Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – #3 – The System Handles a Lot of Outdoor Air in an Extreme Climate

This post continues to build the case for targeting a MAU for additional investigation during an RCx process.

The General Case

When outdoor air is introduced into a facility for ventilation purposes (i.e. to control contaminants and make up for air that is being exhausted elsewhere like at a kitchen hood), then the air has to be treated to protect the equipment inside the facility and ensure safety and comfort. In most cases, the air will need to be heated, cooled, and dehumidified. In some cases, the make-up air will also require humidification.

All of these processes require energy and other resources; how much will depend on the nature of the climate relative to the conditions you are trying to maintain. In a relatively mild climate like San Francisco …

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… the make-up air handling processes will be much less energy intensive than in someplace like St. Louis, Missouri …

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… where it can be extremely hot and humid in the summer and extremely cold and dry in the winter.

The extreme conditions mean that in addition to energy intensity, the make-up air systems could be challenging to operate. Specifically, loss of humidity control in the summer can ruin finishes and create indoor environmental quality problems when condensation happens in places it shouldn’t. And it can be very easy to freeze a coil during sub-freezing weather if things are not properly designed, installed, adjusted, and operated.

If you have ever frozen a coil, it is what FDE’s founding principle, Jay Santos would call “a significant emotional event”. Because of this, as an operator, you may be tempted to take steps to protect your coils during extreme weather that make your energy intensive make-up air system even more energy intensive. In fact, you may have no choice on those nightmarish days when Mother Nature throws everything she has at you and your facility.

Another thing that can make a climate with a wide range between extremes challenging is that you can often see a pretty good portion of the span in one day. Consider January 29, 2008 in St. Louis, MO.

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In one 24 hour period, it went from the mid 50’s°F to a high of 71°F and then plummeted to about 15°F. The purple line is the mixed air temperature for a high performance system serving a surgery that had a 65% minimum outdoor air requirement and a 49°F discharge temperature requirement. In the course of this one 24 hour period, that system went through every single operating mode it was designed to deliver.

Compare the preceding to a day with a huge temperature swing and nearly unbearable heat and humidity in San Francisco, CA.

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My guess is that operating the surgery air handling system in Missouri is more challenging than operating it in San Francisco. And, my guess is there are opportunities to save energy in both locations, but the numbers will be bigger for the system in Missouri because of the climate considerations.

The bottom line is that a make-up air system is a clue about a target for energy savings. But a make up air system in an extreme climate is likely to yield very significant opportunities for improving performance in addition to saving energy and other resources.

Our Target Facility’s Case

In the case of our hotel in Golden, Colorado, we have some extreme conditions to deal with, especially in the winter.

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And it looks like a big swing in 24 hours will be more like the one illustrated in St. Louis than the one illustrated for San Francisco.

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So, it’s likely an effort focused on a makeup air system in this climate has the potential to yield some results, both in terms of energy savings and improved performance.

Incidentally, if you are wondering where the bin and hourly climate data came from, I have done a number of blog posts on the topic including one that hooked you up with most of the others prior to that point. Since then, I have done a few more on the topic including Hourly Weather Data for Times Gone By (Plus Opening a Delimited File with Excel), Bin Weather Data for the United States and International Locations, and Good News about NWS Weather Data, Plus Working with Date and Time in Excel.

If you like the bin plots I do on the psych charts, I am using Akton Psych Chart to do it. the last time I checked, you can get just about all of the functionality of Akton Psych Chart at a discount, including the bin data plot capability by downloading Greenheck’s free electronic psych chart and upgrading it to the professional version.


David Sellers
Senior Engineer – Facility Dynamics Engineering
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