PID Loop Tuning Explained – Part 1 – Proportional Only

Ok so here is the link to the first of I think more than just 3 videos on PID Loop Tuning. I guess after I began recording these videos I realized that 5-10 minutes just doesn’t allow enough time to get into much detail.

Don’t worry that just means this video series will continue as long as it takes to go over everything. The best apart about this is that after each video you can give you me your thoughts on the video and make some comments as to what you might want me to go more in depth with in later videos.

For now, without further ado… Here’s the video:

If you want to see it in HD don’t forget to click the small 480p in the lower right corner and select 720p HD. 

Well as you saw that video was pretty short, so let me write out some comments about that video now.

No Continuous Adjustments

As I mentioned in the video once a Proportional only PID Loop makes an adjustment based on the difference from input to set point it will not make any further adjustments until the input changes. These kinds of loops work well when the input it is trying to control is not affected by anything other than what the controlled device is delivering, such as a humidfier steam valve, chilled water valve or speed of a fan. Honestly, the cases that use proportional only loops the most are usually manufacturing and industrial processes that are very tightly controlled.

On the other hand, most HVAC equipment and systems always have some other ambient or uncontrolled variable that affects the input. An example would be the sun shining through a window creating a heat load to a room’s temperature. Let’s say there was no cooling what-so-ever and the warmth from the window kept the room temp at a steady 80°F instead of the desired 72°F.

We now want to turn on a fan coil unit that has a chilled water valve controlled by a proportional only loop. Based on the difference of the 80°F room temperature and the 72°F set point the P only loop might cause the valve to be at say 75% open.

We Just Need a Lil More Cooling… Please?

Now that the valve is open to 75% and some cold air is pouring in, the room temperature falls to say 75°F. The problem is that the proportional only loop would already be closing down based on the new difference between the input 75°F and the setpoint of 72°F. Using no math at all let’s just say at that point the valve is now closed down to 40% open.

We know that with the hot sun pouring in we need to stay at around 75% open to get the temperature down in to the 70s, but that just won’t happen since that P only loop will focus only on the difference between input and set point and not take into account that the sun is introducing additional load that requires the valve to be more open than normal.

End result

At some point you will find that a proportional only loop will get to a point where it will offset the heat load in such a perfect manner that the room temperature will stay still and not rise or fall. At this point the cooling valve will be offsetting the heat load, but still not maintain set point because the valve or P only loop, only creates a percentage based on the difference of the two temperatures.


I hope that shed a little more light than the quick video. Don’t worry about those office workers next to that window cuz in the next video we are going to change out that P only loop for a shiny proportional and integral (PI) loop that will continually adjust to finally arrive at set point.

Have you had any experiences with tuning PID Loops? How long did it take you to get a good feel for the various settings?


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You can view all the other videos in this series at the PID Loop Tuning Explained Video Training Series post.

One Response to PID Loop Tuning Explained – Part 1 – Proportional Only

  1. Blu Voltaire April 15, 2012 at 19:39

    Great video ! What control system are you using to control this unit ?


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