Economizer Analysis via Scatter Plots–Linking Excel Chart Labels to Data in Cells

As you may recall, this fall, I started a string of posts on using scatter plots to assess building performance.  The 8th post in the series was going to use the technique to assess and economizer by comparing what a perfect economizer would do to what a real world economizer would do by looking at the perfect economizer lines relative to a scatter plot.

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The Excel Trick

When I started writing that post, I realized that it was important to be “on the same page” regarding a few economizer fundamentals.  Economizers, while simple in concept, are actually pretty complicated in terms of what is going on.  You may have developed a sense of that from the previous posts on the physics of a mixed air plenum and the physics of linkage systems.  And there are still a number of topics I feel I need to discuss to set up the scatter plot post, which I enumerate a bit later in this post.

Meanwhile, I realized that the chart I will be using for the scatter plot has a a number of features that are Excel “tricks” that would be useful in the broader context, things like:

  • Chart labels that update to match information in cells in the spreadsheet that support it, or
  • Reference lines that extend across the chart through a specific point.

I actually realized that fairly early on in the development process and made a video clip that shows how to do the “tricks”.  So, I figured I could share that sooner rather than later since the techniques are useful beyond the way I use them in the economizer diagnostic.  So, without further a due, here is the video.


Hopefully that provides some benefit beyond the economizer analysis process.

A Few More Economizer “Details” to Consider

What follows is the list of topics I plan to address to complete the process of “getting us on the same page” regarding economizers so we can discuss the scatter plot diagnostic in the most meaningful manner possible. The current plan is for each of the headings in the list  to become a blog post topic.   I actually have a pretty good start on a number of the posts, so bear with me as I try to get everything pulled together and on line.

Outdoor Air for Ventilation vs. Free Cooling

For one thing, you need to understand the difference between bringing in outdoor air to ventilate and bringing in outdoor air for free cooling.  Ventilation air is reflected in the minimum outdoor air setting and the diagnostic plot will help you assess if your economizer is doing what it should be doing in that regard.  But one of the inputs the plot requires is a reasonable estimate of what the appropriate minimum outdoor air setting should be in the first place.

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Accurately Measuring the Actual Mixed Air Temperature

For the diagnostic to work, you need to make sure you are accurately measuring the mixed air temperature that is one of the inputs to it.  That can be much harder than you might thing.   My previous post on economizer mixed air plenum stratification will give you some perspective on that.  If you get a good mixed air temperature input for your diagnostic plot, it can help you identify if the control system is not as lucky in that regard and it may even be able to help you quantify the value associated with improving the existing sensor system.

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The Importance of the Integrity of the Damper Blade Seals

Most people quickly realize that the integrity of the damper blade seals would be important in a cold climate.  But if you really think about it, you will discover that the impact of poor blade seals can be important in mild climates too.  The diagnostic plot can help you identify issues in this area.  But you need to have a sense of what you are looking for and why it matters to benefit from any insight the diagnostic might provide.

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The Importance of the High Limit Set Point

In most climates, there will come a point in time when the outdoor air is to hot or humid or both to be a viable source of free cooling and economizers need a high limit to shut down the process when that occurs.   The diagnostic plot can show you if that is happening as intended or not.  But to do that, it needs and input from you that is a reasonable estimate of what the high limit set point should be in the first place.  That will vary with both the climate and the nature of the load served by the system, so understanding how to come up with a reasonable assessment of the set point is an important part of using the diagnostic plot.

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Integration of the Economizer Control Process with Other Control Processes

For most HVAC systems, the economizer process is only one of the processes running in the system.  For example, there could be warm-up processes, preheat processes, schedules, mechanical cooling processes, and flow management processes running concurrently in the air handling system.   An economizer process is a cooling process, so, for instance, it should not be running if the system is in preheat or warm-up.

The diagnostic plot can help you understand if the economizer is integrated correctly with many of the other processes in the HVAC system, but you need to furnish reasonable estimates of the trigger points for the other processes to set up the diagnostic plot.  Thus, understanding those processes and how they integrate with the economizer is important.

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David Sellers
Senior Engineer – Facility Dynamics Engineering

This entry was posted in Economizers, Excel Techniques. Bookmark the permalink.

2 Responses to Economizer Analysis via Scatter Plots–Linking Excel Chart Labels to Data in Cells

  1. Erik says:

    Thanks for your writeups – I find them very useful! I was curious if you’ve ever published the “8th Part” of your scatterplot series? I don’t seem to find it on your site, and from the inference at the beginning of this post, I think I’d find it very useful…


    • You know, I really need to finish that. I have it about half written. So, the answer to your question is “no, I haven’t published it”. But let me see if I can juggle it up the stack now that you mention it.

      Thanks for visiting the blog and I’m glad this is useful to you.

      Best regards,

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