to ramp up. Change the three tuners as shown below. 

airflowCFMproportionalKFactor = 0.2
airflowICFMntegralKFactor = 0.8
airflowCFMintegralTime = 5

| Step 2 - Set the Proportional Error Range per Zone

To do this you must first find what the maximum CFM reading is. Don't worry you dont have to be perfect. So when you open up a zone that is cycling like this, hover your tooltip over a peak in the CFM reading and take note of the approximate value.

In this example, we will say it is around 900CFM. Take this value and input as the airflowCFMProportionalRange tuner. 

| Step 3 - Tweak it to find balance

The first two steps should smooth out your damper and CFM swings pretty dramatically. But you may find that your airflowCFMProportionalRange value being set at the maximum caused the zone to never quite get its damper to 100%, or might find that you are responding a little slower than you like. You can play with this value over time to try to balance response time and smoothness.